Lesson overview
Energy changes in reactions: Where does the heat come from?
Run three reactions in a polystyrene cup with one thermometer: magnesium in acid, acid with alkali, and citric acid with sodium hydrogencarbonate. Watch your temperature graph rise or fall, decide which reactions give out thermal energy and which take it in, then read the energy changes from reaction pathway diagrams.
Suggested sequence
- Lesson 1 · Hotter or colder? Exothermic and endothermic reactions and the temperature of the surroundings. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Reaction pathway diagrams. Reaction pathway diagrams; Supplement: ΔH, activation energy, drawing the diagrams. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 5 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. 3 questions, 15 min, lesson closed.
Learning goals
Part 1: Hotter or colder?
- State that an exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases, and that an endothermic reaction takes in thermal energy from the surroundings, so their temperature decreases (5.1.1, 5.1.2)
Part 2: Reaction pathway diagrams
- Interpret reaction pathway diagrams showing exothermic and endothermic reactions (5.1.3)
- Supplement: state that the transfer of thermal energy during a reaction is the enthalpy change, ΔH, negative for exothermic and positive for endothermic reactions (5.1.4)
- Supplement: define activation energy, Ea, as the minimum energy that colliding particles must have to react, and draw and label reaction pathway diagrams with reactants, products, ΔH and Ea (5.1.5, 5.1.6)
Misconceptions to listen for
- “It takes thermal energy from the solution, so it is endothermic.” — If the reaction took thermal energy from the solution, the solution would get colder. It got warmer.
- “The solution gives thermal energy to the reaction, so it is exothermic.” — Exothermic means thermal energy goes out of the reaction. The solution gained energy, so the energy came from the reaction, not from the solution.
- “It gives thermal energy to the solution, so it is endothermic.” — Endo means "in". A reaction that gives thermal energy to its surroundings is exothermic: exo means "out".
- “Fizzing shows that a reaction is exothermic.” — Bubbles show that a gas is made. Citric acid and sodium hydrogencarbonate fizz and get colder. Only the temperature change shows which way the energy goes.
- “A colour change shows that a reaction gives out energy.” — A colour change shows that a new substance has formed. In record 4 the temperature falls, so that reaction takes energy in.
- “An endothermic reaction gives out cold.” — Cold does not flow. Thermal energy flows from the surroundings into the reacting chemicals, so the surroundings are left with less energy and get colder.
- “The potassium hydrogencarbonate is part of the surroundings.” — It is reacting, so it is part of the reaction. The surroundings are everything that is not reacting.
- “Every result goes into the mean.” — An anomalous result does not fit the pattern of the others, so leave it out. With 8.5 °C in, the mean is 11.0 °C, lower than the true rise.
- “The temperature rise is the highest temperature.” — The rise is the highest temperature minus the start temperature.
- “Row A.” — If thermal energy moved into the reacting particles, the solution and the hand would lose energy and get colder. In an exothermic reaction energy moves out.
Scope
Energy changes in reactions (5.1 Core 1–3, Supplement 4–6): exothermic and endothermic reactions and the temperature of the surroundings, reaction pathway diagrams, the enthalpy change ΔH and its sign, activation energy, and drawing and labelling pathway diagrams. Bond breaking, bond making and bond energies are in the next module; collisions and catalysts in topic 6.