Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Benzene
Benzene is an organic molecule with the formula C6H6
...
Each carbon atom is bonded to a single hydrogen atom
...

The Kekule model:
This benzene model was first proposed by the German chemist Friedrich Kekule
...

This was then adapted to say that benzene was constantly changing between two structural
isomers, where the double bonds are between different carbon atoms, in order to explain why
benzene wouldn’t react with bromine water without a halogen carrier present
...
They do this in order to find the most
energetically stable electron arrangement
...

This model states that the p-orbitals of each carbon atom in the ring overlap above and below the
ring
...
The π bonds don’t belong to any particular
carbon atom, so the electrons in the bonds are delocalised in a ring shaped
cloud in the molecule
...







All the bond lengths in benzene are the same
...
However, experiments using X-ray diffraction have shown
that all the bonds in benzene have a length of 140pm
...
They are hybrid bonds: halfway between a double and a single bond
...
The enthalpy of
hydrogenation of cyclohexene (1 double bond) is -120kJ mol-1
...
However, experimentally, the enthalpy of hydrogenation is -208 kJ mol-1
...
The energy
difference of 152 kJ mol-1 is called the resonance energy
...

The increased stability of benzene is due to the delocalised ring
...
In benzene, the delocalised electrons don’t belong to
any specific carbon atom and are spread throughout the molecule evenly
...
The polarised electrophile then accepts an
electron pair from the C=C bond and is added onto the molecule
...
This is because the delocalised ring doesn’t have a high
enough electron density to polarise the electrophile
...
Benzene undergoes electrophilic substitution, which allows it to preserve the stable ring of
delocalised electrons
...
The
electrons released from this broken bond are added back into the ring to replace those lost,
restoring the delocalised ring
...

The electrophile has therefore been substituted for a hydrogen atom
...
A halogen carrier accepts a lone pair of electrons from the electrophile
...
This increases
its electrophilic strength as a permanent dipole is formed
...
Halogen
carriers include aluminium halides, iron halides or iron
...
The halogen carrier acts as a catalyst as it helps form a bromonium ion (in the case of
bromination) which is a much stronger electrophile than bromine
...

Br2 + FeBr3  Br+ + FeBr4H+ + FeBr4-  FeBr3 + HBr
Benzene rings can also be nitrated, by electrophilic substitution, to form nitrobenzene compounds
...
The sulphuric acid is used to form the nitronium ion (NO2+) which acts as
the electrophile that is substituted into the benzene ring
...
The H+ ion released by this
reaction is used to reform the sulphuric acid, fulfilling its catalytic role
...
For multinitration, the temperature must be above 55 degrees C
...
It is a benzene ring with one carbon atom bonded to a hydroxyl
group
...






Phenol reacts with sodium hydroxide to form sodium phenoxide and water
...
In this reaction,
two moles of phenol react with two moles of sodium to form two moles of sodium
phenoxide and one mole of hydrogen
...


Phenol undergoes electrophilic substitution like benzene
...
As the only
difference between benzene and phenol is the hydroxyl group, this must be the factor that
influences phenol’s reactivity
...
These extra electrons increase the
electron density of the ring, especially at carbons 2, 4 and 6
...
It then can undergo the electrophilic substitution reaction to form, in the case of
bromine, 2, 4, 6-tribromophenol
...

2, 4, 6-tribromophenol is insoluble in water and precipitates out of the solution to form a white
solid that smells of antiseptic
...
It was first used as an antiseptic to clean wounds but was only used
for a short time as it was too damaging to tissue
...
Phenol is also used to make polymers such as Kevlar
and polycarbonates
...
Phenol is also used to make plastics such as Bakelite and
other insulating resins
...
Aldehydes
always have the C=O on carbon 1, which is also bonded to a hydrogen, to form a CHO group
...
In ketones, the
C=O group is bonded to two other alkyl groups, to form a C(R)COC(R) group
...

The oxidising agent acidified potassium dichromate (K2Cr2O7/H2SO4) is used to oxidise alcohols to
aldehydes and ketones
...
The solution therefore changes colour from orange to green, which signifies
that the reaction has taken place
...

Primary alcohol + [O]  aldehyde + water
Aldehyde + [O]  carboxylic acid
Primary alcohol + 2[O]  carboxylic acid + water



Secondary alcohols are oxidised to ketones
Secondary alcohol + [O]  ketone + water

Tertiary alcohols cannot be oxidised with potassium dichromate
...

The reaction conditions can be manipulated to give different products
...
At too high a heat, the aldehyde will be oxidised to a carboxylic acid
...
Distillation apparatus
is used so that any aldehyde that evaporates condenses out of the solution before it is further
oxidised
...

Reflux apparatus has a condenser but without anywhere for the condensed product to escape to
...


Reducing Carbonyls
Alcohols can be oxidised to form aldehydes, carboxylic acids and ketones
...
The reducing agent used is NaBH4 (sodium
tetrahydridoborate/borohydride)
...

Aldehyde + 2[H]  Primary alcohol + OH- ion
...
A nucleophile in the
form of a H- ion is added onto the carbon chain
...
The C=O group is polar, due to the electronegativity of the oxygen atom
...

2
...

This forms a bond between the carbon and the hydrogen
...
As the carbon can only have 4 bonds, it breaks one of its bonds with the oxygen atom
heterolytically
...

4
...
The delta positive
hydrogen atoms in the polar water molecules are attracted to the negatively charged
oxygen
...
The oxygen atom donates its lone pair of electrons to the hydrogen atom of the water
molecule
...

This produces an alcohol and an OH- ion
...
It reacts with carbonyl compounds (but not carboxylic acids or esters) to form an
orange crystalline derivative of the DNPH
...

1
...

2
...
This is then filtered and allowed to dry
...
The melting point/boiling point of the purified derivative is measured and recorded
...

For example, if an unknown compound forms an orange crystalline derivative with Brady’s reagent,
the melting/boiling point of the derivative is measured and compared with known values
...

Tollens’ reagent is used to distinguish between an aldehyde and a ketone
...
Tollens’ reagent acts as on
oxidising agent
...
When it reacts with an aldehyde,
the aldehyde is oxidised to a carboxylic acid and the reagent is reduced
...
This causes a silver mirror
to form – solid silver in solution
...

Therefore, Tollens’ reagent only forms a silver mirror when reacted with an aldehyde
...
Carboxylic acids are
weak acids and so partially dissociate in solution
...

Carboxylic acids are polar molecules as the carbonyl and hydroxyl groups are polar
...
This polarity allows for the formation of hydrogen bonds with
polar water molecules, therefore carboxylic acids are soluble
...
The alkyl group is non polar and therefore hydrophobic
...

Carboxylic acids undergo typical acid-base reactions:
RCOOH + NaOH  RCOONa + H2O
2RCOOH + Mg  (RCOO)2Mg + H2
2RCOOH + MgCO3  (RCOO)2Mg + CO2 + H2O
The salts formed are called carboxylate salts and end with the suffix –oate
E
...

2CH3COOH + Mg  (CH3COO)2Mg + H2
Ethanoic acid + magnesium  magnesium ethanoate + hydrogen

Esters
Esters are formed by reacting an alcohol with a carboxylic acid or, less commonly, by reacting an acid
anhydride with an alcohol
...

Esters can be produced by heating an alcohol and a carboxylic acid in the presence of a strong acid
catalyst in reflux
...
This is the esterification reaction
...

Carboxylic acid + alcohol ↔ ester + water
HCOOH + CH3CH2OH ↔ HCOOCH2CH3 + H2O
(Heat, reflux, conc
...
Small esters are volatile so they can be easily distilled out of the
solution
...
High concentrations of acid and alcohol are
used to shift the position of equilibrium to the right
...

An acid anhydride is formed by reacting together two carboxylic acids
...
The acid anhydride is warmed with the alcohol to produce a
carboxylic acid and an ester
...

