Detailed notes on equilibrium I. Notes written using Edexcel Chemistry textbooks, past papers and more. Written by a student with all A*s at GCSE, 3A* predictions at A Level and with an offer for Natural Sciences at Cambridge.
Topic 10: Equilibrium I
1. know that many reactions are readily reversible and that they can reach a
state of dynamic equilibrium in which:
Dynamic Equilibrium
Many reactions are readily reversible and can reach a state of dynamic equilibrium. This means:
The rate of the forwards reaction is equal to the rate of the
backward reaction
The concentrations of the reactants and products remain
constant
A dynamic equilibrium can only occur in a closed system.
An example of a reversible reaction is:
H2(g) + I2(g) ⇌ 2HI 2HI(g)
As the reactants get used up, the rate of the forwards reaction slows down, and as more product
is formed, the rate of the backwards reaction speeds up.
2. be able to predict and justify the qualitative effect of a change in
temperature, concentration or pressure on a homogeneous system in
equilibrium
Le Chatelier’s Principle
Homogenous equilibrium: all the substances involved in the equilibrium are in the same state
Heterogenous equilibrium: the substances involved are in more than one state
Le Chatelier’s principle predicts what will happen to a reversible reaction if the conditions are
changed. If the position of the equilibrium moves to the left, you get more reactants and if the
position of equilibrium moves to the right you get more product.
Concentration
Using the above equation, if the concentration of SO 2 is increased, the equilibrium will shift to
the right. This means more product is made so the new equilibrium will have more SO 3 and less
O2 than the original equilibrium.
Pressure
Changing pressure only affects reactions with gases. Increasing the pressure shifts the
equilibrium towards the side with fewer moles of gas so in the above reaction, the equilibrium is
shifted to the right. This means fewer molecules are produced so the pressure will drop again.
Temperature
Increasing the temperature favours the endothermic reaction so in the above reaction, the
equilibrium shifts to the left. The backwards reaction is favoured and since it is endothermic,
heat is taken in so the temperature will drop again.
Adding a catalyst does not change the position of equilibrium. It speeds up both the forwards
and backwards reaction so that the equilibrium is reached faster.
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