The Effect of Changing PressureOxford AQA IGCSE Chemistry: Revision notes
Section 1
Which reactions are affected by pressure?
Pressure only affects the position of equilibrium for reversible reactions involving gases. If none of the reactants or products are gases, changing pressure has no effect on the equilibrium position.
Section 2
What is the rule for increasing pressure?
In a gaseous reversible reaction, an increase in pressure favours the reaction that produces the smaller number of molecules, as shown by the symbol equation. The system shifts towards the side with fewer gas particles, partially reducing the pressure increase.
Count the total number of gas molecules on each side of the balanced symbol equation before applying this rule — you must count moles of gas, not moles of solid or liquid.
Section 3
What is the rule for decreasing pressure?
A decrease in pressure favours the reaction that produces the greatest number of molecules, as shown by the symbol equation. The system shifts towards the side with more gas particles, partially increasing the pressure back up.
Section 4
How does this apply to the Haber process?
Consider the Haber process:
N2(g) + 3H2(g) ⇌ 2NH3(g)
There are 4 moles of gas on the left (1 + 3) and 2 moles of gas on the right. Because the forward reaction produces fewer gas molecules, increasing pressure shifts the equilibrium to the right, increasing the yield of ammonia. This is why the Haber process uses a high pressure (approximately 200 atmospheres).
If a reaction is A(g) + B(g) ⇌ C(g), there are 2 moles of gas on the left and 1 on the right. Increasing pressure would favour the forward reaction (fewer gas molecules on the product side).
Section 5
What if the number of gas molecules is the same on both sides?
If a reversible gaseous reaction has the same number of gas molecules on both sides of the equation, changing the pressure has no effect on the position of equilibrium, because neither side is favoured over the other.
Students sometimes assume pressure always shifts equilibrium. If gas molecule totals are equal on both sides, pressure changes have no effect on yield.
Must Know
- Pressure only affects equilibrium position in reactions involving gases
- Increasing pressure favours the side with fewer gas molecules
- Decreasing pressure favours the side with more gas molecules
- Count total moles of gas on each side of the symbol equation to apply the rule
- In the Haber process, high pressure favours ammonia (fewer gas molecules on the product side)
- If the number of gas molecules is equal on both sides, pressure has no effect on equilibrium position
That's the notes covered.
Carry on to the next subtopic.