The shape of the lesson
By the end of the lesson, learners can
- Measuring a rate: Run the magnesium and acid experiment in the simulator.
- Five factors that change the rate: Change one factor at a time in the simulator, keeping everything else the same, and compare the curves.
- Faster: higher concentration, pressure, surface area, temperature, or a catalyst.
- Mean rate = change in amount ÷ time taken.
- Same reactant amounts means the same final volume.
- Fair test: change one factor, keep the rest the same.
One picture every idea returns to
The rate of a reaction increases when the concentration of a solution or the pressure of a gas increases, when a solid is broken into smaller pieces (larger surface area), when the temperature increases, and when a catalyst is added. A catalyst increases the rate of a reaction and is unchanged at the end. Rate is measured by following a product or reactant against time, for example the volume of gas given off or the loss in mass.
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, How fast does it go?. Some reactions are over in a flash, others take years. 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: Measuring a rate | Run the magnesium and acid experiment in the simulator. Project tab 3 of the lab (Reaction kinetics lab, measuring a rate) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Five factors that change the rate | Change one factor at a time in the simulator, keeping everything else the same, and compare the curves. Project tab 5 of the lab (Reaction kinetics lab, rate factors) and let a student drive it while the class predicts. | Cooperative Concept Mapping |
| 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, 16 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 Five factors that change the rate, 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.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
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 |
|---|---|
| Changing the rate does not change the amount of product if the same amounts of reactants are used. | On the graph, a faster reaction is steeper at the start but levels off at the same final volume. Only changing the amount of the limiting reactant changes the final volume. |
Rate of reaction: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how you could measure the rate of the reaction between magnesium ribbon and dilute hydrochloric acid.
In a reaction, the volume of gas collected was 0, 24, 40, 50, 55, 56 and 56 cm3 at 0, 10, 20, 30, 40, 50 and 60 s. (a) Calculate the mean rate in the first 20 s. (b) State when the reaction finished and explain how you know.
Explain why the rate of a reaction is fastest at the start and then slows down.
Marble chips react with excess dilute hydrochloric acid. Sketch, on the same axes, the graph of volume of carbon dioxide against time for (a) large chips and (b) the same mass of powdered marble. Explain the differences.
State two ways, other than using a catalyst, of increasing the rate of reaction between zinc and dilute sulfuric acid.
Describe how you would show that copper(II) oxide is a catalyst for the decomposition of hydrogen peroxide.
Answer key and marking notes
- Add the magnesium to the acid in a conical flask connected to a gas syringe.
- Start a stopwatch and record the volume of hydrogen at regular intervals, for example every 10 s.
- Plot volume of gas against time; the gradient shows the rate.
- (a) 40 ÷ 20 = 2 cm3/s.
- (b) At about 50 s,
- because the volume stops increasing (it stays at 56 cm3).
- At the start the concentration (or amount) of reactants is highest.
- As reactants are used up their concentration falls, so the reaction slows and finally stops.
- The powder curve is steeper at the start: faster reaction.
- Both curves level off at the same final volume.
- The powder has a larger surface area, but the mass of marble (the limiting reactant) is the same, so the same amount of gas forms.
- Increase the concentration of the acid.
- Increase the temperature, or use zinc powder instead of larger pieces.
- Compare the rate of oxygen production with and without a weighed mass of copper(II) oxide: it is faster with the oxide.
- At the end, filter off the copper(II) oxide, wash and dry it and weigh it again.
- The same mass is recovered, showing it is unchanged, so it is a catalyst.
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.
Curve X shows the volume of carbon dioxide given off when 2.0 g of marble lumps react with excess dilute hydrochloric acid at 20 °C. Which change to the experiment would give curve Y?
- Ausing 2.0 g of powdered marble instead of lumps
- Busing 4.0 g of marble lumps
- Cusing more dilute hydrochloric acid
- Dcarrying out the reaction at 10 °C
A flask containing marble chips and acid is placed on a balance. In the first 2 minutes the mass falls by 0.88 g as carbon dioxide escapes. What is the mean rate of reaction over this time?
- A0.44 g/min
- B1.76 g/min
- C2.27 g/min
- D0.22 g/min
The volume of hydrogen in a gas syringe is 18 cm3 after 10 s and 30 cm3 after 20 s. What is the mean rate of reaction between 10 s and 20 s?
