Some reactions give out energy to their surroundings, so the surroundings get hotter: they are exothermic (burning, respiration, hand warmers). Others take in energy, so the surroundings get colder: they are endothermic (instant ice packs, photosynthesis, cooking).
Snap an instant ice pack and it goes freezing cold. Shake a hand warmer and it gets hot. Neither is plugged in. Where does the heat go, and where does it come from?
A temperature change is one sign of a chemical change. This lesson asks which way, and why.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: the ice pack
ExploreTry it on the board and compare what happens with your prediction.
Explore: hotter and colder
The temperature is under the board.
This reaction only goes when it's hot already: it needs energy to keep going.
Try it yourselfNothing is saved: start again as often as you like.
Run the hydrogen and oxygen reaction. Note the temperature before and after.
Use the board. This ticks when you get there.
Now nitrogen and oxygen at 600 K (inside a hot engine). Note the temperature before and after.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: exothermic and endothermic
In your book: draw two thermometers, one for an exothermic and one for an endothermic reaction, with arrows showing which way energy moves between the reaction and its surroundings. Add an example under each.
Exothermic or endothermic?
Wood burning
Respiration in your cellsThat's how you stay at 37 °C.
A hand warmer (iron rusting quickly)
An instant ice pack
Photosynthesis (taking in light energy)It takes in energy from sunlight to make glucose.
Bicarb soda reacting with vinegar and fizzingFeel the cup: it cools a little.
Elaborate TypicalUse the same idea in a new context.
Elaborate: hand warmers and ice packs
Try it yourselfNothing is saved: start again as often as you like.
Run the exothermic reaction again.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: why burning keeps going
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Exothermic reactions give out energy to their surroundings, so the surroundings get hotter: burning, respiration, hand warmers.
Endothermic reactions take in energy from their surroundings, so the surroundings get colder: instant ice packs, photosynthesis, bicarb and vinegar.
Energy is never made or destroyed; it moves between the reaction and its surroundings.
There is no "cold energy": colder means less thermal energy.
For teachers
Students explain exothermic and endothermic reactions as energy released to or absorbed from the surroundings, and classify everyday reactions (AC9S8U07, nodes 4 and 5). About 24 minutes.
I can explain what exothermic and endothermic mean.
I can say which way energy moves in each, between the reaction and its surroundings.
I can give an everyday example of each.
Before class
A concept tutorial for introducing or revising the idea; it is best beside a hands-on test with a thermometer (citric acid and bicarb gets colder; magnesium in acid gets hotter). Set the width with ?width=minimum, typical (default) or wider. One lesson for two nodes, after How can you tell a reaction happened?
Neither is a hinge node. The misconception: cold is a thing that flows ("cold energy"). The prediction targets it.
Board notes: the reaction board warms or cools the gas as reactions give out or take in energy. The endothermic reaction (nitrogen + oxygen → nitrogen monoxide) is real but only happens at engine temperatures; it starts at 600 K and cools. Both reactions stop at their target.
Running the lesson
Engage: tally the predictions. 'Makes cold' is the one to discuss.
Explore: ask which way the temperature moved on each board, and what that says about the energy.
Explain: the definition and the sort are the success criteria.
In your own words: the two thermometers with energy arrows are the evidence.
The hard part (on purpose)
Thinking about the surroundings, not the reaction. When a student says 'the reaction gets cold, so it gives out cold', ask where the energy went and let them redraw the arrow.
When students get stuck
A student says the ice pack gives out cold. Ask: Is cold something you can add, or is it less of something? Fix: Less thermal energy. The reaction takes energy in from your hand, so your hand cools.
A student mixes up exo and endo. Ask: Exit means out. Which one sends energy out? Fix: Exothermic: energy out, surroundings hotter.
A student thinks photosynthesis is exothermic because the Sun is hot. Ask: Does the plant take light energy in or give it out? Fix: It takes it in to make glucose: endothermic.
Stretch and support
Stretch: Respiration and photosynthesis are almost the reverse of each other. Why is one exothermic and the other endothermic? (Photosynthesis stores light energy in glucose; respiration releases it.)
Support: Give the frame: 'In an … reaction, energy moves from … to … , so the surroundings get … .'
Where it leads
Next: comparing physical and chemical changes with every criterion (node 6) is a class task. Year 9 energy conservation and Year 10 collision theory build on this.
In class: thermometer readings for citric acid + bicarb (endothermic) and magnesium + dilute acid (exothermic).
What each step is for
Engage: the ice pack: Students often think cold is something added ('cold energy'); the endothermic idea needs energy taken in from the surroundings.
Explore: hotter and colder: Watching the temperature rise in one reaction and fall in another, with the same kind of particle model, makes the direction of energy transfer the difference.
Explain: exothermic and endothermic: Naming the two directions of energy transfer and sorting everyday reactions by them is the node; 'the surroundings' is the word that makes it precise.
Elaborate: hand warmers and ice packs: Matching a reaction's energy direction to a product's job turns the idea into design reasoning.
Elaborate: why burning keeps going: Activation energy explains why exothermic reactions need a start (a match) and then keep themselves going.
Now you can
Explain what exothermic and endothermic mean.
Say which way energy moves in each, between the reaction and its surroundings.