How Pressure Changes Affect Equilibrium Position Revision Notes | GCSE AQA Higher Chemistry

1. Why Pressure Matters in Chemical Equilibrium

Imagine squeezing a balloon full of gas molecules. Where do they go? In chemical reactions involving gases, pressure changes create a similar squeeze that forces equilibrium reactions to respond in predictable ways.

This isn't just theory - industrial processes like the Haber process for making ammonia rely entirely on understanding how pressure affects equilibrium. Get this wrong, and factories lose millions.

2. The Molecule Counting Rule

Here's the key insight: equilibrium systems hate being squeezed. When you increase pressure, the reaction shifts to make fewer gas molecules. When you decrease pressure, it shifts to make more gas molecules.

But how do we count molecules? Look at the coefficients in the balanced equation. In $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$, the left side has 4 molecules (1 + 3), the right side has 2 molecules.

Worked example: Counting Gas Molecules in Chemical Equations

For the reaction $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$, count the gas molecules on each side.

1. $2SO_2(g) + O_2(g)$ — Reactants - everything before the equilibrium arrow
2. $2 + 1 = 3$ gas molecules — Add the coefficients: 2 from $SO_2$ plus 1 from $O_2$
3. $2SO_3(g)$ — Products - everything after the equilibrium arrow
4. $2$ gas molecules — Only $SO_3$ with coefficient 2

Answer: Left side: 3 gas molecules, Right side: 2 gas molecules

This counting is crucial because it tells us exactly which way the equilibrium will shift when pressure changes. Let's see this in action.

3. Le Chatelier's Principle in Action

Le Chatelier's principle explains why this happens. When you increase pressure, you're stressing the system. The equilibrium responds by shifting to reduce that stress - and the best way to reduce pressure is to make fewer gas molecules.

Think of it like a crowded room. If more people squeeze in (higher pressure), some people leave through the exit that leads to a smaller room (fewer molecules). If people leave the room (lower pressure), more people can spread out by going to the bigger room (more molecules).

Worked example: Predicting Pressure Effects Step-by-Step

For $2NO_2(g) \rightleftharpoons N_2O_4(g)$, predict what happens when pressure decreases.

1. Left: 2 molecules, Right: 1 molecule — Always start by counting - this determines everything
2. Pressure decreases — The question states pressure is decreasing
3. Lower pressure favours more molecules — System shifts to oppose the change by making more molecules
4. Equilibrium shifts left — Left side has more molecules (2 vs 1)

Answer: Equilibrium shifts left, producing more $NO_2$ gas

**Trap**: Many students think higher pressure always shifts right. Not true! It depends entirely on which side has fewer molecules. Always count first.

4. Real Industrial Applications

The Haber process demonstrates this perfectly. $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ has 4 molecules on the left, 2 on the right. To maximise ammonia production, factories use high pressure (200-300 atmospheres) to push equilibrium right.

But there's a catch - higher pressure costs more energy and requires stronger equipment. It's a balance between chemistry and economics. This is why understanding equilibrium isn't just academic - it affects real-world decisions worth billions.

Contact process for sulfuric acid uses the same principle: $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$. Left side has 3 molecules, right has 2, so higher pressure increases $SO_3$ yield.

5. Exam Technique and Common Mistakes

Exam questions on pressure effects follow predictable patterns. You'll either get a reaction and need to predict the effect, or you'll get data showing concentration changes and need to explain them.

**Common Exam Mistakes**: Students often forget to count correctly, especially with complex molecules. $CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)$ has 0 gas molecules on left (solids don't count), 1 on right. Only gas molecules matter for pressure effects.

Another trap: thinking pressure changes affect all equilibria. If there are no gases involved, or equal numbers of gas molecules, pressure has no effect. Always check the state symbols.

Exam precision note: count only gaseous species, compare the totals on each side, then state the direction of shift. In pressure questions, say explicitly whether higher pressure favours the side with fewer gas molecules or lower pressure favours the side with more gas molecules.

The coefficient count is a method, not a formula. You inspect the balanced equation, add only the gaseous coefficients on each side, and then decide whether the system would reduce or increase pressure by shifting left or right.