The shape of the lesson
By the end of the lesson, learners can
- Reading the diagram: Switch between exothermic and endothermic reactions in the simulator.
- Labelling ΔH and E a: Extended.
- Products lower: exothermic. Products higher: endothermic.
- Ea: reactants up to the top of the hill.
- ΔH: reactants to products; down is negative, up is positive.
- Catalyst: lower hill, same ΔH.
One picture every idea returns to
On a reaction pathway diagram, if the products are lower in energy than the reactants the reaction is exothermic; if the products are higher it is endothermic. Extended: the enthalpy change ΔH is the difference in energy between reactants and products. The activation energy, Ea, is the minimum energy that colliding particles need in order to react; it is the height of the hill from the reactants to the top.
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, Energy, drawn as a hill.. A reaction pathway diagram shows the energy of the reacting chemicals as the reaction happens. 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: Reading the diagram | Switch between exothermic and endothermic reactions in the simulator. Project tab 1 of the lab (Chemical energetics lab, enthalpy profiles) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Labelling ΔH and E a | Extended. | Jigsaw |
| 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 Labelling ΔH and E a, 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.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
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 |
|---|---|
| The activation energy arrow starts at the reactants, not at the bottom axis and not at the products. | And ΔH is measured between the reactant and product levels, not from the top of the hill. Arrows drawn from the axis lose the mark. |
Reaction pathway diagrams: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Describe how a reaction pathway diagram shows whether a reaction is exothermic or endothermic.
(Extended) Define activation energy.
(Extended) Sketch and label a reaction pathway diagram for the combustion of methane, CH4 + 2O2 → CO2 + 2H2O, which is exothermic.
(Extended) On a reaction pathway diagram the reactants are at 120 kJ, the highest point is at 300 kJ and the products are at 190 kJ. Calculate Ea and ΔH, and state whether the reaction is exothermic or endothermic.
(Extended) State and explain the effect of a catalyst on the activation energy and on the enthalpy change of a reaction.
Explain why a mixture of hydrogen and oxygen does not react until a lighted splint is applied, although the reaction is very exothermic.
Answer key and marking notes
- If the products are at a lower energy than the reactants, the reaction is exothermic.
- If the products are at a higher energy than the reactants, the reaction is endothermic.
The minimum energy that colliding particles must have in order to react.
- Energy on the vertical axis, progress of reaction on the horizontal axis.
- Reactants CH4 + 2O2 at a higher level; products CO2 + 2H2O at a lower level, joined by a curve with a hill.
- Ea: an upward arrow from the reactants level to the top of the hill.
- ΔH: a downward arrow from the reactants level to the products level, labelled as negative.
- Ea = 300 − 120 = 180 kJ.
- ΔH = 190 − 120 = +70 kJ.
- The products are higher than the reactants, so the reaction is endothermic.
- The activation energy is lowered, because the catalyst provides an alternative reaction pathway.
- The enthalpy change is unchanged, because the energies of the reactants and products are not affected.
- The reaction has an activation energy; at room temperature very few collisions have enough energy to react.
- The flame supplies the energy needed; the energy released by the reaction then provides the activation energy for further reaction.
10 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 is shown on the horizontal axis of a reaction pathway diagram?
- Athe progress of the reaction
- Btime in seconds
- Cthe temperature of the mixture
- Dthe mass of the products
Calcium carbonate decomposes only when it is heated strongly and continuously. How would its reaction pathway diagram look?
- Athe products are drawn at a higher energy than the reactants
- Bthe products are drawn at a lower energy than the reactants
- Cthe products and reactants are drawn at the same energy
- Dthe line goes straight down with no hump
On a reaction pathway diagram, the reactants are at 50 kJ and the products are at 170 kJ. What is the overall energy change of the reaction?
- A120 kJ taken in
- B120 kJ given out
- C220 kJ taken in
- D170 kJ taken in
Ethanol burns in oxygen to form carbon dioxide and water. Which substances are shown at the lower energy level on its reaction pathway diagram?
- Acarbon dioxide and water
- Bethanol and oxygen
- Cethanol only
- Doxygen and water
The diagrams show the reaction pathways for two processes, P and Q. Which diagram represents ammonium nitrate dissolving in water, which makes the water colder, and why?
- AQ, because its products are higher in energy than its reactants
- BP, because its products are lower in energy than its reactants
- CQ, because its hump is the same height as in P
- DP, because its reactants start at a higher energy
On a reaction pathway diagram, what does the vertical distance between the reactants level and the products level represent?
