KS3 · Chemistry

Exothermic and endothermic reactions

An ice pack isn't full of cold — it's full of a reaction pulling energy out of your injury.

Chemistry · Reading the thermometer

Up or down? Follow the energy

You've done the cup test and have your two readings. Pick what the thermometer did, then follow the energy to the verdict.

What the thermometer did → Which way the energy moved → The verdict

2 possible routes.

On The cup test is done. 2 branches to choose from.

Here's the big idea: the thermometer isn't measuring the reaction. It's sitting in the solution around the reacting chemicals — the surroundings. So the question to ask is simple: where did the energy go?

Chemistry · What's really going on?

Does a cold reaction give out cold?

Citric acid is added to sodium hydrogen carbonate solution in a polystyrene cup. The thermometer reading drops.

Which is closest to what you think is happening right now?

The reaction is giving out cold, and the cold is spreading through the solution.The thermometer went down, so the reacting chemicals must have lost energy.Energy is moving from the solution into the products, so the solution gets colder.If the solution gets colder, the whole classroom will get colder too.

How sure are you?

I'm sureI think soI'm guessing

The reaction is giving out cold, and the cold is spreading through the solution.

This is a common misconception

It really does feel that way — but cold isn't a substance that can flow anywhere. 'Colder' just means less thermal energy. The energy actually moved the other way: out of the solution and into the products. That's what makes this reaction endothermic.

The thermometer went down, so the reacting chemicals must have lost energy.

This is a common misconception

Here's the catch: the thermometer isn't measuring the chemicals. It's sitting in the surroundings — the solution. The solution lost energy and the products gained it. So the chemicals didn't lose energy at all; they took it in.

Energy is moving from the solution into the products, so the solution gets colder.

This matches the evidence

Spot on. The solution is the surroundings, and energy has been transferred from it to the products. The falling thermometer is your evidence that this reaction is endothermic.

If the solution gets colder, the whole classroom will get colder too.

This is a common misconception

The solution is the surroundings; the classroom is the environment — a far bigger place. Thermal energy only moves from warmer to cooler, so energy flows from the warmer room into the cold cup, not the other way. And the room is so large that its temperature barely changes.

Nothing gives out cold. Thermal energy only moves from somewhere warmer to somewhere cooler — so when the solution cools, it's because energy left it and went into the products.

Chemistry · The cup test

Get the order right, or the readings mean nothing

The order you carry out the steps of the cup test

1 · First4 · Last

Add reactant 2 and stir
Record the highest or lowest temperature as the end temperature
Place reactant 1 in a polystyrene cup supported in a beaker
Measure and record the start temperature

Chemistry · Temperature change

End minus start — then read the sign

Two tests with illustrative readings. Test A: start 20 °C, highest 27 °C. Test B: start 21 °C, lowest 16 °C. Work out each temperature change and decide: exothermic or endothermic?

Test A. Temperature change = end temperature − start temperature. The end temperature is the highest reading, 27 °C.End first, then take away the start.
missing step

Which line is step 2?

27 − 20 = +7 °C20 − 27 = −7 °C27 °C

Mirror images

ExothermicvsEndothermic

Read across each row — every exothermic fact has an endothermic twin pointing the other way.

Focus

Which way the energy moves

Exothermic

From the reactants to the surroundings

Endothermic

From the surroundings to the products

The insight

Same two places, opposite arrow. That's the whole difference.

What the thermometer in the solution does

Exothermic

Goes up — the surroundings get warmer

Endothermic

Goes down — the surroundings get colder

Sign of the temperature change (end − start)

Exothermic

Positive (+)

Endothermic

Negative (−)

After the reaction has finished

Exothermic

Thermal energy dissipates into the room, and the warm mixture gradually cools to room temperature

Endothermic

Energy from the room warms the cold mixture back up to room temperature

Chemistry · Spot the type

Exothermic or endothermic?

Put each reaction or product in the right group, then read why it belongs there.

Still to sort

Exothermic (0)

Energy is transferred to the surroundings — they warm up.

Where the line is: Don't be fooled by a flame or a hot lab. Ask which way the energy goes: out of the reacting chemicals (exothermic) or into them (endothermic).

Endothermic (0)

Energy is transferred from the surroundings to the products — they cool down, or energy has to keep being supplied.

