Guided lesson: The Position of Equilibrium

SyllabAI HubIGCSE & IAL revision

Sign in Practise

Chemistry

Checks

0 of 9 attempted · 0 correct

1.Heating an equilibrium mixture favours which direction?
2.The forward reaction of an equilibrium is exothermic. What does cooling the mixture do?
3.What does it mean for the position of equilibrium to shift right?
4.Iodine monochloride and chlorine form iodine trichloride, with the forward reaction exothermic. On heating, the brown colour deepens. Why?
5.Which change would shift an equilibrium towards the exothermic direction?
6.State what a shift to the right means for an equilibrium mixture.
Written response · recall
Your answer
7.Which direction of shift does raising the temperature cause at equilibrium?
Written response · recall
Your answer
8.Explain why lowering the temperature favours the exothermic direction at equilibrium.
Written response · explain why
Your answer
9.A closed system's equilibrium shifts left after the conditions change. State what forms more as a result.
Written response · application
Your answer

2.Pressure and the molecule count

4CH1-3.22C

Squeezing a gaseous equilibrium changes the balance too, but only gases feel the squeeze. This lesson counts molecules on each side of the equation and predicts the shift from the count alone.

Key points

Changes in pressure only affect gases in an equilibrium mixture.
Increasing the pressure favours the reaction producing the fewest gas molecules.
Decreasing the pressure favours the reaction producing the most gas molecules.
Equal numbers of gas molecules on both sides mean pressure has no effect on the position.
Pressure changes affect only the position of equilibrium, never the rates equally matched by a catalyst.

First pass

Push on a gaseous equilibrium and the balance slides towards whichever side packs fewer gas molecules. If both sides hold the same count, squeezing changes nothing about the balance.

Analogy — It is like a crowded train: squeeze the carriage and people move to whichever end has fewer passengers standing.

Exam pointer — Count the gas molecules on each side before predicting any pressure shift.

Going deeper

Raising the pressure of a gaseous equilibrium favours the direction that produces the fewest gas molecules, while lowering the pressure favours the direction producing more. When the gas counts match on both sides, pressure changes leave the position of equilibrium unchanged.

Analogy — A lift preferentially fills with the smallest group first; if both floors hold equal groups, squeezing the lobby sends no signal.

Exam pointer — Write the molecule counts over each side, then quote the direction that the pressure change favours.

Checks

0 of 9 attempted · 0 correct

1.Why does increased pressure favour fewer gas molecules?
2.In 2NO2 (g) turning into N2O4 (g), what does increased pressure do?
3.A gaseous equilibrium has three molecules on the left and three on the right. What does increased pressure do?
4.Which observation would show a gaseous equilibrium shifting right after compression?
5.Pressure changes do not affect the position when which condition holds?
6.Which side of an equilibrium does increasing the pressure favour?
Written response · recall
Your answer
7.State which substances in an equilibrium mixture are affected by a pressure change.
Written response · recall
Your answer
8.Explain why changing the pressure has no effect on an equilibrium with equal numbers of gas molecules on both sides.
Written response · explain why
Your answer
9.The pressure over an equilibrium mixture is decreased. State which reaction direction is favoured.
Written response · application
Your answer

3.Catalysts and dynamic equilibrium

4CH1-3.21C

A catalyst speeds reactions, so surely it should push the balance towards products. It does not, and the reason is elegant: it speeds both directions equally. This lesson proves that on the rate graph.

Key points

Catalysts leave the equilibrium position exactly where it was.
A catalyst increases the rate at which equilibrium is reached.
Forward and backward rates rise together, each sped up equally.
The concentrations of reactants and products at equilibrium are the same with or without a catalyst.
On a rate graph the catalysed run flattens sooner but at the identical final level.

First pass

A catalyst helps both directions equally, so the balance point never moves; it is simply reached sooner. The final mixture is exactly the same, only faster in arriving.

Analogy — It is like two escalators running at double speed in opposite directions: everyone moves faster, but the crowds on each floor end up the same.

Exam pointer — Say faster equilibrium, same position, whenever a question pairs catalysts with equilibrium.

Going deeper

Because the catalyst lowers the barrier for both directions by the same amount, the forward and backward rates rise together and the position of equilibrium is unchanged. The equilibrium concentrations therefore match the uncatalysed case, with equilibrium simply arriving earlier.

Analogy — Two queues served by faster tills shorten at the same pace, so the split of shoppers between them never changes.

Exam pointer — Explain the unchanged position through equal speeding of both directions, then read the flattening graph.

Checks

0 of 9 attempted · 0 correct

1.What effect does a catalyst have on the position of equilibrium?
2.Why does a catalyst fail to move the equilibrium position?
3.What does a catalyst change about an equilibrium system?
4.On a graph of product amount against time, how does the catalysed run compare with the uncatalysed one?
5.Why is the equilibrium composition the same with and without a catalyst?
6.State the effect of a catalyst on the time needed to reach equilibrium.
Written response · recall
Your answer
7.State what a catalyst does to the position of an equilibrium.
Written response · recall
Your answer
8.Explain why the concentrations at equilibrium are the same whether or not a catalyst is present.
Written response · explain why
Your answer
9.Two identical rate graphs are drawn, one with a catalyst and one without. State how the catalysed curve differs.
Written response · application
Your answer