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The structure of the periodic table
Periodicity is the repeating pattern in chemical and physical properties
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This is because elements across a period have
the same number of subshells e
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elements in period 2 have 2 sub shells
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Elements down a group have the same number of outer electrons which are
used in chemical reactions and the same type of orbitals e
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all elements in group 2 have 2 outer electron and an s²
An orbital is a region within an atom that can hold up to two electrons with opposite spins
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For example, atoms of group 2 elements
lose one electron to from 2+ ions with a noble gas configuration
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Mg: 1s²2s²2p⁶ 3s² → [Ne] 3s²
First ionisation energy is the energy needed to remove 1 mol of electrons from 1 mol of gaseous atoms
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The atomic radius decreases because the increased nuclear
charge pulls the electrons in towards the nucleus
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Therefore there is greater nuclear attraction on the electrons
and the outer electrons are more strongly attracted to the nucleus
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This means the nuclear attraction decreases
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This increases the atomic radius and the electron shielding as p-orbitals require a little less energy which
means there is slightly less nuclear attraction, so it’s easier to remove an electron
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It’s easier
to remove an electron from a singly-occupied orbital so less energy is needed, decreasing the nuclear attraction
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The
number of protons also increases which increases the
nuclear charge
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The nuclear attraction from the increase in atomic radius
and electron shielding outweighs the nuclear attraction from
the increased nuclear charge
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The number of electrons in each shell of an
atom is worked out by counting how many
electrons there are before there is a big
jump in ionisation energies
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The group of the element can be found by
counting the number of electrons before the
first big jump in successive ionisation
energies
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Periodic trend in structure and melting point
Metallic bonding is the strong electrostatic attraction between cations (positive ions) and delocalised electrons
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There are positive metal ions or
cations fixed to the lattice with a ‘sea’ of delocalised electrons that are free to move around the structure
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They are held together by strong electrostatic attractive forces
or metallic bonds which require a lot of energy to be overcome
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They are good electrical conductors because they have delocalised electrons which can move freely within the metal
lattice, which can carry charge
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Metals are malleable and ductile because the delocalised electrons are free to move through the structure allowing the
layers of atoms to slide past each other
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Diamond has a tetrahedral structure held together by strong covalent bonds throughout the lattice
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They
are hard because the tetrahedral shape allows external forces to spread throughout the lattice
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Graphite is made up of layers of carbon atoms held together by strong covalent bonds with weak van der Waals’ forces
between the layers, forming a strong hexagonal structure
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It has high melting and boiling points because it has strong covalent
bonds which require a lot of energy to break
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Graphene has a hexagonal layer structure, like graphite
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It’s a good electrical conductor
because the delocalised electron is free to move
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Silicon or silicon dioxide (SiO₂) has a similar structure and bonding to diamond but with a few oxygen atoms
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The elements in period 2 and 3 generally increase until element four where it decreases
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This leads to a higher charge
density, attracting the ions together more strongly
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These are very strong and require a lot of energy to break
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The
more atoms the molecule has, the stronger the induced dipole-dipole forces
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When a group 2 element reacts, it is oxidised from a state of 0 to +2:

Group 2 elements react vigorously with oxygen to from an ionic oxide
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Group 2 elements react with water to from hydroxides and hydrogen gas
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Group 2 elements react with dilute acids to from a salt and hydrogen gas
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When reacted with water the solid metal disappears (solid to aqueous) and bubbles are formed (release of hydrogen
gas)
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So, nuclear attraction decreases because the increased nuclear charge is outweighed by the increase in
atomic radius and shielding, making it easier to remove outer electrons
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Reactions of Group 2 compounds
Group 2 oxides react with water to form a solution of the metal hydroxide
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The solubility of the hydroxides increases down the group so the solutions are more alkaline
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CaO, which is known as quicklime, is made through the thermal decomposition of CaCO₃
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Radium decomposes at the
highest temperature
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Mg(OH)₂ and CaCO₃ are ‘antacids’ and are used to treat indigestion by neutralising any excess hydrochloric acid in the
stomach
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The boiling point increase down the group because the numbers of electrons increase, leading to stronger van der Waals’
forces/induced dipole-dipole forces between the molecules which require more energy to break down
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Redox reactions and reactivity of halogens and their compounds
A displacement reaction happens when a more reactive element displaces a less reactive element
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Orange
Orange
Bromine will displace iodine
Br₂
Br₂(aq) + 2I¯(aq) → 2Br¯(aq) + I₂(aq)
(violet)
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The organic solvent helps to distinguish between
bromine and iodine
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The halogens get less reactive down the group because the atomic radius increases and the electron shielding increases,
so the ability to gain an electron into the p sub-shell decreases to from a halide ion, there is less nuclear attraction → they
become less oxidising
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Chlorine and water reacts to from hydrochloric acid and chloric (I) (known as hypochloric acid)
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Chlorine reacts with cold and dilute sodium hydroxide to form sodium chlorate (I) solution or bleach, NaClO (aq)
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Sodium chlorate (I) solution is used in water treatment, to bleach paper and textiles and clean toilets
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Chlorine gas is toxic and harmful when breathed in
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Precipitates are often formed
when two aqueous solutions are mixed together
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Dilute nitric acid (HNO₃) is first added to
get rid of any unwanted ions and then aqueous silver nitrate (AgNO₃) is added
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Aqueous silver nitrate
Dilute aqueous ammonia
Concentrated aqueous ammonia
Chloride White precipitate
Dissolves, colourless
Bromide Cream precipitate
Doesn’t dissolve, cream precipitate
Dissolves, colourless
Iodide
Yellow precipitate
Insoluble (doesn’t dissolve), yellow Insoluble (doesn’t dissolve), yellow
precipitate
precipitate
Test for ions
Carbonates react with dilute acids making carbon dioxide which we see as effervescence
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It doesn't matter which acid you use because the Chloride ions from HCl or Nitrate ions from HNO₃ would stay in solution
