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Unit 2 Chemistry

Topic 7 Page 1

Enthalpy change
28 September 2014

19:19

7
...
2 Enthalpy
• Heat change
• Constant pressure

7
...
18 g-1k-1

7
...
7 Bond enthalpies

 Draw Displayed formula
Mean bond enthalpy
→ average bond dissociation enthalpy for a particular bond in a range of
different compounds
...
2 Equilibrium conditions
Features of an equilibrium
Catalyst
• Concentrations of the reactants and products
remains the same
...

Factors

Concentration of reactant

Pressure

Increase

• Equilibrium will shift to the right
to the increase in concentration
...

endothermic reaction

Yield
products

increases

Increases

Decrease

• Opposite

• Equilibrium would shift to where • Equilibrium would shift
there is more gas moles
• exothermic reaction

Yield
products

Decrease

Decreases

Says the
same

• Gas moles on both sides has no
effect on equilibrium position

Topic 9 Page 5

Temperature

Decreases

Increases

Equilibrium in industry
Importance of equilibrium
• Sometimes low temperature will give
more yield but slow the reaction down
...

• Le charters principle can find the best
conditions
...

CH4(g) + H2O(g) 3H2(g) + CO(g)
2) Ammonia is manufactured : combining nitrogen and hydrogen and is fed in to an
iron catalyst
...
The remaining hydrogen and nitrogen is recycled
...

Slow rate of reaction
Equilibrium would shift to the right to oppose the decrease in temperature
...

• Decreases yield of ammonia
High pressure (20,000 KPa)
• More moles on left side of the gases
• Favours the forward reaction( fewer
moles)
• Equilibrium moves to right to oppose
the Increase in pressure
...

Uses

Fermentation
Glucose is converted into ethanol and carbon dioxide
...

C66H12O6(aq) → 2C2H5OH(aq) + 2CO2(g)
• slow process, anaerobic conditions
• renewable ( uses existing crops)
• CO2 is produced

Hydration
• High temperature (570k)
• High pressure (6500 KPa)
• Exothermic reaction
• 5% yield but can be recycled over the catalyst until there is a 95% conversion

Ethanol can also be produced by reacting ethene with steam and can be a
continuous process
...

C2H4(g) + H2O(g)

C2H5OH(g)

Maximum yield
• High pressure, equilibrium will move to the right ( fewer molecules)
• Low temperature , equilibrium will move to the right and give out heat

Topic 9 Page 7

•
•
•
•
•
•

Alcohol drinks
Varnishes
Perfumes
Drugs
Medicines
Biofuel

Methanol production
Synthesis gas
1
...

H2O(g) + C(g) CO(g) + H2(g)
CO(g) + H2O(g) CO2(g) + H2(g)
2
...

3
...
The reactions are both
exothermic
...
Identify the atom being oxidised or reduced and make sure there are the same
number of that atom on both sides (by balancing)
2
...
(on the left if reduction, on the right if oxidation)
4
...
balance Hydrogen atoms by adding H+

Topic 10 Page 9

Topic 11

Halogen

Melting point /oC Boiling point /oC Electronegativity Pure form

F

-220

-188

4
...
0

Pale green gas

Br

-7

59

2
...
5

Grey solid

Electronegativity is the ability of an atom to attract electrons in a covalent bond
...

Cl2(g) + H2O(l)

HCl(aq) + HClO(aq)

HClO is used in bleach and swimming pools:
• oxidising agent and kills bacteria by oxidation
...
16

922

[Ne] 3s2

Ca

20

0
...
215

1042

[Kr] 5s2

Ba

56

0
...


Solubility
Metal Hydroxides Solubility Observation

Metal sulphates Solubility observation

Mg(OH)2

White ppt

MgSO4(S)

Ca(OH)2

Faint white ppt

CaSO4(s)

Faint white ppt formed

Sr(OH)2

_

SrSO4(s)

thick white ppt formed

_

BaSO4(s)

insoluble thick white ppt formed

Ba(OH)2

Insoluble

soluble

Soluble

_

Test for sulphate ions
1
...
Add barium chloride:
Positive result

BaSO4 is washed to remove excess sulphuric acid

White ppt( barium sulphate is formed)
BaSO4 is insoluble and is used in x rays
...
This is achieved by roasting them in air:
 2ZnS + 3O2(g)  2ZnO(s) + 2SO2(g)
 2PbS + 3O2(g)  2PbO(s) + 2SO2(g)
 Sulphur dioxide is one of the principal causes of acid rain
...


REDUCTION OF METAL OXIDES WITH CARBON
Metals which are below carbon in the reactivity series are often extracted from their ores by
reacting the ores with carbon and carbon monoxide – a cheap and effective method
...

1
...
This is an
exothermic reaction and keeps the furnace hot:
 C(s) + O2(g)  CO2(g)
b) making the reducing agent
The carbon dioxide rises up the furnace and reacts with more coke to produce carbon monoxide
...