Acid anhydride + alcohol  ester + carboxylic acid
(CH3CO)2O + CH3OH  CH3COOCH3 + CH3COOH
As the esterification reaction is a condensation reaction, it can be reversed by adding a molecule of
water, which breaks the ester bond
...
Using just water can be very slow, so an
acid or a base is used
...
The ester is heated under reflux with the dilute acid and water
...

Ester + water ↔ alcohol + carboxylic acid
(Heat, reflux, dilute acid catalyst)
Base hydrolysis is uses a dilute alkali
...


Ester + OH- ion ↔ alcohol + carboxylate ion
(Heat, reflux, dilute alkali)
Esters have many uses, but principally they are used in perfumes and in food as flavourings and
odours
...


Fatty acids and triglycerides
Fatty acids are carboxylic acids
...

Saturated fatty acids don’t have any C=C bonds in their carbon chain, whereas unsaturated fatty
acids have at least one C=C bond in their carbon chain
...
They are made up of three fatty acids and
one molecule of glycerol (propan-1, 2, 3-triol)
...
This releases three molecules of water in a
condensation reaction
...

Using the systematic method, the fatty acid is named just as any other carboxylic acid e
...

tetradecanoic acid
...
E
...
if there are C=C bonds on carbons 5, 7 and 12,
tetradecanoic acid becomes tetradeca-5, 7, 12-trienoic acid
...

E
...
tetradecanoic acid would become 14, 0 for 14 carbons and 0 double bonds
...

As unsaturated fatty acids possess C=C bonds, they can form cis-trans isomers due to the lack of
molecular rotation around the C=C bond
...
This results in bent or curved
molecules
...
Trans fatty
acids have similar shapes to saturated fatty acids
...
This shape allows them to overlap with one and other and adhere together due to the
formation of van der Waal’s forces between the molecules
...

Trans fatty acids also raise the levels of LDL’s in the blood and lower the levels of HDL’s in the blood
...
Triglycerides are what are commonly
known as fats
...

Trans fatty acids increase the proportion of LDL’s (low density lipoproteins) in the bloodstream,
which carry cholesterol from the liver into the bloodstream
...
Too much cholesterol in the bloodstream can clog blood vessels,
increasing the risk of heart attacks and strokes
...
Trans fatty acids
therefore represent a serious health hazard, as measures are taken to reduce the amount of trans
fats in food and decrease consumption of processed foods, which often contain trans fats
...
Partial hydrogenation of unsaturated fatty acids can distort
the molecule and cause it to adopt a trans shape
...
HDL’s (high
density lipoproteins) carry cholesterol from the bloodstream to the liver to be broken down
...

Fats can be used as a renewable source of fuel
...

Potassium hydroxide is used as a catalyst
...
Most normal car engines can function
with B20 fuel, which contains 20% biodiesel and 80% normal diesel, however they must be
converted in order to function on B100 fuel
...
This would not only result in deforestation, but also could result
in food shortages, as less land is used to produce edible crops
...
This forms primary, secondary, tertiary amines and quaternary ammonium ions
...
This is because there is a lone pair of electrons on the nitrogen atom
...
As a result, the
amine acts as a proton acceptor and therefore qualifies as a base
...
When an amine is bonded to an aliphatic (no benzene ring) organic group, it is
a stronger base than when bonded to an aromatic group
...
This makes it more difficult to attract and
bond with the H+ ions, decreasing its basic strength
...
g
...
A primary amine and an amine salt form
2NH3 + CH3CH2Br  CH3CH2NH2 + NH4+BrHowever, in reality a mixture of primary, secondary, tertiary and quaternary amines form, as it is
likely that there is more than one substitution
...

When the haloalkane is in excess, it is more likely that a primary amine formed will react with
another haloalkane molecule to form a secondary/tertiary/quaternary amine instead
...

To produce aromatic amines, an aromatic nitro compound is reduced to form a phenylamine
...
g nitrobenzene) is reacted with tin (Sn) and HCl in reflux
...
g C6H5NH3+Cl-)
...
Then an alkali
(NaOH) is added to form phenylamine and water
...

C6H5NO2 + 6[H]  C6H5NH2 + 2H2O (Reflux, Sn/HCl, then NaOH)

Azo Dyes
Azo dyes are man-made compounds that have the azo functional group –N=N-
...
The resultant molecule is an azo dye
...
The electron density is therefore well spread throughout the molecule, increasing its
stability
...
This is because different wavelengths of light are
absorbed by the different delocalised electron systems in each azo dye
...
This is called diazotisation
...
Diazonium salts are weak electrophiles, so they are then
reacted by something that is susceptible to electrophilic attack, like phenol
...

To make a diazonium salt, phenylamine can be reacted with nitrous acid and hydrochloric acid to
form benzenediazonium chloride and two molecules of water
...
Sodium nitrite is reacted with hydrochloric acid to form nitrous acid and sodium
chloride
...

Phenylamine + nitrous acid + hydrochloric acid  benzenediazonium chloride + 2 x water
NaNO2 + HCl  HNO2 + NaCl
Next the diazonium salt is reacted with phenol
...
The azo dye then precipitates out of the solution, and
sodium hydroxide and water are also formed
...
The p orbitals of the oxygen atom in the
hydroxyl group overlap with those in the benzene ring, therefore adding an extra electron pair into
the ring
...

There are many uses for azo dyes
...
As they are very stable, they are used in lightfast
paints: paints that don’t fade in light
...
There is some controversy over their use in food as some azo dyes have been
shown to decrease attention and activity levels in children
...


Polymers
Addition polymers are formed from alkenes
...
This process is called addition polymerisation
...
The polymers are made of repeat units, which is the
part of the molecule that repeats over and over again
...

Condensation polymers are usually polymers that are made up of more than one type of monomer
...
When this link forms,
a molecule of water is produced, so the reaction is a condensation reaction
...





Polyesters are formed from dicarboxylic acids and diols
...

Polyamides are formed from dicarboxylic acids and diamines
...
Amino acids have both an amine and a carboxyl
group
...
An amide group is a group where the carbon atom is double bonded to an oxygen
atom and single bonded to a nitrogen atom (-CON-)
...

Polyesters form when a dicarboxylic acid reacts with a diol
...
This
forms an ester bond that links the two molecules
...
Therefore, this
reaction can be reversed by a hydrolysis reaction – addition of a water molecule will break the
ester/amide link, reforming the original monomers
...
Acids or alkalis can be used for both polyesters and polyamides, but polyesters are
hydrolysed more easily with alkalis and polyamides are more easily hydrolysed with acids
...

Polyester + 2nNaOH  n(dicarboxylate salt) + n(diol)
The catalyst used is usually a metal hydroxide, such as sodium hydroxide
...
In the case of sodium hydroxide, the salt formed would be
sodium dicarboxylate
...
The ester and amide links are present in nature, and the
hydrolysis reaction can be carried out by many bacterial and fungal enzymes
...
Alternative polymers
are being develop that don’t take very long to break down and so don’t accumulate as waste, such
as poly(lactic acid) PLA
...

Addition polymers are even more of a problem, as hydrolysis reaction will not break the monomer
linkages, so natural enzymes cannot degrade them
...

Polymers are being developed that are photodegradable
...
As the bonds
on either side of carbonyl groups are always bonds that link two molecules, the polymers are broken
down into monomers
...
There
are different types of amino acids
...
They also have a hydrogen group and a side chain (R) attached to the central
carbon
...

The general formula for an α-amino acid is RCH(NH2)COOH
...
The carboxyl group gives
them acidic properties as the COOH group can be depronated to form COO-, releasing a H+ ion into
the solution
...

Due to their amphoteric properties, amino acids can exist as zwitterions
...
Zwitterions have a positive charge on one side of the
molecule and a negative charge on the other side
...
This is the pH where the overall charge on the amino acid is 0,
as it is dipolar
...
When the amino acid becomes
a zwitterion, the amine group accepts an H+ ion to become –NH3+ and the COOH group donates a
proton to become COO-
...
The carboxyl group will not depronate,
so the amino acid will have an overall positive charge
...
A zwitterion forms and the overall charge on the amino acid is 0
...

