Lesson overview
Rates of reaction: What makes a reaction go faster?
Race marble against hydrochloric acid three times while a gas syringe fills and your graph plots the volume of carbon dioxide every 20 seconds: lumps, then powder, then lumps in half-strength acid that you set up as a fair test. Predict each curve, find out why the powder makes no more gas, and work out a rate from the graph.
Suggested sequence
- Lesson 1 · The gas syringe race. Measuring a rate: following the gas against time, reading the curve and working out a rate. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Five ways to speed it up. Five things that change a rate, catalysts and enzymes, and a fair test. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 3 · Why collisions matter. Supplement: collision theory, activation energy and how a catalyst works. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 6 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 4 questions, 15 min, lesson closed.
Learning goals
Part 1: The gas syringe race
- Measure the rate of a reaction by collecting a gas or by the loss of mass, and interpret rate data and graphs: a steeper curve means a faster reaction, a rate is the change in amount divided by the time, and the curve levels off when one reactant is used up (6.2.3, 6.2.4); Supplement: evaluate the two methods (6.2.8)
Part 2: Five ways to speed it up
- Measure the rate of a reaction by collecting a gas or by the loss of mass, and interpret rate data and graphs: a steeper curve means a faster reaction, a rate is the change in amount divided by the time, and the curve levels off when one reactant is used up (6.2.3, 6.2.4); Supplement: evaluate the two methods (6.2.8)
- Describe the effect on the rate of concentration, the pressure of gases, surface area, temperature and a catalyst (including enzymes), plan a fair test, and state that a catalyst increases the rate and is unchanged at the end (6.2.1, 6.2.2)
Part 3: Why collisions matter
- Describe the effect on the rate of concentration, the pressure of gases, surface area, temperature and a catalyst (including enzymes), plan a fair test, and state that a catalyst increases the rate and is unchanged at the end (6.2.1, 6.2.2)
- Supplement: describe collision theory (particles per unit volume, collision frequency, kinetic energy, activation energy), use it to explain the effect of the five factors, and state that a catalyst lowers the activation energy (6.2.5, 6.2.6, 6.2.7)
Misconceptions to listen for
- “The rate goes up because the volume goes up.” — The volume keeps rising, but by less each time. The rate is how fast it rises, and that falls.
- “Rate = time ÷ volume.” — That gives the seconds for each cm³. A rate is how much gas is made each second: volume ÷ time.
- “The total volume is the top of the axis.” — The axis only gives the scale. The total is the height where the curve goes flat.
- “A flat line means the reaction is going steadily.” — A flat line means the volume is not changing: no more gas is being made.
- “The height of the flat part shows how fast the reaction was.” — The height shows how much gas was made in total. How fast is shown by the steepness.
- “A less steep curve means the reaction has stopped.” — While the curve still rises, gas is still being made, only more slowly. Stopped means flat.
- “Readings that stay the same mean the reaction stopped for a while.” — A reaction slows down smoothly; it does not stop and start again. A stuck plunger stays put, then jumps when the gas pushes it free.
- “Gas lost before the bung goes in makes the readings jump.” — Gas lost at the start makes every reading a little low, but the curve stays smooth.
- “A balance can follow any gas.” — Only a gas heavy enough to change the reading, such as carbon dioxide. A little hydrogen weighs almost nothing.
- “Extra mass lost means more gas was made.” — The amount of gas is fixed by the reactants. Mass lost beyond that is something else leaving the flask: acid spray.
Scope
Rate of reaction (6.2 Core 1–4, Supplement 5–8): the effect of concentration, gas pressure, surface area, temperature and a catalyst, catalysts and enzymes, practical methods for following a rate and interpreting its data, collision theory and activation energy. Moles and gas volumes are in an earlier module; reversible reactions and equilibrium in 6.3.