The shape of the lesson
By the end of the lesson, learners can
- Successful collisions: Extended content.
- How catalysts work: Extended content.
- Concentration, pressure, surface area: more frequent collisions.
- Temperature: more frequent collisions and more with energy ≥ Ea.
- Catalyst: alternative pathway, lower Ea, unchanged at the end.
- Say "frequent", not just "more".
One picture every idea returns to
Extended. Particles react only when they collide with at least the activation energy. Higher concentration, higher pressure and larger surface area put more particles in contact, so the frequency of collisions increases. Higher temperature makes particles move faster: they collide more often and, more importantly, more collisions have energy at least equal to the activation energy. A catalyst provides a pathway with a lower activation energy, so more collisions are successful.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Collide hard, collide often.. Particles can only react when they hit each other, and only if they hit hard enough. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Successful collisions | Extended content. Project tab 1 of the lab (Reaction kinetics lab, collision theory) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: How catalysts work | Extended content. Project tab 4 of the lab (Reaction kinetics lab, catalysts) and let a student drive it while the class predicts. | Think-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 14 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in How catalysts work, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| "More collisions" is not enough for the marks: say more frequent collisions (more collisions per second). | For temperature, you must also say that more particles have energy greater than or equal to the activation energy; that is the bigger effect. |
Collision theory and catalysts: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
(Extended) State the two conditions needed for a collision between particles to result in a reaction.
(Extended) Use collision theory to explain why hydrochloric acid of concentration 2 mol/dm3 reacts faster with calcium carbonate than acid of concentration 1 mol/dm3.
(Extended) Use collision theory to explain why increasing the temperature increases the rate of a reaction.
(Extended) Explain how a catalyst increases the rate of a reaction.
(Extended) Explain why flour dust in the air of a flour mill can explode, although a heap of flour only burns slowly.
(Extended) Sketch reaction pathway diagrams for an exothermic reaction with and without a catalyst, and label the activation energy for each.
Answer key and marking notes
- The particles must collide.
- The collision must have energy greater than or equal to the activation energy.
- There are more acid particles in the same volume of the more concentrated acid.
- So collisions between acid particles and the calcium carbonate are more frequent.
- The particles gain kinetic energy and move faster.
- So they collide more frequently.
- A greater proportion of collisions have energy greater than or equal to the activation energy, so there are more successful collisions per second.
- It provides an alternative reaction pathway with a lower activation energy.
- So a greater proportion of collisions have enough energy to react, and the frequency of successful collisions increases.
- Dust particles are very small, so the surface area in contact with oxygen is very large.
- Collisions between flour and oxygen particles are therefore extremely frequent, so the reaction is very fast (explosive).
- Reactants at a higher energy level than products, joined by a curve with a hill.
- The catalysed curve has a lower hill between the same reactant and product levels.
- Each Ea labelled as an arrow from the reactants level to the top of its own hill.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What name is given to the proteins that act as catalysts in living things?
- Aenzymes
- Bhormones
- Cindicators
- Dpolymers
Which change increases the number of acid particles in each cm3 of a solution of hydrochloric acid?
- Ausing acid of a higher concentration
- Busing a larger volume of the same acid
- Cstirring the acid
- Dwarming the acid
What is meant by the activation energy of a reaction?
- Athe minimum energy that colliding particles need in order to react
- Bthe energy given out when the products form
- Cthe total energy stored in the reactants
- Dthe energy change between the reactants and the products
The diagram shows the reaction pathway for an exothermic reaction with and without a catalyst. Which arrow represents the activation energy of the catalysed reaction?
- Aarrow Q
- Barrow P
- Carrow R
- Dthe difference in length between arrows P and Q
Two reactant particles collide but do not react. What is the most likely reason?
- AThe collision had less energy than the activation energy
- BThe particles collided too frequently
- CThe collision had more energy than the activation energy
- DThe reaction is exothermic
Raising the temperature of a reaction from 20 °C to 30 °C roughly doubles its rate, yet the particles collide only a few per cent more often. What best explains the large increase in rate?
- AA much greater proportion of collisions have energy equal to or greater than the activation energy
- BThe activation energy is lowered by the higher temperature
- CThere are more particles in each unit volume
- DThe particles become larger and easier to hit
The pressure of a gas-phase reaction mixture is doubled at constant temperature. Which statement is correct?
- AThe number of particles per unit volume increases, so collisions are more frequent, but the energy of each collision is unchanged
- BEach particle has more kinetic energy, so more collisions are successful
- CThe activation energy decreases, so more collisions are successful
- DThe particles move more slowly, so collisions are gentler
Why does using a catalyst in an industrial process usually reduce energy costs?
- AIt gives an acceptable rate at a lower temperature, so less fuel is needed for heating
- BIt increases the yield, so less raw material is heated
- CIt raises the activation energy, so the reaction is easier to control
- DIt is used up quickly, so less of it has to be heated
Quiz key
Enzymes are biological catalysts: they speed up reactions in cells, for example in digestion and in fermentation by yeast, and are not used up. Hormones are chemical messengers, indicators change colour with pH, and polymers is a general name for long-chain molecules; none of those is defined by speeding up reactions.
A higher concentration means more acid particles dissolved in each unit volume, so they are more crowded and collide more often. A larger volume of the same acid adds particles but spreads them through more water, so the number per cm3 is unchanged. Stirring and warming make particles move about more but do not add any more to each cm3.
Activation energy, Ea, is the minimum energy a collision must have for the particles to react. The energy given out on forming products and the difference between reactant and product energies describe the enthalpy change, ΔH, not the energy barrier. The total energy stored in the reactants is not a threshold at all.
The catalysed route is the dashed curve with the lower peak, and its activation energy is the rise from the reactants level to that lower peak, which is arrow Q. Arrow P reaches the higher, uncatalysed peak, so it is the activation energy without a catalyst. Arrow R runs from the reactants down to the products, so it is the enthalpy change, which the catalyst does not alter. The difference between P and Q is how much the catalyst lowers the barrier, not the barrier itself.
Only collisions with energy greater than or equal to the activation energy lead to reaction; weaker collisions bounce apart. Colliding frequently makes reaction more likely, not less, and a collision with more than the activation energy would react. Whether the overall reaction is exothermic does not decide whether a particular collision succeeds.
At the higher temperature the particles have more kinetic energy, so a much larger fraction of collisions reach the activation energy and are successful; this is the main reason the rate rises sharply. Temperature does not change the activation energy (that needs a catalyst), heating does not add particles to each unit volume, and particles do not grow when heated.
Squeezing the same gas into a smaller volume raises the number of particles per unit volume, so collisions become more frequent and the rate increases. Because the temperature is constant, the particles have the same average kinetic energy, so the energy of the collisions is not what changes. Pressure does not alter the activation energy, and the particles do not slow down.
By providing a pathway with a lower activation energy, a catalyst lets a large proportion of collisions succeed even at a lower temperature, so the plant can run cooler and burn less fuel. A catalyst does not change the yield of an equilibrium reaction, it lowers rather than raises the activation energy, and it is not used up in the reaction.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.