Some of the haematite is reduced directly by the carbon:
 Fe2O3(s) + 3C(s)  2Fe(s) + 3CO(g)
The ore often contains other oxides such as Fe3O4 or FeO, which are also reduced by carbon:
 Fe3O4(s) + 4C(s)  3Fe(s) + 4CO(g)
 FeO(s) + C(s)  Fe(s) + CO(g)
2
...
Advantages and disadvantages of the process

Advantages

Disadvantages

raw materials are very cheap

carbon dioxide is released

reaction is exothermic, so less energy is metals which are higher than carbon in the reactivity
needed to keep the furnace hot
series (eg aluminium) cannot be extracted in this way
continuous process which means iron
can be made more efficiently

metals which react with carbon to form carbides (eg
titanium and tungsten) cannot be extracted in this way
Some need contains significant quantities of carbon

Topic 13 Page 14

Aluminium, Tungsten
04 April 2015

15:28

REDUCTION OF OXIDES BY ELECTROLYSIS
1
...
Aluminium is one such example
...
It occurs predominantly as bauxite, which is an impure form of aluminium oxide,
Al2O3
...


Al2O3 has a melting point of 2000oC, which is too high to enable it to be electrolysed
...
(lower
melting point)

1
...
Aluminium is produced at the cathode
• Al3++ 3e  Al(l)
3
...
Extraction of tungsten
Tungsten is reacted with hydrogen gas at a high temperature:
WO3 + 3H2  W + 3H2O
2
...

 TiO2(s) + 3C(s)  TiC(s) + 2CO(g)
 2WO3(s) + 9C(s)  W2C3(s) + 6CO(g)
A reactive metal could be reacted with the oxide, such as magnesium:
TiO2(s) + 2Mg(s)  Ti(s) + 2MgO(s)
However it is very difficult to achieve pure titanium by this method as the magnesium forms an alloy
with the titanium
...
Extraction of titanium

i) conversion of ore into chloride
TiO2 is heated to 900 with stream of chlorine, in the presence of coke(carbon), to produce TiCl4
...
It can be separated from the other products by fractional distillation
...

This process is known as the Kroll process
...

2
...
This adds to the cost of the process
...

1
...

2
...

3
...
The ingots are rolled into sheet metal from which more cans are cut
...

Because of the large size of these objects, the collection is done by specialist metal reclaiming
companies
...
The objects are cut into a manageable size
...
The iron is separated from the other metals using a large magnet
...
The scrap is melted and purified in the Basic Oxygen Process
...


15:40

-

free radical substitution
methane with chlorine

-

electrophilic addition
alkenes with hydrogen bromide
alkenes with bromine
alkenes with sulphuric acid

2
...


nucleophilic substitution
haloalkanes with hydroxide ions
haloalkanes with cyanide ions
haloalkanes with ammonia

4
...

The lower arrow is the halide leaving the group
...
Made from sugars from sugar cane plants where it can be fermented
2
...
Mass spectrometry can help identify compound
○ Used to find the relative isotopic abundance of different isotopes
○ Relative molecular mass, Mr of a compound
2
...
Beam Inferred(IR) radiation is passed through a sample from a chemical
2
...
Infrared frequencies of different bonds absorb

Fingerprint region identifies a molecule
•
•
•
•

Region between 1000cm-1 and 1550cm-1
It is unique for each particular compound
Matches with a known sample
Extra peaks can identify any impurities: how pure the compound is?

Topic 17 Page 29

Glossary
05 May 2015

19:23

Enthalpy change: change in heat energy under constant pressure
Standard conditions: 100 kPa and a stated temperature
Exothermic: a change during which heat is given out by the particles
Endothermic: a change during which heat is absorbed by the particles
Standard enthalpy of formation: heat energy change when one mole of a compound is formed from
its elements in their standard states under standard conditions
Standard enthalpy of combustion: heat energy change when one mole of a substance is completely
burned in excess oxygen under standard conditions
Activation energy: minimum energy required for a collision to be successful
Collision frequency: number of collisions between particles per second
Collision energy: combined kinetic energy of the colliding particles
Rate of reaction: change in concentration of a species per unit time
Catalyst: substance which increases the rate of a chemical reaction without itself being chemically
unchanged at the end of the reaction
Dynamic equilibrium: a reaction in which the forward and reverse reactions are proceeding at the
same rate, so the concentrations of reactants and products are constant
Le Chatelier’s principle: if a change in conditions is imposed on a system at equilibrium, the system
will react in such a way as to oppose the effect of that change
Oxidation: loss of electrons

Reduction: gain of electrons
Oxidising agent: electron acceptor
Reducing agent: electron donor
Oxidation number: charge that would be on an atom if the bonding were completely ionic
Redox reaction: reaction in which electrons are transferred from one species to another
Geometrical Isomerism: the same molecular formula, but a different spatial arrangement of atoms
around a double bond
Addition: Combination of two or more molecules to form a single molecule
Substitution: Replacement of one atom or group of atoms by another
Elimination: Loss of a small molecule from a larger molecule to form an unsaturated compound
Electrophile: Species which can accept an electron pair from another species

Glossary Page 30

Nucleophile: Species which can use a lone pair of electrons to form a covalent bond
Free Radical: Species with an unpaired electron
Homolytic Fission: Breaking of a covalent bond in such a way that one electron goes to each atom
Heterolytic Fission: Breaking of a covalent bond in such a way that both electrons go to the same
atom
Hydration: Addition of water to a species
Hydrolysis: Use of water to break covalent bonds
Oxidation: Addition of oxygen or removal of hydrogen
Addition Polymerisation: Combination of a large number of monomers to form a single chain
Biofuel: A fuel derived or produced from renewable biological sources

Glossary Page 31



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