Rate tells you how quickly equilibrium is reached. Position tells you how much product there is once it has been reached. A change can affect one, the other or both, and keeping them apart is what earns the marks in this part of reversible changes and equilibrium.
This lesson builds on predicting shifts and on the collision ideas in rates of reaction.
Two different questions
| Question | About | Answer words |
|---|---|---|
| How quickly does it get there? | rate | faster, slower, sooner, later |
| How much product is there at the end? | position | more product, less product, shifts to the right/left |
Write two headings in your rough working, “rate” and “position”, and answer each change once under each heading.
The four changes compared
| Change | Effect on rate | Effect on position |
|---|---|---|
| Catalyst added | faster, both directions equally | no change |
| Temperature increased | faster, both directions | towards the endothermic direction |
| Pressure increased (gases) | faster | towards the side with fewer gas molecules, if the counts differ |
| Concentration of a reactant increased | faster at first | towards the products |
The catalyst row is the one examiners like to test, because the tempting answer “it increases the yield” is wrong. A catalyst does not help the reaction get further; it helps it get there sooner.
Worked example
For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the forward reaction is exothermic. Describe the effect of each change on (i) the rate at which equilibrium is reached and (ii) the equilibrium yield of ammonia.
A. Add an iron catalyst. (i) Faster, because the catalyst provides a lower-energy route for both reactions. (ii) No change, because both reactions are sped up equally.
B. Raise the temperature. (i) Faster, because particles have more energy and collide more often with enough energy. (ii) Lower yield, because higher temperature favours the endothermic reverse reaction.
C. Raise the pressure. (i) Faster, because the gas particles are closer together, so collisions are more frequent. (ii) Higher yield, because 4 gas molecules on the left and 2 on the right means the position shifts towards NH₃.
The same three changes give “faster” every time, but the yield goes no change, down, up. That contrast is the whole lesson.
The mistake to watch for
Mistaken answer: “Raising the temperature increases the yield of ammonia because the reaction is faster.”
This uses a rate fact to answer a position question.
The correction is to give two separate statements: “Raising the temperature makes the reaction faster. But the forward reaction is exothermic, so the position shifts towards the reactants and the equilibrium yield of ammonia falls.” If the question asks only about yield, give only the second statement. If it asks about how quickly, give only the first.
Check yourself
1. A catalyst is added to a reversible reaction at equilibrium. Describe the effect on the position of equilibrium and on the time to reach equilibrium.
Show answer
Position: no change, because both directions are sped up by the same amount. Time: shorter, because the catalyst lowers the energy needed for both reactions.
2. For H₂(g) + I₂(g) ⇌ 2HI(g), pressure is increased. State the effect on rate and on position.
Show answer
Rate: faster, because particles are closer together and collide more frequently. Position: no change, because there are 2 gas molecules on each side.
3. For the exothermic forward reaction 2SO₂ + O₂ ⇌ 2SO₃, the temperature is lowered. State the effect on rate and on the yield of SO₃.
Show answer
Rate: slower, because fewer particles have enough energy to react when they collide. Yield: higher, because cooling favours the exothermic forward reaction.
Where this leads next
Now you can handle the real industrial question in interpreting a yield-rate compromise descriptively, where conditions that raise yield and conditions that raise rate pull in different directions.
If “faster” and “more” still blur in your written answers, that is something a teacher in online one-to-one Chemistry tuition can untangle with you on mixed examples.