- A1.2 cm3/s
- B1.5 cm3/s
- C3.0 cm3/s
- D1.8 cm3/s
Sodium thiosulfate solution reacts with hydrochloric acid to form a cloudy precipitate. The time for a cross under the flask to disappear is 80 s at 20 °C, 40 s at 30 °C and 20 s at 40 °C. What conclusion can be drawn?
- AThe rate increases as the temperature increases
- BThe rate decreases as the temperature increases
- CThe temperature has no effect on the rate
- DMore precipitate forms at 20 °C than at 40 °C
A student investigates how the concentration of hydrochloric acid affects the rate of its reaction with marble chips. Which pair of variables must be kept the same?
- Athe temperature, and the mass and size of the marble chips
- Bthe concentration of the acid and the temperature
- Cthe volume of gas produced and the time taken
- Dthe mass of the chips and the concentration of the acid
A 0.05 g piece of magnesium ribbon reacts completely with 50 cm3 of 1 mol/dm3 hydrochloric acid, which is in large excess. The experiment is repeated with 100 cm3 of the same acid. What happens?
- AThe rate and the final volume of hydrogen stay about the same
- BThe rate doubles and the final volume stays the same
- CThe rate stays the same and the final volume doubles
- DThe rate and the final volume both double
On a graph of the volume of gas produced against time, what shows the rate of reaction at a particular moment?
- Athe gradient (steepness) of the curve at that time
- Bthe height of the curve at that time
- Cthe final volume reached
- Dthe time at which the curve starts
Coal dust in a mine can explode, but a lump of coal burns steadily. Which factor explains the difference?
- Athe much larger surface area of the dust
- Bthe higher temperature of the dust
- Ca catalyst present in the dust
- Dthe greater mass of coal in the dust
Quiz key
Curve Y is steeper at the start but levels off at the same final volume, so the reaction is faster but the amount of marble (the limiting reactant) is unchanged. Powdering the same 2.0 g of marble increases the surface area, which does exactly that. Using 4.0 g of marble would double the final volume. More dilute acid and a lower temperature would both make the curve less steep, not steeper.
Mean rate = 0.88 g ÷ 2 min = 0.44 g/min. Multiplying 0.88 by 2 gives 1.76, dividing 2 by 0.88 gives 2.27 (the inverse), and 0.22 comes from dividing by 4 instead of 2; none of those is the mass lost per minute.
Between 10 s and 20 s the volume rises by 30 − 18 = 12 cm3 in 10 s, so the mean rate is 12 ÷ 10 = 1.2 cm3/s. 1.5 cm3/s is 30 ÷ 20, the mean rate from the start. 3.0 cm3/s divides the total volume by the length of the interval, and 1.8 cm3/s is the mean rate over the first 10 s only.
The time gets shorter as the temperature rises, and a shorter time means a faster reaction, so the rate increases with temperature. Reading a longer time as a faster rate gives the opposite conclusion. The times clearly change, so temperature does have an effect. Each run stops at the same cloudiness (the cross just disappearing), so the results give no evidence that more precipitate forms at 20 °C.
Concentration is the variable being changed, so it cannot be kept the same; any option that controls it is wrong. The volume of gas and the time are what is measured. Temperature and the mass and size (surface area) of the marble also affect the rate, so they must be kept constant for a fair test.
The acid concentration is unchanged, so the acid particles are no more crowded and the rate stays about the same. Magnesium is the limiting reactant and its mass is the same, so the same volume of hydrogen forms. Doubling the volume of acid is not the same as doubling its concentration, which is the misconception behind the options where the rate doubles; the final volume could only increase if more magnesium were used.
Rate is the change in volume per unit time, which is the gradient of the curve: a steeper curve means a faster reaction. The height of the curve is the total volume collected so far, and the final volume depends on the amount of limiting reactant, so neither measures speed. Every curve starts at time zero, so that tells you nothing about the rate.
Fine dust has a huge surface area, so much more coal is in contact with oxygen at once and the reaction can be so fast that it becomes explosive. The dust is not hotter and contains no catalyst, and the lump may well have the greater mass; it is the size of the pieces, not the amount of coal, that changes the rate.
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.