At conditions more basic than the isoelectric point, the carboxyl group donates a proton,
and the amino acid behaves as an acid
...


Amino acids have both amine and carboxyl groups, so they can react together in a condensation
reaction to form a dipeptide
...
Many amino acids can
join together to form a polypeptide, which each reaction releasing a water molecule
...
This
accounts for the millions of different proteins, which are made up of polypeptides
...
This is because, as there are two functional groups on each molecule that could
react together, the amino acids could form a dipeptide with either functional group
...
g
...
Heating proteins with HCl for 24 hours is used
to hydrolyse them
...
This is useful for working out the specific sequence of amino
acids in proteins
...
V
...
H2SO4 catalyst, 170 degrees, reflux)
Primary alcohol + [O]  aldehyde + water (K2Cr2O7/H2SO4, distillation)

Secondary alcohol + [O]  ketone + water (K2Cr2O7/H2SO4, reflux)

Aldehyde + [O]  carboxylic acid (K2Cr2O7/H2SO4, reflux)
Carboxylic acid + alcohol  ester + water (conc
...
H2SO4 catalyst, reflux, excess water, acid hydrolysis)
Ester + OH-  carboxylate salt + alcohol (dilute NaOH, reflux, base hydrolysis)
Acid anhydride + alcohol  ester + carboxylic acid (gentle heat)

Aromatic compounds:
Benzene + Cl2/Br2  chloro/bromobenzene + H+ (halogen carrier catalyst, electrophilic substitution)
Benzene + HNO3  nitrobenzene (H2SO4 catalyst, below 55 degrees for mononitration, above for
multinitration)
H2SO4 + HNO3  HSO4- + NO2+ + H2O

Nitrobenzene  phenylamine (Sn, HCl and reflux, then add NaOH)
Phenylamine + nitrous acid + HCl  benzenediazonium chloride + 2H2O
(below 10 degrees)
(NaNO2 + HCl  HNO2 + NaCl)
Benzenediazonium chloride + phenol  azo dye (chilled, NaOH)

Phenol + NaOH  sodium phenoxide + water
2 x Phenol + 2Na  2 x sodium phenoxide + hydrogen
Phenol + 3Br2  2, 4, 6-tribromophenol + 3HBr

Stereoisomerism and Chirality
Compounds with C=C double bonds can form stereoisomers
...

Stereoisomers occur in alkenes as the C=C bond restricts rotation and movement of other molecules
...
Molecules in a particular arrangement will stay in that arrangement
...
It occurs when the high priority groups are on the same
or different sides of the C=C bond
...
When the priority groups attached to each carbon are
on different sides of the C=C bond, the result is an E isomer
...

Cis/trans isomerism is a type of E/Z isomerism
...
If the two groups are on opposite sides of the C=C bond, the
result is a trans isomer
...

Optical isomerism is another type of stereoisomerism
...
A chiral carbon is a carbon that is attached to four different groups
...
They are optical isomers of each other, or enantiomers
...
One enantiomer will
rotate it in one direction (e
...
clockwise) and the other will rotate it in the other direction (e
...

anticlockwise
...
In nature, enantiomers are very
common
...


Chirality and Drug Development
A racemate is a mixture that contains equal quantities of two enantiomers
...
This is because, as they contain two enantiomers of each other, the two
enantiomers rotate plane polarised light opposite directions, and the effect is cancelled out
...
Equal quantities are produced because there is the
same chance that one enantiomer will be produced as the other
...
g
...
The resultant 2chlorobutane is a 1:1 mix of two enantiomers of 2-chlorobutane
...
This is due to the specificity
of the active site of the enzyme
...
If a drug is produced,
generally only one enantiomer is desired
...
In order to do this, they bind to enzymes and receptor molecules that have very
specific binding sites
...
The other enantiomer will
bind to a different complementary enzyme or receptor and can result in side effects
...
One enantiomer helps to treat TB, while the other
causes blindness
...
One enantiomer of thalidomide relieves morning
sickness, while the other enantiomer causes severe deformities in developing foetuses
...
Ibuprofen is an example of a racemate
...
However, for the same effect to be had, the dose of the drug must be
doubled as half of the drug is ineffective
...
However, there are methods of producing single
enantiomer drugs:






Using natural enzymes or bacteria that only produce one enantiomer
Using reactants that are single enantiomers, such as amino acids and saccarides
Using chemical chiral synthesis
...
This is done by arranging the molecule so that it can
only be attacked from one side, by sticking it to a polymer support or making it into a cyclic
molecule
...
These are catalysts that provide a reaction pathway which only
produces a single enantiomer
...


Thin Layer Chromatography
Chromatography is used to separate substances out in a mixture
...

Thin layer chromatography uses a mobile phase and a stationary phase
...
The stationary phase can be a solid or a
liquid on a solid support
...
Substances that adsorb strongly to the solid
stationary phase remain attached to the stationary phase for longer, and so travel only a short
distance up the plate
...
Similarly, substances
that are very soluble in the liquid stationary phase will dissolve more easily into the liquid and
remain attached to the stationary phase for longer, so are carried a shorter distance up the plate
...
Draw a pencil line at the bottom of the plate
...
Put a spot of the mixture to be separated on the base line
3
...
As the mobile phase spreads up the plate, it carries the different substances in the plate
with it
...

5
...
The resultant pattern of
spots on the plate is the chromatogram
To work out what each substance is, find the Rf value
...

Rf = distance travelled by spot/distance travelled by solvent
Different substances have different Rf values, so the Rf value obtained from the chromatogram can
be compared with known Rf values and the identity of the compound can be deduced
...
It uses the same
principle of TLC but is more accurate and separates different substances more precisely
...

The stationary phase in GC is usually a viscous liquid, such as a long chain alkane, or a solid
...
The tube and beads act as the solid support for the liquid stationary phase
...

GC separates out substances in a mixture by using the different levels of solubility/adsorption in
each substance
...
Substances with high levels of solubility will spend more time dissolved in the
stationary phase and take longer to leave the coil
...
If the stationary phase is a solid, then the different substances have different levels of
adsorption to the phase, the same as in TLC
...
The sample is vaporised and injected into the machine, along with the carrier gas
...
The carrier gas moves through the coil, carrying with it the vaporised sample
...
Depending on whether the stationary phase is a liquid or a solid, the different substances
are separated out by the different levels of solubility/adsorption
...
Each substance therefore spends a different amount of time in the machine, which gives
varying retention times (time take for the component to pass from the inlet to the detector)
...
The retention times are given on a graph
...
A
peak is produced when the detector senses something other than the carrier gas leave the coil
...
The area under each peak can be used to determine the relative
proportions of each substance in a mixture
...
g
...

Each compound has a unique retention time, so the retention times that are obtained from the gas
chromatogram can be compared with the retention times of known compounds in order to identify
them
...

GC has some limitations
...
This is because many compounds
have very similar retention times that are not always easily distinguished on a graph
...
Another problem is that compounds can only be identified if you have reliable retention
times with which to compare your observed retention times
...


Mass Spectrometry
Mass spectrometry is used to identify compounds
...
The compound is vaporised and then bombarded with electrons
...
It also breaks some molecules of the compound
into fragments
...
The ionised molecules are then
accelerated using electric fields and then passed through the spectrometer
...
Ions are deflected according to their charge and mass
...
Ions with a higher charge will be
deflected more than ions with a smaller charge
...
The voltage of the current is specific to each ion, which allows them to be
identified
...

The peak at the highest mass/charge ratio is the original compound, so the highest mass/charge
value is the molecular mass of the original compound
...
Each compound has a unique
fragmentation pattern, so two compounds with the same molecular mass and formula will have
different mass spectra
...
Gas chromatography separates
out substances very efficiently, but using retention times to identify the constituent compounds can
give inaccurate data, as different compounds may have the same retention times and retention
times must be compared with retention times recorded under the same conditions
...

They are therefore used together
...
These compounds then hit a detector at the end of the coil, which is
connected to a mass spectrometer
...

