AQA AS Chemistry Unit 2 Revision Notes

Revision Notes on Unit 2 AQA AS Level Chemistry

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  • Created on: 28-01-10 21:37
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1) An enthalpy change is a change in energy at constant pressure (i.e. in normal `open container' conditions where
there is no change in pressure during the reaction).
2) A positive enthalpy (H) signifies an endothermic reaction, whereas a negative enthalpy will be given by an
exothermic reaction.
3) The energy released (or absorbed) in a reaction can be calculated by E = mcT, where c is the specific heat
capacity of the reaction mixture (usually taken as water, i.e. 4200 J kg­1K­1). The enthalpy for the reaction in J
mol­1 can then be calculated by dividing E by the number of moles reacting.
4) A simple calorimeter measures the energy released in a reaction by using it to heat a mass of water ­ the
increase in temperature of the water gives the energy released by the reaction. For combustion of alcohols, the
mass of the burner before and after gives the mass of alcohol used, which can be used to calculate the moles
5) Standard enthalpy of combustion is the enthalpy change when one mole of a substance in its standard state is
completely burned in air or oxygen to produce the products in their standard states, under standard conditions
(298K, 101kPa). This is Hoc.
6) Standard enthalpy of neutralisation is the enthalpy change when one mole of water is produced when an acid
neutralises an alkali under standard condition (298K, 101kPa). This is Ho neut.
7) Standard enthalpy of formation is the enthalpy change when one mole of a substance in its standard state is
formed from its constituent elements in their standard states under standard conditions (298K, 101kPa). This is
8) Hess' Law states that the enthalpy change of a reaction depends only upon the initial and final states of the
reaction and is independent of the route the reaction takes.
9) Bond dissociation enthalpy is the enthalpy change when one mole of a substance in its gaseous state is split into
individual gaseous atoms.
10) The mean bond enthalpy is the average enthalpy required to break or make one mole of a bond (e.g. C­H, C­C
etc.). This is taken as an average from many (but not all) molecules containing the bond.
11) Mean bond enthalpies can be used to estimate enthalpy changes by calculating the enthalpy needed to break
existing bonds (endothermic), and that when new bonds are made (exothermic).
12) Mean bond enthalpies can be calculated using the bond dissociation enthalpy for a species.

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Reaction Kinetics And Equilibria
1) The rate of a reaction can be explained using collision theory:
a) The molecules must have a minimum amount of energy to start the reaction ­ the activation energy (EA).
b) The molecules must approach each other at the correct angle and alignment.
c) Molecules can only react if they collide, and not all collisions lead to a reaction.
d) Increasing the frequency of collisions will increase the rate of reaction.…read more

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A reversible reaction is one that goes in both directions ­ Reactants Products. `Left to right' is the forward
reaction, and `right to left' is the reverse (or backward) reaction.
10) As a reversible reaction takes place, the rate of the forward reaction decreases as the reactants are used up,
and the rate of the reverse reaction increases as more products are produced. Eventually the rate will be the
same for both reactions, as the overall concentration of reactants and products will stop changing.…read more

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Redox Reactions
1) A redox reaction is one in which reduction and oxidation occur at the same time (you cannot get one without the
a. Reduction involves a gain of electrons and a decrease in oxidation state.
b. Oxidation involves a loss of electrons and an increase in oxidation state.
2) An oxidising agent will cause oxidation of another species and in doing so will itself be reduced.…read more

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Group VII ­ The Halogens
1) The halogens are the most reactive non-metal group, with names ending in ­ine. These will form halide ions, with
names ending in ­ide.
2) The electronegativity decreases down the group:
a) The nuclear charge increases, which should provide a greater attraction for electrons.
b) There is increased shielding of the nuclear charge from complete inner shells.
c) The atomic radius is increased, so the electrons are further from the nucleus and the force felt by them is less.…read more

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Chlorine will react with water to produce hydrochloric acid and chloric (I) acid. This is a disproportionation
reaction, as the chlorine is simultaneously reduced and oxidised:
Cl2 (aq) + H2O (l) HCl (aq) + HOCl (aq)
14) Chloric (I) acid will firstly turn damp litmus paper red, as it is an acid, and will then turn it white, as it is also a
15) The products of the disproportionation reaction will then neutralise a base, such as NaOH.…read more

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Extraction Of Metals
1) The following ores are used to extract metals:
a) Haematite ­ contains Fe2O3.
b) Magnetite ­ contains Fe3O4.
c) Iron pyrites ­ contain FeS2.
d) Bauxite ­ contains Al2O3.
e) Rutile ­ contains TiO2.
f) Ilmenite ­ contains FeTiO3.
2) Iron is extracted from its ore in the blast furnace:
a) The furnace is lined with heat resistant bricks ­ the temperature ranges from 2000°C at the bottom, to 400°C
at the top.…read more

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Electrolysis ­ produces a very pure product, but is very expensive in terms of running costs.
11) Uses of the extracted metals:
a) Iron ­ very common and cheap, widely used.
b) Aluminium ­ cooking foil, drinks cans, aeroplane parts etc.
c) Titanium ­ alloys in aircraft, missiles and car engines (very strong and low density).
12) Pollution problems:
d) Carbon monoxide produced is poisonous, but is burnt during the extraction process.
e) Carbon dioxide is a greenhouse gas, and constitutes to global warming.…read more



Cheers for these, they're helpful :D

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