The shape of the lesson
By the end of the lesson, learners can
- Measure the temperature change: Run the neutralisation in the calorimeter and watch the thermometer rise.
- Everyday examples: Learn a few examples of each type.
- The enthalpy change, ΔH: Extended.
- Exothermic: energy to the surroundings; temperature rises.
- Endothermic: energy from the surroundings; temperature falls.
- Extended: ΔH negative for exothermic, positive for endothermic.
- Reverse the reaction, reverse the sign of ΔH.
One picture every idea returns to
An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases. An endothermic reaction takes in thermal energy from the surroundings, so the temperature of the surroundings decreases. Extended: the transfer of thermal energy during a reaction is the enthalpy change, ΔH. ΔH is negative for exothermic reactions and positive for endothermic reactions.
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, Hotter or colder?. Every chemical reaction moves thermal energy. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Measure the temperature change | Run the neutralisation in the calorimeter and watch the thermometer rise. Project tab 2 of the lab (Chemical energetics lab, calorimetry) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Everyday examples | Learn a few examples of each type. | Pairs |
| 21 to 30 min | Teach and check: The enthalpy change, ΔH | Extended. | Quiz-Quiz-Trade |
| 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, 15 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 The enthalpy change, ΔH, 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.
Quiz-Quiz-Trade
Each student holds a card with a question on the front and the answer on the back. They pair up, quiz each other, coach if needed, trade cards and find a new partner. Three rounds covers the whole set.
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 |
|---|---|
| In an endothermic reaction the chemicals gain energy, but the thermometer shows a fall, because it measures the surroundings, which have lost energy. | Do not say an endothermic reaction "gives out cold": cold is not a form of energy. |
Exothermic and endothermic reactions: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by an exothermic reaction and an endothermic reaction.
A student mixes pairs of solutions and records the temperatures.
| Mixture | Start / °C | End / °C |
|---|---|---|
| A | 21 | 34 |
| B | 21 | 16 |
| C | 20 | 27 |
| D | 22 | 22 |
Calculate the temperature change for each and classify each as exothermic, endothermic or no detectable energy change.
Give one example of an exothermic reaction and one example of an endothermic reaction.
Describe how a student could show that the reaction between hydrochloric acid and sodium hydroxide solution is exothermic.
(Extended) Define enthalpy change, ΔH, and state its sign for an endothermic reaction.
(Extended) Hydrogen burns in oxygen with ΔH = −286 kJ/mol of water formed. State the value of ΔH for the decomposition of water into hydrogen and oxygen, per mole of water, and explain your answer.
Answer key and marking notes
- Exothermic: a reaction that transfers thermal energy to the surroundings, so the temperature of the surroundings increases.
- Endothermic: a reaction that takes in thermal energy from the surroundings, so the temperature of the surroundings decreases.
- A: +13 °C, exothermic.
- B: −5 °C, endothermic.
- C: +7 °C, exothermic.
- D: 0 °C, no detectable energy change.
- Exothermic: for example combustion of a fuel, neutralisation, or a metal reacting with an acid.
- Endothermic: for example thermal decomposition of calcium carbonate, photosynthesis, or dissolving ammonium nitrate.
- Measure a known volume of acid into a polystyrene cup and record its starting temperature with a thermometer.
- Add the sodium hydroxide solution, stir, and record the highest temperature reached.
- A rise in temperature shows that the reaction is exothermic.
- The transfer of thermal energy during a reaction.
- ΔH is positive for an endothermic reaction.
- +286 kJ/mol.
- The decomposition is the reverse reaction, so the energy change has the same size but the opposite sign.
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.
Which of these everyday products depends on an endothermic process?
- Aan instant cold pack for a sports injury
- Ba self-heating can of coffee
- Ca reusable hand warmer
- Da camping gas stove
Zinc powder is added to copper(II) sulfate solution. The temperature changes from 18.5 °C to 26.0 °C. What is the temperature change and what type of reaction is this?
- A+7.5 °C; exothermic
- B−7.5 °C; exothermic
- C+7.5 °C; endothermic
- D+44.5 °C; exothermic
When citric acid solution is mixed with sodium hydrogencarbonate in a beaker, the beaker feels cold to the touch. Which explanation is correct?
- AThe reaction takes in thermal energy from its surroundings, including your hand
- BThe reaction releases cold into the surroundings
- CThe products have less energy than the reactants
- DThe reaction transfers thermal energy to the surroundings
Which list contains only exothermic processes?
- Acombustion, neutralisation, respiration
- Bcombustion, photosynthesis, respiration
- Cneutralisation, thermal decomposition, combustion
- Drespiration, dissolving ammonium nitrate, neutralisation
The graph shows how the temperature changed during two reactions, A and B, that both started at room temperature. What was the largest temperature change in reaction B, and what type of reaction is B?