- Athe overall energy change of the reaction
- Bthe activation energy
- Cthe rate of the reaction
- Dthe temperature of the surroundings
On a reaction pathway diagram, the products are drawn 90 kJ below the reactants. What happens to the temperature of the surroundings during the reaction?
- AIt rises, because 90 kJ of energy is given out
- BIt falls, because 90 kJ of energy is taken in
- CIt falls, because the products have less energy
- DIt does not change, because energy cannot be lost
Two fuels X and Y both burn exothermically. Burning one mole of X releases 50 kJ and burning one mole of Y releases 300 kJ. How do their reaction pathway diagrams differ?
- AThe products of Y are drawn much further below its reactants than those of X
- BThe products of Y are drawn above its reactants, but those of X are below
- CThe hump for Y is taller than the hump for X
- DThe two diagrams are identical, because both reactions are exothermic
Use the reaction pathway diagram to find the activation energy, Ea, and the enthalpy change, ΔH, of the reaction.
- AEa = 150 kJ, ΔH = +70 kJ
- BEa = 190 kJ, ΔH = +70 kJ
- CEa = 80 kJ, ΔH = +70 kJ
- DEa = 150 kJ, ΔH = −70 kJ
The reaction pathway diagram for an exothermic reaction has four arrows labelled A, B, C and D. Which arrow represents the enthalpy change, ΔH, of the reaction?
- AC
- BA
- CB
- DD
Quiz key
The horizontal axis shows the progress of the reaction (sometimes called the reaction pathway), from reactants on the left to products on the right. It has no scale of time, so it cannot be read as seconds. Temperature and mass are not plotted on this diagram; the vertical axis shows energy.
Thermal decomposition needs a continuous energy input, so it is endothermic. The energy taken in is stored in the products, so they are drawn higher than the reactants. Products drawn lower would show an exothermic reaction, and the same level would mean no overall energy change. Every reaction pathway rises to a hump before reaching the products, so a straight line down is never correct.
Difference = 170 − 50 = 120 kJ. The products are higher than the reactants, so the chemicals have gained 120 kJ from the surroundings: energy is taken in and the reaction is endothermic. 'Given out' would need the products to be lower. 220 kJ adds the levels instead of subtracting, and 170 kJ is just the energy of the products.
Combustion is exothermic, so the products are lower in energy than the reactants: carbon dioxide and water are at the lower level. Ethanol and oxygen are the reactants, drawn at the higher level. The levels always show complete sides of the equation, so 'ethanol only' and 'oxygen and water' mix up reactants and products.
The water cools, so dissolving ammonium nitrate is endothermic: the chemicals take in energy and the products end up higher than the reactants, which is diagram Q. Diagram P has products below reactants, so it shows an exothermic process. The height of the hump does not decide exothermic or endothermic, and what matters is the change from reactants to products, not how high the reactants start.
The difference in height between reactants and products shows how much energy is given out or taken in overall: the energy change of the reaction. The activation energy is measured from the reactants up to the top of the hump, not to the products. The diagram does not show rate or temperature directly.
The products have 90 kJ less energy than the reactants, so 90 kJ has been transferred to the surroundings, which become warmer: the reaction is exothermic. Energy taken in would put the products above the reactants. Products having less energy is exactly why the surroundings warm, not cool. Energy is conserved, but it moves from the chemicals to the surroundings, which is why the temperature changes.
The overall energy released is shown by how far the products are below the reactants. Y releases six times as much energy, so its drop is much larger. Both are exothermic, so both have products below reactants. The height of the hump relates to the activation energy, which the stem gives no information about. Being exothermic tells you the direction of the change, not its size.
Ea = peak − reactants = 190 − 40 = 150 kJ. ΔH = products − reactants = 110 − 40 = +70 kJ; it is positive because the products are higher, so the reaction is endothermic. 190 kJ reads the peak value from the axis instead of measuring from the reactants; 80 kJ (190 − 110) measures from the products, which is the activation energy of the reverse reaction; a negative ΔH would need the products to be below the reactants.
ΔH is the energy difference between the reactants and the products, shown by arrow C, which starts at the reactants level and points down to the products level (downwards because energy is released, so ΔH is negative). Arrow A, from the reactants to the top of the hump, is the activation energy. Arrow B, from the products to the peak, is the activation energy of the reverse reaction. Arrow D only shows the energy of the products measured from the axis.
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.