Energy changes in reactions · Process
Exothermic reactions
Every fire you have ever seen gave out far more energy than the match that started it. Nobody keeps feeding it energy — so where does the heat come from?
Start here
A hand warmer is a sealed plastic pouch. Snap the metal disc inside and it reaches 50 °C, in a pocket, on a cold day.
No battery, no flame, nothing plugged in, and nothing added from outside. The pouch was sitting at the same temperature as the room a moment before. It stays hot for about an hour, then goes cold — and it can be reset by boiling it.
Where does the heat come from?
From energy that was already stored in the chemicals. Snapping the disc starts a change, and as the contents rearrange they release energy that was locked in the arrangement they had before. Nothing is created — the energy was there all along, stored where a thermometer could not see it. A change that gives energy out to its surroundings is called exothermic.
An exothermic change transfers energy to its surroundings. The surroundings get warmer — which is why the test for one is a thermometer in the beaker, not in the flame.
Most reactions you have met are exothermic. Combustion, neutralisation, respiration, a metal reacting with acid, and every reaction in the displacement grid gave out energy. It is common enough that people forget it needs explaining at all.
Your turn · five beakers, one thermometer
Predict the temperature change, then run it.
0 of 5 run
Four of these five do the same thing. Run all five — the odd one out is the point. The temperatures are typical classroom values chosen to show the pattern, not readings from one particular afternoon.
Burning magnesium
The set-upA coil of magnesium ribbon lowered into a beaker of water, lit at the top with a Bunsen. Thermometer in the water.
Predict before you run it.
Start
21 °C
Highest reading
60 °C
Combustion, and one of the fiercest exothermic reactions in a school lab. It needed a flame to start — and then gave out enough energy to be dangerous to look at directly.
Acid + alkali
The set-up25 cm³ of hydrochloric acid with 25 cm³ of sodium hydroxide stirred in. Thermometer in the mixture.
Predict before you run it.
Start
20 °C
Highest reading
27 °C
Neutralisation is exothermic, and you noticed this without being told: the beaker warmed while you were titrating in the acids unit. A rise of seven degrees from two clear liquids that look unchanged afterwards.
Magnesium + acid
The set-upA strip of magnesium dropped into 25 cm³ of dilute hydrochloric acid. Thermometer in the acid.
Predict before you run it.
Start
20 °C
Highest reading
34 °C
A metal reacting with an acid is strongly exothermic. This is the reaction that produced the squeaky pop, and the warm tube was evidence you had already collected without naming it.
Hand warmer
The set-upA commercial reusable hand warmer, snapped and placed against the thermometer bulb.
Predict before you run it.
Start
21 °C
Highest reading
50 °C
Sodium ethanoate crystallising out of a supersaturated solution. Not strictly a chemical reaction — it is a change of state — but the energy accounting is identical: the particles fall together and release what was stored.
Citric acid + baking soda
The set-upCitric acid solution with sodium hydrogencarbonate stirred in. Thermometer in the mixture.
Predict before you run it.
Start
20 °C
Highest reading
12 °C
The temperature fell by eight degrees. Energy went from the surroundings into the reaction rather than out of it — the opposite of every other beaker on this bench, and the subject of the next lesson.
Four went up. One went down.
Combustion, neutralisation, metal with acid and the hand warmer all warmed their surroundings — they are exothermic, and between them they cover most of the chemistry you have done so far. The fifth got colder, which means energy went the other way. That one has its own name and its own lesson.
Note what the thermometer is actually in. It is in the mixture, and it is reading the surroundings getting warmer. Exothermic is defined by what happens outside the reaction, not inside it.
Key fact
An exothermic change gives energy out to its surroundings, so the temperature of the mixture rises. The energy was stored in the chemicals before the reaction started.
Three judgements
Exothermic on purpose, and exothermic by accident
0 of 3 decided
A disposable hand warmer contains iron powder, salt and sawdust, and gets warm when the packet is opened to the air. What is the reaction?
The iron oxidising — rusting, deliberately made fast. Salt speeds it up and the sawdust holds everything in contact with air. Rusting is exothermic like any oxidation; normally it is so slow that the energy escapes unnoticed, and the trick here is simply making it fast enough to feel.
Why do power stations, cars and your own body all rely on exothermic reactions?
Because getting energy out is the entire point. A power station burns fuel, a car burns petrol, and your cells respire glucose — three very different settings running the same accounting. An endothermic reaction would take energy in, which is useful for cooling but no use for driving anything.
A student says fireworks prove chemical reactions create energy. Are they right?
No, and this is the important sentence in the lesson. Energy cannot be created. It was stored in the chemicals before the firework was lit, and the reaction released it as light, heat and sound. The firework is a container for energy that was put there when it was manufactured — the same accounting as a battery.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“A reaction that needs heating to start cannot be exothermic.”
A Bunsen really is needed to light the magnesium. Commit before you read on.
Starting a reaction and running it are two different accounts. Petrol needs a spark; a match needs striking; magnesium needs a flame. That initial energy gets the reaction going — and once it is going, it gives out far more than the spark ever supplied, which is why the flame keeps burning after you take the match away.
The test is the balance, not the beginning. Exothermic means more energy comes out than went in. If the spark were the whole story, a car would need to be lit continuously — and a forest fire would go out the moment the lightning stopped.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What happens to the temperature of the surroundings during an exothermic change?
Rung 2 · The one that catches people
Burning methane needs a spark to start. Does that make it endothermic?
Rung 3 · Explain
A student mixes an acid and an alkali and the thermometer rises by 7 °C. Explain what this shows, where the energy came from, and why the beaker cools down again after a few minutes.
Rung 4 · Take it somewhere new
A company wants to sell a self-heating food can. Explain what they need from the chemistry, and give two safety problems they would have to solve.
Key note
An exothermic change transfers energy to the surroundings, so the temperature of the mixture rises. Combustion, neutralisation, respiration and metals reacting with acids are all exothermic. Needing energy to get started does not stop a reaction being exothermic — what matters is that more energy comes out than went in.
Going further
Self-heating cans of coffee use the reaction between calcium oxide and water, which is exothermic enough to bring a drink to serving temperature in three minutes from a chemical reaction in a sealed compartment. The same reaction is a genuine hazard on building sites: quicklime dust in a wet eye releases its energy exactly where you would least want it.
Compost heaps are exothermic on a scale that surprises people. Bacteria respiring their way through grass cuttings release enough energy to hold the middle of a heap at 60 °C through a frost, and a large enough badly-managed pile of hay can reach the point of catching fire on its own. Farmers have lost barns to it. The energy is coming out of the chemical store in the plant material, and it makes no difference to the arithmetic that the reaction is being run by bacteria rather than a Bunsen.
Before this lesson
Next in this unit
At GCSE this becomes
- Energy level diagrams, activation energy, and bond breaking and making as the reason a reaction gives energy out.
Where to next
Ask Mr Badmus AI
Still not sure why a match does not disprove this?
These are demonstrations. Burning magnesium is blinding to look at directly and is watched through the eye protection everyone in the room is already wearing. A reusable hand warmer is reset by boiling it in a pan of water on a hob, never in a microwave and never by an unsupervised student.
Lesson content © MrBadmusAI.