Comprehensive study guide for Chemistry A Level, made by an Oxford Biochemistry student with all 9s at GCSE and 3 A*s at A Level! Information arranged by spec point. Notes written using past papers, textbooks and more.
1. Know that many reactions are readily reversible and that they can reach a state of
dynamic equilibrium in which:
i. The rate of the forward reaction is equal to the rate of the backward reaction.
ii. The concentration of reactants and products remain constant.
Reversible changes are processes which can be reversed by altering the conditions.
- Some changes go in only one direction.
o These are irreversible processes.
o For example, once you bake bread in an oven, there is no way to reverse the
process and revert it back into flour and water etc.
o Burning a fuel is another example of a one-way process.
- Many other reactions involve reversible changes.
o You can often revert these processes by altering the conditions, for example the
temperature and pressure.
o E.g. Hb + 4 O2 ⇌ Hb4O8
o E.g. CoCl2·6H2O (s) ⇌ CoCl2 (s) + 6 H2O (g)
- This reaction involving cobalt (II) chloride and water can be used as a very sensitive and
accurate test for the presence of water.
o CoCl2 is blue and CoCl2·6H2O is pink
o If water is present, the cobalt chloride will turn pink.
- NH4Cl (s) ⇌ NH3 (g) + HCl (g)
o At room temperature, the two gases combine to make a white smoke of
ammonium chloride.
o Heating reverses the reaction and ammonium chloride decomposes at high
temperatures to give hydrogen chloride and ammonia.
- 3 Fe (s) + 4 H2O (g) ⇌ Fe3O4 (s) + 4 H2 (g)
o Supplying lots of steam and ‘sweeping away’ the hydrogen means that the
forwards reaction continues until all the iron changes to iron oxide.
o Altering the conditions brings about the reverse reaction. A stream of hydrogen
reduces all the iron (III) oxide to iron, so long as the flow of hydrogen sweeps
away the steam that has formed.
- Reversible changes often reach a state of balance, or equilibrium.
- Balance points exist in most reversible reactions where neither the forward nor the
reverse reaction is complete.
- Reactants and products are present and the reactions appear to have stopped.
Important features of equilibrium features:
- At equilibrium, the concentration of reactants and products
does not change.
- The same equilibrium state can be reached from either the
‘reactant side’ or the ‘product side’ of the equation.
- Both the forward and backward reactions happen at the
same rate.
- This is dynamic equilibrium.
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