Where the line is: A reaction that only keeps going while you supply energy — by heating, light or electricity — is taking energy in. That makes it endothermic, however hot the Bunsen flame is.

11 of 11 still to sort.

Reactions you'll meet, and everyday products that use them. For each one, ask: does energy go out to the surroundings, or come in from them?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Hotter or colder? The thermometer tells you which way the energy went.

What you need to know

Exothermic: energy is transferred from the reactants to the surroundings, so the temperature of the surroundings increases.
Endothermic: energy is transferred from the surroundings to the products, so the temperature of the surroundings decreases.
A thermometer in a reaction mixture is measuring the surroundings (the solution), not the reacting chemicals themselves.
Temperature change = end temperature − start temperature. A positive change shows an exothermic reaction; a negative change shows an endothermic one.

The big picture

Some reactions make the things around them warmer, and some make them colder. An exothermic reaction transfers energy from the reacting chemicals to the surroundings, so the temperature of the surroundings goes up. An endothermic reaction transfers energy from the surroundings to the products, so the temperature goes down. You find out which by measuring the temperature before and after mixing and working out end temperature − start temperature.

Key points

1Surroundings warmer → energy came out of the chemicals → exothermic.

2Surroundings colder → energy went into the products → endothermic. Nothing 'gives out cold'.

3Thermal energy only ever moves from somewhere warmer to somewhere cooler.

4The surroundings are where the reaction happens (the solution); the environment is where the mixture sits (the room).

5Once a reaction stops, the mixture drifts back to room temperature, but the room hardly changes because it is so big.

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Worked example

Problem

Illustrative readings: a student puts reactant 1 in a polystyrene cup and records 19.5 °C. After adding reactant 2 and stirring, the lowest temperature reached is 12.0 °C. Is the reaction exothermic or endothermic? Explain how you know.

⚠ Watch out

Working out start − end instead of end − start. Get the order the wrong way round and the sign flips — and a warming reaction suddenly looks like a cooling one. Always begin with the END temperature.

🧠

Memory hook

EXothermic = energy EXits the chemicals, so the thermometer goes up. ENdothermic = energy ENters the products, so the thermometer goes down.

✓

Check yourself

A classmate says: "The thermometer went up, so the reaction must have taken in energy." Which part of that is wrong, and how would you put it right?

Flashcards

(15)

In an exothermic reaction, what happens to the temperature of the surroundings — and why?

It rises, because energy is transferred from the reactants to the surroundings.

In an endothermic reaction, what happens to the temperature of the surroundings — and why?

It falls, because energy is transferred from the surroundings to the products.

In a reaction in a cup, what is the thermometer actually measuring?

The surroundings — the solution the reaction happens in — not the reacting chemicals themselves.

What is the difference between the surroundings and the environment?

Surroundings: where the reaction takes place (e.g. the solution). Environment: where the reaction mixture is (e.g. the classroom).

Which way can thermal energy be transferred?

Only from somewhere warmer to somewhere cooler.

Does an endothermic reaction give out cold?

No. Cold is not something that flows. The surroundings get colder because energy leaves them and goes into the products.

What happens to the mixture after an exothermic reaction has finished?

Thermal energy dissipates into the environment, and the mixture gradually cools to the temperature of the room.

What happens to the mixture after an endothermic reaction has finished?

Energy from the environment warms it back up to room temperature.

Why does a reaction in a beaker barely change the temperature of the classroom?

The environment is so large that the effect on room temperature is very small.

How do you work out the temperature change, and what does its sign tell you?

Temperature change = end temperature − start temperature (in °C). Positive → exothermic. Negative → endothermic.

In the cup test, which reading do you record as the end temperature?

The highest temperature reached if it rises, or the lowest temperature reached if it falls.

Name three types of exothermic reaction.

Combustion (burning), neutralisation, and displacement reactions — for example, magnesium powder added to copper sulfate solution.

Give two everyday uses of exothermic reactions.

Single-use hand warmers and self-heating meals.

Name four endothermic reactions.

Photosynthesis, electrolysis, the thermal decomposition of copper carbonate, and citric acid added to sodium hydrogen carbonate solution.

Give two everyday uses of endothermic reactions.

Self-cooling drinks and instant ice packs for sports injuries.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Up or down? Follow the energy

Does a cold reaction give out cold?

Get the order right, or the readings mean nothing

End minus start — then read the sign

Exothermic or endothermic?

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