High performance liquid chromatography is another type of chromatography that is combined with
mass spectrometry to separate and identify compounds
...
g
...
The mobile phase is a solvent
...

Uses:





GC-MS is used in forensics, for identifying compounds found at crime scenes, and matching
them to compounds (DNA, cloths fibres) collected from suspects
...
The MS can be programmed to
test for a particular substance, which makes the process fast
...

GC-MS is used in environmental analysis to monitor air pollution, water pollution or
pesticide levels in food or ecosystem
...

All nucleons (protons and neutrons) have spin – they spin in a particular direction
...
In the nucleus,
nucleons that spin in the opposite direction pair up, causing the overall spin to be cancelled out
...

Atoms with an odd number of nucleons (isotopes) have overall spin, due to the unpaired nucleons,
which generates a small magnetic field
...
When a magnetic field is applied to the nuclei, they align themselves with the field or
against the field
...
This difference between the energy aligned
with the field and the energy aligned against the field is the energy gap
...
They then release this energy
slowly and revert back to their lower energy state, aligned with the field (relaxation)
...
Nuclei continue to flip
back and forth between these two energy states and alignments, absorbing radio waves and
emitting energy
...
This flipping is called resonance
...
This results in different carbon environments – different carbons absorbing different
amounts and frequencies of energy according to the position in each molecule
...
E
...
a carbon atom bonded to another carbon
has more shielding than a carbon bonded to an oxygen, as the electronegativity of the oxygen pulls
the electrons in the bond away from the carbon
...
The NMR spectrum shows the different
nuclear environments by showing the different amounts of energy absorbed by each atom in each
environment relative to tetramethylsilane
...
Chemical shift
is measures in ppm (parts per million) with tetramethylsilane being 0 ppm
...
Each peak represents a different carbon environment in the
molecule, so the number of peaks gives the number of different carbon environments
...
Data for each carbon environment
and its corresponding chemical shift value are used to image a molecule
...

H1 NMR spectroscopy is where hydrogen nuclei absorb radio waves, instead of carbon nuclei
...
H1 NMR spectroscopy produces similar spectra
to C13 spectra but provides more information:






The number of peaks shows the number of different hydrogen environments in the
molecule
...

The relative areas of each peak show the relative numbers of hydrogen atoms in each
environment
...
g
...
The
areas of each peak can be represented with an integration trace, which shows the area as
the relative heights of each peak
...

The splitting pattern shown on the spectra can be used to find the number of hydrogens
attached to the adjacent carbon
...
This is due
to the way in which adjacent protons align their magnetic fields
...
These
peaks are called multiplets
...
This is the n + 1 rule
...
There are no hydrogens on the adjacent carbon
...
There is one hydrogen on the adjacent carbon
...
There are two hydrogens on the adjacent carbon
...
There are three hydrogens on the adjacent carbon
...
Deuterium is an isotope of hydrogen that
has a neutron
...
This means that deuterium isn’t affected by the magnetic field and doesn’t absorb the
radio wave energy, so doesn’t produce a peak on the NMR spectrum
...


OH and NH peaks have a very large range of chemical shift values that overlap with many other
hydrogen environments
...
To overcome this deuterium is used again in the form of deuterium oxide (D2O) is
used
...
Two spectra are run on the compound – for the second one, D2O is added
...
As the deuterium doesn’t
absorb radio waves, the second spectra should not have a peak, if the peak is due to an OH/NH
group
2
...
If it doesn’t,
then it isn’t
...
A beam of infrared radiation is passed
through a sample of a compound
...
E
...
the –OH
group in alcohols absorbs a different frequency of IR than the –OH group in carboxylic acids
...
Spectra can be used to identify molecules, as
absorption data is used to identify which bonds and functional groups are present in a molecule
...
Most
reactions take place in solution, so the units will be concentration per second
...

There are many ways to measure the progress of a reaction:






Gas evolved
...
The gas produced in the reaction is captured in
the gas syringe, which can be used to find the volume of gas produced
...

Colour change
...
If only the reactants
are coloured, the solution will become colourless over time
...
If both reactants and products are
coloured, there will be a colour change
...

Electrical conductivity
...
If the reactants are ions, the conductivity will decrease over time
...
To find the reaction rate, the
gradient at a certain time in the reaction must be found, which gives the rate of reaction at this time
in mol dm-3 s-1
...
If the graph is plotting
the concentration of the reactants, the y-value will be negative as the reactant concentration will be
decreasing
...

If the graph is curved, the gradient is difficult to find
...
A tangent is just a straight
line
...


Rate Equations
A rate equation shows how the rate of the reaction is affected by the concentrations of reactants at
a particular temperature
...
m and n are the
orders of the two reactants and k is the rate constant
...
Reactant orders determine how much the concentration
of a reactant affects the rate of a reaction
...
If the concentration of a zero order reactant increases or decreases, the reaction
rate stays the same
...
If the concentration of a first order reactant doubles,
the reaction rate will double
...
5, the reaction rate will
decrease by 1
...

Reactions where the rate increases by the square of the reactant concentration involve
second order reactants
...
If the concentration decreases by 3, the
rate will decrease by 3-2
...
The rate
determining step of a reaction is the step that takes the longest and therefore determines the
overall rate of the reaction
...
If one molecule is involved in the rate determining step, then the
reactant is first order
...
Catalysts are reactants that affect the
reaction rate, even though they are regenerated
...

Reactant orders can be calculated if the rates at different concentrations are known
...

The rate constant is a specific value for each reaction at a particular temperature
...
K changes at different temperatures, because a change in temperature affects
the reaction rate
...

At higher temperatures, particles have higher kinetic energy so collide more frequently and with
more energy, so the reaction rate is higher and k is larger
...
Just
rearrange the rate equation for that reaction:
K = r/[A]m[B]n
To find the units of k, divide the units of the reaction rate by the units of the concentrations

E
...
if the rate equation for k was:
K = 1
...
A
rate concentration graph can be made from a concentration-time graph:
1
...
Record the concentration of the reactant at these points
...
Plot the reaction rate and concentration for each point in time on a graph to create the rateconcentration graph
...
For the rate-conc graph, it is a horizontal line
...
For the rate-conc graph,
it is a steady increase, as the rate is directly proportional to [X]
For second order reactants, the conc-time graph is a steep curve downwards
...
The half life of a first
order reactant is independent of the concentration, so it doesn’t matter what the concentration of a
reactant is, the half life will always be constant
...


Initial Rates
Looking at the shapes of graphs is one way of finding out the reactant order
...
For these experiments, two or more reactions are run
...
The data can then be compared to find the reactant order
...
Measure your first initial concentration of reactant and carry out a reaction
...

2
...
Use the concentration time graph
to find the initial rate of the reaction (tangent method)
...
Carry out another reaction but this time use a different initial concentration of reactant (e
...

double what was first used)
...

4
...
Use this information to find the reactant orders
...
g
...

Clock reactions, such as the iodine clock reaction, can be used to find the initial rate
...
The faster the colour changes, the faster the initial rate
...

E
...
After a certain amount of time, enough iodine is formed to
turn the solution dark blue
...

Reaction 1:

H2O2 + 2I− + 2H+ → I2 + 2H2O

Reaction 2:

2S2O32− + I2 → S4O62− + 2I−

First, the hydrogen peroxide reacts with the iodide ions to form iodine and water
...
These reactions
continue until all the thiosulphate is used up
...
Reaction 1 is the rate determining step, while reaction 2 is the
fast step
...
The rate
determining step is the step in the mechanism that is the slowest
...
The rate determining step doesn’t have to be the first step of the
reaction, only the slowest
...
If two reactants for a reaction appear in the
rate equation, they must affect the rate, and so must be involved in the rate determining step
...

Reactant orders are also linked to the rate determining step
...

If a reactant is first order, then one molecule of the reactant is involved in the rate
determining step
...

If a reactant is second order, then two molecules are involved in the rate determining step
...


