Nitrogen and hydrogen react to form ammonia, with four molecules of gas becoming two. The pressure on the equilibrium mixture is raised. What happens?

1. Pressure comes from gas molecules striking the walls of the container. 2. Fewer gas molecules in the same volume means fewer collisions with the walls, and so a lower pressure. 3. Le Chatelier's principle says the equilibrium shifts to reduce the effect of the change applied. 4. So raising the pressure favours the side of the equation with fewer gas molecules. 5. Count the molecules: one molecule of nitrogen plus three of hydrogen gives four on the left. 6. Those four form two molecules of ammonia, so there are only two gas molecules on the right. 7. The forward reaction reduces the number of gas molecules, so it is favoured and the yield of ammonia rises. 8. Answering less ammonia is the trap for counting the sides the wrong way round, and the equilibrium never stops reacting because both reactions continue at equal rates. _Source: Siyavula Physical Sciences Grade 12 (Everything Science, CC BY 4.0), Chapter 8: Chemical equilibrium_