- Aa fall of 5 °C; endothermic
- Ba fall of 15 °C; endothermic
- Ca fall of 5 °C; exothermic
- Da rise of 10 °C; exothermic
A student compares the temperature rise when different metals react with dilute hydrochloric acid. Which of these must be kept the same to make it a fair test?
- Athe volume and concentration of the acid
- Bthe temperature rise
- Cthe highest temperature reached
- Dthe type of metal
A student measures the temperature rise when an acid neutralises an alkali, first in a polystyrene cup and then in a thin glass beaker. How does the temperature rise measured in the glass beaker compare, and why?
- AIt is smaller, because more thermal energy is lost to the surroundings through the glass
- BIt is larger, because glass conducts heat into the mixture
- CIt is the same, because the same reaction takes place
- DIt is smaller, because less energy is released by the reaction in glass
Which observation shows that a reaction is endothermic?
- AA thermometer in the reaction mixture shows a fall in temperature
- BBubbles of gas are given off
- CThe mixture gives out light
- DThe colour of the solution changes
A disposable hand warmer contains iron powder that reacts with oxygen from the air when the packet is opened. Which statement is correct?
- AThe reaction is exothermic: it transfers thermal energy to the surroundings
- BThe reaction is endothermic, because it needs oxygen from the air
- CThe reaction is exothermic, because it takes in energy from the air
- DThe reaction is endothermic, because the packet gets warm
When sodium hydrogencarbonate reacts with ethanoic acid, the temperature of the mixture falls. Which statement about ΔH for this reaction is correct?
- AΔH is positive, because the reacting chemicals gain energy from the surroundings
- BΔH is negative, because the temperature falls
- CΔH is positive, because the reacting chemicals lose energy to the surroundings
- DΔH is zero, because no heat is added
Quiz key
An instant cold pack works because the process inside it takes in thermal energy from the surroundings (including the injured skin), so the temperature falls: it is endothermic. A self-heating can and a hand warmer rely on reactions that release thermal energy, and a gas stove burns a fuel; combustion is exothermic.
Change = 26.0 − 18.5 = +7.5 °C, a rise. The surroundings (the solution) got warmer, so thermal energy was transferred to them: the reaction is exothermic. −7.5 °C subtracts the wrong way round; an endothermic reaction would make the temperature fall; 44.5 °C adds the two temperatures instead of finding the difference.
Your hand feels cold because thermal energy flows out of it into the beaker. The reaction is endothermic: it takes in energy from the surroundings, so their temperature falls. 'Cold' is not a form of energy that can be released. If the products had less energy than the reactants, energy would have been released and the beaker would feel warm. Transferring energy to the surroundings describes an exothermic reaction.
Combustion, neutralisation and respiration all release thermal energy to the surroundings, so they are exothermic. Photosynthesis needs a continuous supply of light energy, thermal decomposition needs continuous heating, and dissolving ammonium nitrate makes the water colder: all three are endothermic, so each of the other lists contains one endothermic process.
Line B starts at 20 °C and falls to a lowest value of 15 °C: a fall of 20 − 15 = 5 °C. The surroundings got colder, so B took in thermal energy: it is endothermic. The later slow rise is just the mixture warming back up from the room, not part of the reaction. 15 °C is the lowest reading, not the change; a fall in temperature cannot mean exothermic; a rise of 10 °C describes reaction A, which is exothermic.
The metal is the variable being changed, and the temperature rise (worked out from the highest temperature) is what is being measured, so neither can be kept the same. For the comparison to be fair, the same volume and concentration of acid must be used each time (along with the same mass of metal and the same insulated container).
The reaction releases the same amount of energy in both containers. Glass is a much better conductor than polystyrene, so more of that energy escapes to the surroundings instead of warming the mixture, and the measured rise is smaller. The glass does not add energy to the mixture, and the container does not change how much energy the reaction releases; it only changes how much is lost.
An endothermic reaction takes in thermal energy from its surroundings, so a thermometer in the mixture shows the temperature falling. Bubbles and colour changes show that a chemical reaction is happening but say nothing about energy direction. Giving out light means energy is being released, which is a sign of an exothermic reaction.
The packet warms your hands, so thermal energy is transferred from the reaction to the surroundings: the rusting reaction is exothermic. Needing a reactant from the air has nothing to do with the direction of energy transfer. Taking in energy would describe an endothermic reaction, and a packet that gets warm is a sign of an exothermic reaction, not an endothermic one.
The surroundings cool because energy has flowed from them into the reacting chemicals. The chemicals gain energy, so ΔH is positive: the reaction is endothermic. A negative ΔH would mean the chemicals lost energy and the mixture warmed up. Losing energy to the surroundings is also the exothermic case. ΔH is not zero, because energy has clearly been transferred.
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.