If the reaction mechanism and the rate determining step is known, the rate equation can be worked
out
...
If two molecules of a reactant are involved in the step, then the reactant is second order,
whereas if only one molecule is, the reactant is first order
...
All the reactants in
the rate equation will be involved in the rate determining step
...

The chemical equation can be misleading when writing the rate equation
...

When suggesting a mechanism:




The molecules in the rate equation must be present in the rate determining step in the
proportions indicated by their reactant orders
The equations in the mechanism must balance
...


The Equilibrium Constant
Reversible reactions are chemical reactions that can be reversed
...
Initially, the concentration of the reactants will be high, so
the rate of the forward reaction will also be high
...
But
when the product concentration increases, the products react to form reactant molecules, so the
rate of the backward reaction increases
...
This can only take place at a closed system and at a constant temperature
...
The concentrations are multiplied to the power of the number of moles in the
reaction, so if there are two moles of a particular reactant or product involved in a reaction, the
product/reactant is multiplied to the power of 2
...

If the product concentrations are low and the reactant concentrations are high, Kc will be a small
value
...
Only an
increase or decrease in temperature will affect Kc as the forward and backward reactions are
exothermic and endothermic respectively
...
Find the number of moles at equilibrium from the chemical equation
...
Divide the number of moles by the equilibrium volume, which gives the equilibrium
concentrations
...
This will result in different amounts of product and reactant being produced
...

The forward and backward reactions in a reversible reaction are thermically antagonistic – one will
be endothermic and one will be exothermic
...
If the temperature is decreased, the rate of the exothermic
reaction will increase in order to replace the lost heat, and the position of equilibrium will move in
the direction of this reaction
...
E
...
in the reaction 2SO2 + O2 ↔ 2SO3 the forward reaction is exothermic and the
backward reaction is endothermic
...

Any reaction where a temperature change causes a decrease in product concentration will decrease
the value of Kc
...

Changing the concentration of reactant will only affect the position of equilibrium and not Kc
...
If there is an increase in the product concentration, the position of
equilibrium will shift to the left, as the product molecules will react to reform reactant molecules at
a higher rate
...

Changing the pressure affects reactions involving gases
...
This is because an increase in pressure causes the gas molecules to react at
a higher rate, producing more molecules of product
...

Using a catalyst changes neither the position of equilibrium nor Kc
...
As the reactant and product concentrations remain the same, Kc doesn’t
change either
...


Acids and Bases
A Bronsted-Lowry acid is a proton donor – it releases protons (H+ ions) into a solution
...
In solution, it is very rare that H+ ions exist as
just protons (the concentration of free protons in water is estimated to be 10-130 mol dm-3)
...
The water molecules have two lone pairs of
electrons, which can accept the H+ ion to form a dative covalent bond
A Bronsted-Lowry base is a proton acceptor – it accepts H+ ions in solution
...

Reactions of acids:





Acids react with metals to form a metal salt and hydrogen
...
The metals are oxidised and the H+ ions are reduced, forming a salt
...

2H+ + 2Na  2Na+ + H2
Acids react with carbonates to form a salt, carbon dioxide and water
...
The
ions reacting in this reaction are the OH- ions or the O2- ions, depending on whether the
base is a metal hydroxide or a metal oxide
...
Strong acids
completely dissociate, meaning that nearly all of their H+ ions are released
...
Weak acids only partially
dissociate, meaning that only some of their H+ ions are released
...
This is why strong acids produce more acidic solutions – they release more H+ ions into
the solution than weaker acids, as they dissociate completely, which lowers the pH
...

Acids can be mono-, di-, or tribasic
...

Monobasic acids release one proton: HCl  H+ + ClDibasic acids have two protons and release H+ ions in two stages:
H2SO4 ↔ H+ + HSO4HSO4- ↔ H+ + SO42Tribasic acids have three protons and release them in three stages:
H3PO4 ↔ H+ + H2PO4H2PO4- ↔ H+ + HPO42HPO42- ↔ H+ + PO43-

Acids and bases can react to form conjugate pairs
...
Therefore the two species can be transformed into
each other by gaining or losing a proton
...
Two conjugate pairs will be formed, with each species in a pair on
opposite sides of the chemical equation
...
Ammonia has accepted the proton to form NH4+ so it must be the base
...

The water and OH- ion are separated by the difference of one proton so they must be a conjugate
pair
...
The OH- ion can accept a proton to
reform water, meaning that it must be the conjugate base
...
As the H+ concentration
can vary enormously, the pH scale is a logarithmic scale, in order to account for the varying [H+]
...
pH 7 is generally neutral, which means that the [H+] and the
[OH-] are roughly equal
...

To calculate pH, you need the [H+]
...
Some
examples are HCl and HNO3
...
As each molecule only releases one H+ ion, the concentration of the acid is the same as the
concentration of the H+
...


The Acid Dissociation Constant
Weak acids only partially dissociate in solution
...
The
[H+] and pH of a weak acid cannot be calculated with only the acid concentration (like with a strong
monobasic acid) because not all of the H+ ions dissociate
...
All weak acids have a specific acid dissociation constant at a specific
temperature
...
But as only a very small proportion of HA dissociates, we can assume
that the acid concentration at equilibrium is the same as at the concentration of
undissociated acid
...
This
means that the equation can be simplified to put [H+]2 on the top, as the values for [H+] and
[A-] will be the same
...
A small Ka value means that the acid hasn’t dissociated
much at all and is weak
...
pKa and Ka have exactly the same relationship as H+ and pH and are derived in the
same way:
pKa = -log10(Ka) and Ka = 10-pKa
Percentage dissociation is the percentage of H+ ions that have dissociated from the acid, by dividing
the [H+] by the acid concentration and multiplying this value by 100:
Percentage dissociation = ([H+]/[HA]) x 100

The Ionisation of Water
In solution, water dissociates into H+ ions and OH- ions
...
This can be shown in the equilibrium
reaction H2O ↔ H+ + OH-
...
The Kc value of water is shown as:
Kc = [H+] x [OH-] / [H2O]
However, the number of water molecules that dissociate is so small that the concentration of
dissociated water at the start of the reaction can be assumed to be the same as the concentration of
dissociated water at equilibrium – it has a constant value
...
At 298 K, the value for Kw
is 1 x 10-14 mol2 dm-6
...
At pH 7, a solution is neutral, so the
concentrations of H+ and OH- ions must be equal
...
At a pH higher than 7, the concentration of H+
ion must be lower than the concentration of OH- ions
...
At higher temperatures, water molecules have higher kinetic
energy and so more molecules dissociate
...
As the H+ concentration has increased, the pH of neutrality is lowered
...
At 37 degrees, the pH of neutral water is 6
...

Pure water has a neutral pH, meaning that [H+] and [OH-] are exactly the same, so Kw can be
expressed as: Kw = [H+]2
Kw can be used to calculate the pH of strong bases and to find the [H+] of strong bases
...
Weak bases only partially dissociate, releasing a small amount of OH- ions
...
This is because, as 1 mole of
strong base completely dissociates to release 1 mole of OH- ions, the concentration of the base will
be the same as the OH- ion concentration (the same as with strong monobasic acids)
...
Then the equation can simply be rearranged to find
the [H+] and then the pH
...
They
do not completely prevent a change in pH, but they minimise the change
...
There are two ways to make buffers: you
can add a weak acid and a salt of the weak acid together
...
For example, a buffer could be a solution of ethanoic acid and
sodium ethanoate
...
The sodium ions are spectator ions and therefore don’t affect buffer action
...
The acid will react with
the alkali to form a salt, which dissociates in solution to release the conjugate base of the weak acid
...
E
...
react excess ethanoic acid and sodium hydroxide
...
The sodium ethanoate
then dissociates in solution, releasing ethanoate ions
...
The solution will therefore be made up of ethanoic acid and ethanoate ions: a weak
acid and its conjugate base, constituting a buffer
...
In
the buffer, the equilibrium HA ↔ H+ + A- exists
...
The acid in the buffer solution only partially dissociates as it is a
weak acid, but the salt completely dissociates, releasing all of its conjugate base ions
...

