The short answer
What changes a reaction's speed — and how Section III tests it with graphs and data.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Kinetics is about how fast, not how far. Section III rarely asks you to recall a rate law — it gives you data or a graph and asks you to reason about the speed.
Collision theory in a line
Reactions go faster when particles collide more often or more energetically. Everything that speeds a reaction does one of those two things.
What speeds a reaction — and why
Increases the rate
- ↑ Concentration / pressure (more collisions)
- ↑ Temperature (faster collisions, and more of them clear the activation energy — usually the biggest single effect)
- ↑ Surface area (more contact)
- Add a catalyst (lower activation energy)
What a catalyst does NOT do
- Does not get used up
- Does not change the products
- Does not shift the equilibrium position
- Only provides a lower-energy pathway
Reading the slope
Two questions ask about the graph above: where is the reaction fastest, and what is happening to the rate over time?
Doubling the concentration does not always double the rate
How the rate responds to concentration is the reaction's order, and order is something you read off the data — never something you assume. Double [A] and the rate doubles: first order in A. Double it and the rate quadruples: second order. Double it and nothing changes: zero order. When a question hands you a table of concentrations and rates, that comparison is the question.
| Experiment | [A]₀ (mol/L) | [B]₀ (mol/L) | Initial rate (mol/L/s) |
|---|---|---|---|
| 1 | 0.10 | 0.10 | 2.0 × 10⁻³ |
| 2 | 0.20 | 0.10 | 4.0 × 10⁻³ |
| 3 | 0.20 | 0.20 | 1.6 × 10⁻² |
Check yourself
On the concentration-time graph above, how does the reaction rate change as time goes on?
Key takeaways
- Kinetics = how fast; equilibrium = how far. Don't confuse them.
- Faster rate = more frequent or more energetic collisions.
- On a concentration-time graph, rate is the gradient — steepest at the start.
- A catalyst lowers activation energy without being consumed or shifting equilibrium.
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