The Effect of Changing Pressure (Oxford AQA IGCSE Chemistry)
Revision Note
Written by: Alexandra Brennan
Reviewed by: Stewart Hird
The Effect of Changing Pressure
We can predict the effect of changes in pressure on systems in equilibrium
Changes in pressure only affects gases
In gaseous reactions:
An increase in pressure will favour the reaction that produces the least number of molecules
A decrease in pressure will favour the reaction that produces the greatest number of molecules
If there are the same number of moles of gases on either side of the equation, then there is NO effect on the position of equilibrium when the pressure is changed
Increasing the pressure will increase the rate of the forward reaction and backward reaction equally which is why the position of equilibrium is unchanged
Worked Example
Nitrogen dioxide molecules can dimerise and form dinitrogen tetroxide in the following equilibrium reaction:
2NO2 (g) ⇌ N2O4 (g)
dark brown colourless
What will the colour change be if the pressure is increased? Explain your answer.
Answer:
The number of gas molecules produced by the forward reaction = 1
The number of gas molecules produced by the reverse reaction = 2
An increase in the pressure will favour the reaction that produces the least number of molecules
This is the forward reaction
This means that the mixture will appear increasingly colourless as the concentration of N2O4 increases
Examiner Tips and Tricks
Remember, you are counting the number of molecules on each side of the equation, not the number of atoms.
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