When acid is added, the H+ ion concentration increases
...
The position of equilibrium
of the dissociation reaction is shifted to the left, as more acid molecules are made
...

When alkali is added, the OH- ion concentration increases
...
This causes the position of equilibrium to shift to the right, as acid molecules
dissociate to release more H+ ions to replace those lost
...

Buffers have biological importance as they help to keep the pH of the internal conditions of the body
constant
...
In the blood, the pH is maintained by a buffer formed
from carbonic acid and hydrogen carbonate ions (H2CO3 and HCO3-)
...
An
increase in H+ ions causes the position of equilibrium to shift to the left and more carbonic acid to
be produced
...
If the blood pH is not controlled, enzymes lack
optimum conditions for reactions and may denature
...


Calculating the pH of Buffer Solutions
In order to calculate the pH of a buffer solution, the concentrations of the acid and its conjugate
base must be known, as well as the Ka value for the buffer
...
The concentration of the acid at the beginning of the
reaction will be pretty much the same as the concentration at equilibrium, so it is assumed that the
concentrations are the same
...
From here, the Ka equation can be rearranged:
[H+] = (Ka x HA)/AThen use the –log[H+] equation to find the pH
...
For
this, subtract the amount of alkali from the amount of acid, as all the alkali reacts with the acid, to
find the new acid concentration
...


Titrations and pH Curves
Titrations are used to found out how much acid is needed to neutralise a known quantity of base, or
vice versa
...
Add a small amount of indicator to a known concentration of base in a conical flask
2
...

3
...
Release the acid until the indicator changes colour, and read the
amount of acid added at this point
...

4
...
Then add the acid drop by drop until a colour change occurs
...
The volume of acid used is the amount taken to neutralise the base at the
used concentration
...
The mean of the readings for each titration is then calculated
...
As acid/base is added, the pH of the initial solution changes
...

This is for a strong acid and a strong base
...

This is for a strong acid and a weak base
...

This is for a strong base and a weak acid
...
There is very little change in pH, as the pH starts off
between 4-6 (weak acid) and rises to 8-9 (excess weak base)
All the graphs apart from the weak acid-weak base graph have sections that are almost vertical
...
The midpoint of the
vertical section is the end point of the reaction – this is the volume of acid that exactly neutralises
the alkali
...
This is put into the titration solution and
measures the pH change as acid/alkali is added
...
In a titration, the colour change would occur very slowly
...
This is because different indicators change
colour in different pH ranges
...
If the titration is between a weak acid and a strong base, for example, an indicator with a
colour change that occurs in the pH range of the vertical range of the curve must be used
...
As the
indicator is used to signify the end of the reaction, the indicator must change colour when the end
point is reached
...
A pH meter must be used to determine the midpoint, and the value taken is then
used to find a suitable indicator
...
In
an acid-base titration, the volume of acid taken to neutralise the alkali is then used to find the
concentration of the alkali itself
...

After the acid volume and concentration is found, this information is then used to find the
concentration of the base, using the equation:
Moles = concentration x volume
...
The units of enthalpy change are in kJ mol-1
...
Standard
conditions are 100kPa (one atmospheres pressure) and 298 K (25OC)
...
This means that heat energy is evolved and
released in the reaction, typically causing a rise in temperature of the surroundings
...
Endothermic reactions have a positive enthalpy change
...
Energy
is taken in in order to form chemical bonds, so the energy taken in to break the bonds of the
reactants in endothermic reactions is greater than the energy released by the formation of the
product bonds
...
m is the mass of the surroundings in grams
...
18 J g-1 K-1)
...

Neutralisation reactions usually have a negative enthalpy change
...
The definition for the enthalpy change of
neutralisation is the enthalpy change that occurs when 1 mole of water is formed by the reaction
between an acid and a base in their standard states under standard conditions
...

1
...

2
...

3
...

4
...
Strong dibasic acids release
almost double this amount and strong tribasic acids release even more
...


Enthalpy Definitions
Enthalpy change of formation is the enthalpy change when one mole of a compound is formed from
its constituent elements in their standard states under standard conditions
...

Enthalpy change of atomisation of a compound is the enthalpy change when one mole of gaseous
atoms is formed from a compound in its standard state under standard conditions
First ionisation enthalpy is the enthalpy change when one mole of gaseous 1+ ions is formed from
one mole of gaseous atoms under standard conditions
...

First electron affinity is the enthalpy change when one mole of gaseous 1- ions is formed from one
mole of gaseous atoms under standard conditions
Second electron affinity is the enthalpy when one mole of gaseous 2- ions are formed from one mole
of gaseous 1- ions under standard conditions
...


Born-Haber Cycles
Ionic lattices can be theoretically formed in two ways
...
This is very exothermic as many
ionic bonds are formed, and the energy released is the lattice enthalpy
...
The
second way is the indirect route
...
This can be plotted on a Born-Haber cycle
...
This means that the lattice enthalpy is the same as the enthalpy change of the indirect
route
...

The enthalpy change of atomisation of the first element is endothermic
...
The enthalpy change of atomisation is representative to one
atom, so if there are more than one, the value for the enthalpy change must be multiplied
by the number that there are
...
Energy must therefore be put in to overcome the repulsion
...
The enthalpy
change that occurs when the bonds in the ionic compound break is the reverse of the lattice
enthalpy – the lattice enthalpy is the energy released upon the formation of these bonds, so the
energy taken in to break them must be the same value, only positive
...

Bonds form between the ions and the water molecules because the water molecules are polar, sand
so negative ions are attracted to the slightly positive hydrogen atoms on the water molecules, and
positive ions are attracted to the slightly negative oxygen atoms on the molecule
...
If the energy taken in to break the lattice bonds is greater than the energy
released when bonds are formed between the water and the ions, the reaction is endothermic
...
The overall
enthalpy change is the enthalpy change of solution
...

Enthalpy change of solution = ( - lattice enthalpy) + (enthalpy of hydration for positive and negative
ions)
The enthalpy of hydration is affected by two factors, similar to the way lattice enthalpy is affected by
two factors: ionic radius and charge




A small ionic radius will result in a more negative enthalpy of hydration
...
The formation of stronger bonds releases more energy
...

A higher ionic charge means that the ions attract water molecules more strongly, and form
stronger bonds with them
...


Entropy
Entropy is the amount of disorder or randomness in a system
...
Entropy can also be defined as the dispersal of energy within in
a system, as energy is moved from a localised area to being more spread out
...
This explains why many chemical processes happen
...
Entropy is measured in J K-1 mol-1






Changing physical state increases overall entropy, as there is more disorder and greater
energy dispersal in gaseous atoms than in liquids or solids, as the particles in liquids and
especially solids are fixed in position, and therefore have lower entropy
...

Reactions where more moles are produced increase entropy, as more molecules mean more
particle arrangements and energy dispersal
...
g
...


A spontaneous or feasible reaction is a reaction that can take place by itself at a certain temperature
and doesn’t need any energy put into it
...
This is
because the overall entropy is increased, allowing the reactions to take place spontaneously at a
given temperature without an energy supply being needed
...
Theoretically, at 0 K,
particles can have no entropy, but 0 K is physically impossible
...
If the total entropy change isn’t positive, it won’t be possible at
the given temperature
...
However, it may be feasible at a higher temperature, meaning that
temperature affects the feasibility of reactions
...
When calculating the enthalpy change of surroundings, the
enthalpy change must be converted into Joules
...

Total entropy change = entropy change of system + surroundings

Free Energy Change
Free energy is a calculation that is used to determine if a reaction is feasible at a certain temperature
or not
...
The free energy change of a reaction must be negative
or zero for a reaction to be feasible
...
Even if the reaction is theoretically at the given temperature, it
may have a very high activation energy or take place so slowly that it will not appear to be reacting
...

Free energy change = enthalpy change – (temperature x entropy change)
The temperature is measured in K
...

