Collisions, Catalysts, Curves — study notes

Leave a bowl of ice cubes on the counter and they melt slowly; drop the same ice into a hot pan and it's gone in seconds. Same substance, wildly different speed — and chemistry has a name for exactly this: rate of reaction. Once you know what actually controls that speed, questions that look tricky in an exam turn into one repeated idea applied five different ways.

The One Idea Behind Every Rate Question

Nothing reacts unless particles physically bump into one another, and even then a gentle bump achieves nothing — each collision must carry a certain minimum amount of energy, a threshold chemists call the activation energy, before old bonds can break and new ones can form. So every question about "why does X speed up the reaction" boils down to one of two effects: either the particles are meeting more often, or a bigger share of those meetings carry the energy needed. Squeeze reactant particles closer together (by raising concentration, cramming a gas into less space, or crushing a solid into grains) and meetings happen more often. Warm the mixture up and the particles speed up, which pulls both levers at once: they meet more often, and more of their collisions pack the energy to clear that threshold — the second effect being the one weaker answers forget. A substance can also open up an easier route between starting materials and end products, one whose energy hurdle sits lower, so far more meetings succeed without anything being heated — that's what a catalyst does, and it comes out of the reaction chemically the same as it went in. Learn that one sentence — frequency and energy are the only two levers — and the whole topic stops being five things to memorise and becomes one thing to apply.

Worked Example — Working Out a Rate From Data

1. A learner collects the gas given off by a fizzing reaction and records 80 cm³ collected after exactly 20 seconds.
2. Rate is defined as the amount produced divided by the time it took, so set up: rate = 80 cm³ ÷ 20 s.
3. Carry out the division: rate = 4 cm³ per second.
4. Sanity-check the units — "cm³ per second" is amount-per-time, matching what rate is actually measuring, so the answer makes sense.

That's the arithmetic side of rate sorted — but the harder marks come from explaining why concentration, particle size, pressure and heat each push that number up or down, plus how a real experiment is built to test one factor fairly. The full lesson below walks through each factor with its own worked example, a listen-along audio explainer, and a practice worksheet.