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Sulphates don't usually react to make gases but barium sulphate is insoluble whereas most other sulphates (except
calcium sulphate) are soluble
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If you are doing a sequence of tests to discover which anion is in a substance you should obviously split your sample and
do the tests separately to avoid confusion
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This will prevent false results of as both BaCO₃ and
Ag₂SO₄ are insoluble
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This would give a false positive test for sulphate ions
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Barium Nitrate would be a suitable soluble alternative
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This is because solutions can absorb carbon dioxide to
form carbonate ions
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We couldn't use Hydrochloric acid to acidify due to the extra Chloride ions
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If
ammonium ions are present, ammonia gas will be given off:
Ammonia gas can be identified by its smell or by turning red litmus paper blue
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Enthalpy changes: ∆H of reaction, formation, combustion ad neutralisation
Enthalpy, H, is the heat content that is stored in a chemical system
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The surroundings are what are outside the chemical system
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In an exothermic reaction, the enthalpy of the products is smaller than the enthalpy of the reactants
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ΔH has a negative sign because heat has been lost by the
chemical system
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There is a heat gain
to the chemical system from the surrounding (ΔH + ve)
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Activation energy is the minimum energy required to start a reaction by the breaking of bonds
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0 mol dm¯³ (for reactions with aqueous solutions)
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Enthalpy change of reaction is the enthalpy change associated with a state equation under standard conditions
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The standard enthalpy change of combustion is the complete combustion of 1 mol of a substance under standard
conditions
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Standard enthalpy changes determined experimentally can be less/more exothermic or endothermic than the calculated
theoretical values because heat may have been released to the surroundings/absorbed from the surroundings,
incomplete reactions may have occurred (reactants not completely reacted) or it may not have occurred under standard
conditions
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Every species must be a gas
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Bond enthalpy is an endothermic change with bonds being broken
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Bond breaking absorbs energy and bond making releases energy
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In endothermic reactions, more energy is absorbed than released (bonds formed
are weaker)
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Simple collision theory
Increasing the concentration increases the rate of reaction because there are more particles per (unit)/ in the same
volume so they are closer together and have a greater chance of colliding
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For a particular length of time, more frequent collisions will take place with greater energy than the activation energy
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Catalysts
Catalysts have greater economic importance and benefits for increased sustainability because they aren’t used up by the
overall reaction so can be used again reducing costs and produces less hazardous chemicals
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Reactions can take place at lower temperatures which reduces energy demand from the combustion of fossil fuels
resulting in lower carbon dioxide emissions and allows fuels to last longer
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Homogeneous catalysts are catalysts with a different physical state to the reactants, frequently reactants are gases whilst
the catalyst is solid
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Reaction rates can be measured by measuring the change in mass
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It’s accurate
and easy to use but must be done in a fume cupboard if a gas is given off
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Use a gas syringe to record the amount of gas given off
at time intervals
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Other methods such as measuring changes in pressure (for gases), changes in colour (for solutions) or changes in
conductivity can also be used to measure reaction rates
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There are no molecules with no energy so the curve starts at the origin
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Only the molecules with energy greater than the
activation energy are able to react, so not all collisions lead to a reaction
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Higher proportions of
molecules have energy greater than the activation
energy and are able to react
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The rate of reaction increases in the presence of a
catalyst because the activation energy is reduced
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On collision, more molecules will
overcome the new lower activation energy of the
reaction
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Dynamic equilibrium and le Chatelier’s principle
Dynamic equilibrium exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse
reaction and the concentrations of reactants and products don’t change
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Increasing the temperature adds heat energy
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Decreasing temperature removes heat energy
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If the forward reaction is endothermic, the reverse will be exothermic, and vice versa
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Increasing the pressure shifts the equilibrium to the side with
fewer gas molecules
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Decreasing the pressure shifts the equilibrium to the side with more gas
molecules, raising the pressure again
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Increasing the concentration of a reactant (or adding more reactant) makes equilibrium position shift to the right to get rid
of the extra reactant, making more products
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This makes the reverse reaction go faster
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Catalysts increase the rate of both forward and reverse reactions in equilibrium by the same amount resulting in an
unchanged position of equilibrium
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However
equilibrium is reached faster
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Top carry out the
experiment, add equal amounts of the equilibrium solution to three to four test tubes
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Test tube 2 and 3 contains a higher concentration of the reactant(s)
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Example
Test tube 1 – control
Test tube 2 – few drops of iron (III) nitrate solution
Test tube 3 – few drops of potassium thiocyanate solution
Test tube 4 – few drops of iron (III) thiocyanate solution
The forward reaction is favoured and more product is made in test tubes 2 and 3 (both are a deep red colour)
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Changes to the position of equilibrium can be investigated through the changes in temperature
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The changes in colours of the mixtures are observed
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The tube in the ice bath will go a
paler colour as the equilibrium shifts to the right to replace the lost
heat by favouring the exothermic reaction
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The
conditions have to be appropriate so that the process isn’t expensive or needs complicated equipment
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The equilibrium constant, Kc
A homogenous reaction is where all the reactants and products are in the same physical state
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It gives you an idea of how far to the left or right
the equilibrium is
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The units of Kc vary depending on the reaction
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The Kc is often asked at a
specific temperature that is given so that the temperature is shown as being constant
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The higher above 1 the
Kc is, the further to the right the equilibrium is
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The further below 1 Kc is, the further to the left the equilibrium is

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