If a reaction is exothermic (negative enthalpy) and has a positive entropy change, the free energy
change is always negative
...
If
the reaction is endothermic (positive enthalpy) and has a negative entropy change, the free energy
change is always positive and the reaction isn’t feasible at any temperature
...

Even if the reaction isn’t feasible at a given temperature, this doesn’t mean that the reaction cannot
take place, only that it won’t take place by itself and needs an added energy input for the reaction to
occur
...
These two
processes happen simultaneously
...
Reduction is the gain in
electrons
...
Reduction results in a decrease
in the oxidation number
...
A reducing agent is a substance that
helps other molecules become reduced by donating electrons to them, itself becoming oxidised
...

E
...
Zn  Zn2+ + 2e- and

Ag+ + e-  Ag

becomes Zn + 2Ag+  Zn2+ + 2Ag
H+ ions and H2O can be added to these ionic equations to help balance them
...
First, combine
the two reactions as one single reaction and find the oxidation changes of the reactants
...

E
...
in the reaction 2HI + H2SO4  H2S + I2 the iodine is oxidised from -1 to 0 and the sulphur is
reduced from +6 to -2
...
This gives 8HI + H2SO4  H2S + 4I2 + 4H2O

Electrochemical Cells
Redox reactions involve the transfer of electrons
...
Electrochemical cells can be set up that control the flow of electrons in order to create
usable electricity
...

A half cell is comprised of an element in two oxidation states
...
An equilibrium is set up between the metal and its ions
...
The electrode potential is the ability of the element to
lose electrons and become oxidised
...
A less reactive element has a less negative or positive electrode
potential – it is less easily oxidised
...

Two half cells can be combined to form an electrochemical cell, each half cell with different
elements at different electrode potentials
...
The
electrode will be submerged in a solution of ions
...
Platinum can also be used, for when there is no solid form of the element that
conducts electricity or for when the solution is made up of ions in two different oxidation states
...
g strip of filter paper soaked in KNO3) that connects
the two solutions and allows ions to be transferred between them
...
The half cell with the more negative electrode potential will be the one that is
oxidised, so the equilibrium for this reaction will shift to the left
...
This half cell will be reduced, so the electrode will accept the
electrons
...

E
...
a Cu2+/Cu and a Zn2+/Zn electrochemical cell can be set up by connecting the zinc half cell and
the copper half cell with a wire and the solutions with a salt bridge
...
The position of
equilibrium for the Zn2+(aq) + 2e- ↔ Zn(s) reaction shifts to the left
...

E
...
an Fe2+/Fe3+ half cell can be used
...
Platinum is used as it is an inert solid that conducts electricity
...
Depending on the other half cell used, the equilibrium
either shifts to the left or right
...
g
...
A
hydrogen half cell is set up by bubbling a hydrogen gas into a solution of H+ ions
...
An equilibrium is set up between the
hydrogen and the H+ ions: 2H+(aq) + 2e-  H2 (g)
...
The standard conditions for a standard hydrogen
electrode potential are:




Both solutions (hydrogen and the other half cell) must be 1 mol dm-3
Pressure must be 100kPa
Temperature must be 298 K

The standard electrode potential is the voltage produced by a half cell when compared to a standard
hydrogen half cell
...
To measure the
standard electrode potential, a half cell is connected to a hydrogen half cell and the voltage
produced is measured
...

The cell potential shows the total voltage produced by the electrochemical cell
...
The electrochemical series shows the electrode potential for
each half cell reaction
...

The electrode potentials of each half cell are found and the cell potential calculation is carried out
...
In the electrochemical cell, the half cell with the more
negative electrode potential will be oxidised and the half cell with the less negative electrode
potential will be reduced
...
Metals that
are highly reactive want to lose electrons, so the more negative the electrode potential of a metal,
the higher its reactivity
...

The electrode potentials can also be used to predict the outcome of reactions
...

The electrode potentials can also be used to determine if two reactants will react
...
Find the two half equations for the redox reaction, and write them with the reduction
reaction on the left hand side
...
Put the half equation with the more negative electrode potential on top of the other – this is
the oxidative equation
...
Draw an arrow from the products to the reactants of the oxidation equation
...
Draw an arrow from the reactants to the products of the reduction equation
...
The substances at the non pointy ends of the arrows will react, while the substances at the
non pointy ends of the arrows won’t react
...
g
...
37 V)
Ag+(aq) + e- ↔ Ag (s) (EP = +0
...

However, aqueous magnesium ions won’t react with solid silver
...
If the conditions are not standard,
any change in concentration, pressure or temperature will change the electrode potential, as the
position of equilibrium of the half equations will change
...
g
...
If the concentration of silver increased, the position of equilibrium
would shift to the left, which would cause the electron flow and hence the electrode potential to
increase
...

Energy Storage Cells
Energy storage cells are batteries
...
It is possible
to work out the voltage produced by these cells by working out the cell potential (the difference of
the electrode potentials of the half cells)
...

Energy storage cells can be recharged
...
The current oxidised the chemical that was reduced and
reduces the chemical that was oxidised, returning them both to their original state
...


Fuel Cells
Fuel cells produce electricity by reacting a fuel with an oxidant
...

A fuel cell is made up of an anode and a cathode, separated by a polymer electrolyte membrane
...
The cathode is where the
oxygen is added
...
At the anode, the platinum catalyst causes the hydrogen to split into H+ ions and electrons
...
The polymer electrolyte membrane allows H+ ions to pass through it into the cathode but
doesn’t allow the electrons to pass through
...

3
...
This is used
to power an appliance
...
At the cathode, the oxygen, H+ ions and electrons all react to form water
...
O2 + 4H+ + 4e-  2H2O
Fuel cell vehicles are vehicles that are powered by fuel cells
...
As well as hydrogen, they can also be powered
by hydrogen rich fuels, such as methane or methanol
...
Hydrogen powered vehicles produce much less pollution than combustion engine
vehicles, as only water is produced as a waste product
...

Hydrogen fuel cells are difficult to make
...
Disposing of these chemicals is an expensive process
...

Storing and transporting hydrogen is also difficult
...

It can be stored as a liquid but this requires expensive fridges which consume lots of energy,
as the boiling point of hydrogen is so low
...

These problems don’t apply to hydrogen rich fuels, which are transported and stored easily
...
Hydrogen is rare in its gaseous form, so to produce usable hydrogen, it must be extracted
from hydrogen sources (molecules that contain hydrogen)
...
The electrolysis of water is an energy intensive process, so
lots of fossil fuels are consumed in order to produce the electricity required
...
The reaction itself also produces
CO2
...
For this to happen, people need to accept hydrogen
as a fuel
...
Hydrogen must be able to be produce renewably and without huge energy
demands
...


Transition Metals
Transition metals are located in the d-block of the periodic table
...

As the d-subshell can take ten electrons, a transition metal can form one or more stable ions with
between 1 and 9 electrons in the d-subshell
...
This means
that the 4s subshell will fill up before the 3d subshell, and the 3d orbitals will fill up singly at first
before they start to share
...




Chromium (Cr) will have 1 electron in each orbital of the 3d subshell and one electron in the
4s subshell – this gives it greater stability
...
1s22s22p63s23p63d104s1

Transition metals form positive ions when they are oxidised
...
The electron configurations of the transition metals
change when they are oxidised as electrons are removed from their orbitals
...
They lose electrons from their 4s orbital before they
lose electrons from the 3d orbitals
...
The electron configuration of scandium is 1s22s22p63s23p63d14s2
...
As it loses three electrons, its electron configuration
changes to 1s22s22p63s23p6, therefore the 3d subshell is empty, so it can’t be a transition metal
...
Zinc only forms one ion: Zn2+, so it loses
two electrons
...


Transition Metal Properties
Transition metals have many chemical and physical properties afforded to them by their incomplete
d-subshells
...

They form coloured ions
...
The colour that we see is the light wavelengths that have not
been absorbed
...
The wavelengths they
absorb depend on the electron configuration of the d-subshell
...
All transition metals can form at least 2 stable ions
...

They are good catalysts
...
The first way is providing a
surface on which the reaction can take place
...
The other way is by donating and accepting electrons using their varying oxidation
states
...


Transition metal ions have a variety of different colours
...
When transition metal
ions react with sodium hydroxide, the metal ion and the hydroxide ion react to form a coloured
precipitate
...
A complex ion consists of a central metal ion
surrounded by dative covalently bonded ligands
...
For example [Fe(H2O)6]2+ is a complex ion where six water molecules have each donated
an electron pair each to form six coordinate bonds with the iron ion
...
In this case it is
six
...
Monodentate ligands only form one coordinate bond with
the metal ion and therefore generally only have one lone pair of electrons
...
Multidentates are molecules that form more
than one coordinate bond with the metal ion, and therefore at least two lone pairs of electrons
must be present on the molecule
...
This forms
two coordinate bonds with a metal ion as there are two lone pairs on the molecule: one on each
amine group
...
g
...
EDTA4- is a hexadentate ligand
...

The overall charge of the complex ion is affected by the ligands that bond to it
...
If the ligands are charged (Cl-, CN-), the overall charge is
changed
...
g
...

The shape of complex ions changes depending on how many ligands are bonded to the metal
ion
...
This is because the electron repulsion of
larger ligands is greater than that of smaller ligands
...
Complex ions with 4 ligands are tetrahedral in shape and
have 109
...


Isomerism in Complex Ions
Complex ions can show optical isomerism and E/Z isomerism
...
E
...
[Ni(en)3]2+
...
E
...
[Fe(en)2Cl2]
...
E
...

[Cu(EDTA)]2-

The optical isomers formed will be non-superimposable mirror images of each other
...
An equal mixture of both isomers will be
optically active as they will both rotate the plane polarised light in opposite directions, cancelling
each other out
...
Complex ions with four monodentate ligands, with two
identical pairs of ligands can be cis or trans isomers, depending on which side of the metal ion the
two ligands are on
...
If they are on the same side of the metal ion as each other, then it is a cis
isomer
...
Cis-platin is where the chloride and ammonia ligands are
on the same sides of the platinum ion
...
Cis-platin enters cells and easily releases its chloride ligands, allowing it to
form coordinate bonds with the lone pairs of nitrogen atoms in the nitrogenous bases of DNA
...
This works for all cells but has a
much greater effect on cancer cell than normal body cells as they divide at a much faster rate than
normal body cells
...

Octahedral complexes with a 4 ligands of one type and 2 ligands of another type (e
...
[Ni(H2O)4Cl2])
can also show cis-trans isomerism
...
If they are next to each other (adjacent) then it is a cis isomer
...
Any
changes to the complex ion as a result depend on the type of ligand used
...

E
...
When excess ammonia is added to a copper-aqua complex:
[Cu(H2O)6]2+ + 6NH3  [Cu(NH3)6]2+ + 6H2O
OH- ions can also be substituted for water or ammonia and not change the shape of the molecule as
they are a similar size
...

Cl- ions are much larger ligands than water or ammonia, and therefore when they are substituted
into the complex, the shape of the complex changes, as less Cl- ions can fit around the metal ion
...
The concentrated hydrochloric acid provides a high
concentration of chloride ions
...
This is because they have stronger electron repulsions than the
water molecules
...
An intermediate solution with
a pale green colour forms
...
The resultant
solution is a mixture of pale blue copper-aqua complexes and yellow copper-chlorine complexes
...
This means that water can be added to the copper-chorine complex to reform the
copper-aqua
...
The shape changes from octahedral to tetrahedral
...
This reaction is also in equilibrium so the concentrations of
reactant and products can be manipulated to shift the position of equilibrium
...

Sometimes, ligands are only partially substituted, meaning that not all of the original ligands are
substituted out of the complex
...
The shape changes from octahedral to distorted
octahedral
...
The colour changes from pale blue to deep blue
...
First, when a small
amount of ammonia is added, the colour changes from pale blue to light blue
...
This is due to a precipitate of copper hydroxide forming
...

Haemoglobin is a complex ion
...
On one end of the
molecule, the iron ion is bonded to a nitrogen that is attached to a globin (protein) molecule
...
The last coordinate bond is made either with oxygen or water
...
At the lungs, the oxygen
concentration is high and the oxygen affinity of haemoglobin is increased due to the increased pH
...
At the tissues,
the oxygen concentration is low, due to the oxygen being consumed by respiring cells, and the
oxygen affinity of haemoglobin is also low
...
The decrease in pH changes the
protein structure of the globin, which causes the haemoglobin to bind less strongly to the oxygen
ligand
...

Carbon monoxide is dangerous as it can act as a ligand and bind to haemoglobin
...
This can cause headaches, unconsciousness and death
...
The definition of the
stability constant is the equilibrium constant of the formation of a complex ion from its constituent
ions in solution
...
The
stability constant is shown in the same way as the equilibrium constant: concentrations of products
divided by the concentrations of the reactants
...

E
...
Cu2+ + 4Cl- ↔ [CuCl4]2-  Kstab = [[CuCl4]2-]/[Cu2+][4Cl-]
In ligand substitution, the same process occurs
...
This is because
the reaction takes place in solution, so the concentration of water is practically constant at
equilibrium
...
g
...
The larger the stability constant, the more stable the ion
...
For the stability constant to increase, the number of product moles
must be greater than the number of reactant moles
...
A large stability constant also shows that the
position of equilibrium is shifted to the right in the reaction
...
This is because the energetic stability of the ion is high, and
molecules move to increase their energetic stability
...
This is
because the entropy increases when a multidentate ligand is substituted for a number of
monodentate ligands, as the number of moles produced increases
...
This
makes them good oxidising and reducing agents
...
It therefore donates electrons to help other
species become reduced
...
The MnO4- ion is purple and the [Mn(H2O)6]2+ is colourless, so when the
reaction takes place, there is a colour change from purple to colourless
...
g
...
It contains dichromate ions (Cr2O72-)
which can oxidise a reducing agent in a redox reaction
...

E
...
Cr2O72- oxidise zinc in a redox reaction: Cr2O72- + 14H+ + 3Zn  2Cr3+ + 7H2O + 3Zn2+
When the Cr3+ ions are released in solution, they form a chromium-aqua complex, which is green
...
This is the same as
when potassium dichromate is used to oxidise alcohols
...
These are done to
find out the exact quantity of oxidising agent needed react with a known quantity of reducing agent,
or vice versa
...

1
...
E
...
Fe2+ ions
...
Excess dilute sulphuric acid is added to the flask
...

3
...

4
...

5
...
This is the rough titration
...
Repeat the titration
...

An example of a colour change is when KmnO4 is used
...
The end point of the reaction is when the
solution turns purple, as it this point all of the reducing agent will have reacted and there will be
nothing less to remove the MnO4- ions from the solution
...
It’s the same calculation as in
acid-base titrations
...
It can be used to find the concentration of iodine and the amount of oxidising agent
required to oxidise iodide ions to iodine
...

I2 (aq) + 2S2O32- (aq)  2I- (aq) + S4O62- (aq)
The concentration of iodine in a solution can be determined in a titration where this reaction takes
place
...
The oxidising agents could be iodate
ions, chlorate ions, copper(II) ions or dichromate ions
...
A redox reaction takes place that
produces iodine
...
This concentration is then used to work out the concentration of the oxidising
agent
...
The iodide is reacted with the investigated oxidising agent
...
g
...
The iodide ions are in excess
...

2
...
The thiosulphate
ions react with the iodide ions to produce iodine
...
There isn’t a clear colour change, so when the solution becomes pale yellow, starch
solution is added which will turn dark blue, showing that iodine is still present
...
This means that all the iodine has
reacted
...
The volume and concentration of the sodium thiosulphate are used to calculate the moles of
thiosulphate used
...

4
...
For example, in the equation for the reaction between iodide ions and
iodate ions (the oxidising agent), three moles of iodine are produced from every mole of
iodate, so the moles of iodate is the moles of iodine divided by three
...
This value is then used to work out the oxidising agent concentration



---