Energy changes in reactions · Contrast
Endothermic reactions
Two powders, both at room temperature, stirred together in a beaker. Thirty seconds later the thermometer has fallen from 20 °C to 12 °C, and nothing was cooled.
Start here
Nothing was cooled. Nothing was refrigerated. The beaker got cold on its own.
Citric acid and sodium hydrogencarbonate, both at 20 °C, both dry, are stirred into water. The thermometer falls to 12 °C — eight degrees down, the same drop the fifth beaker made in the last lesson — and the outside of the beaker mists over cold.
Where has the energy gone?
Into the reaction. This one needs energy to proceed, and it takes it from the nearest available source — the water, the beaker, the bench and the air. Those things lose energy, so they get colder, and the thermometer reports it. A change that takes energy in from its surroundings is endothermic. It is the mirror image of last lesson, and it is far rarer.
An endothermic change takes energy in from its surroundings, so the surroundings get colder. The thermometer falls, and nothing has been refrigerated.
Both kinds obey the same law. Energy is never created and never destroyed — it is transferred. Exothermic moves it out of the chemicals and into the surroundings; endothermic moves it the other way. The only question is which direction.
Your turn · eight changes
Exothermic or endothermic? The thermometer decides.
0 of 8 sorted
Four of the eight are endothermic, and three of those four are changes you have already met under another name.
Burning natural gas
a hob
Exothermic — the surroundings warm.
Combustion is exothermic, always. The whole reason for burning anything is the energy it gives out.
Thermal decomposition of copper carbonate
a Bunsen
Endothermic — the surroundings cool.
Endothermic. The Bunsen has to keep running: take the flame away and the reaction stops, because it can only proceed while energy is being supplied. That is why the decompositions you did with a Bunsen all needed the flame kept on.
Neutralising an acid with an alkali
a beaker
Exothermic — the surroundings warm.
Exothermic — the mixture warms by several degrees. You measured this in the acids unit without naming it.
Photosynthesis
a leaf
Endothermic — the surroundings cool.
Endothermic, and the biggest one on the planet. The energy comes from sunlight, which is why a plant in the dark cannot do it however much carbon dioxide and water it has.
Respiration
every living cell
Exothermic — the surroundings warm.
Exothermic. It is the reverse of photosynthesis, so it releases what photosynthesis stored — and it is why a crowded room warms up.
Ice melting
a drink
Endothermic — the surroundings cool.
Endothermic. Melting takes energy in, taken from the drink — which is exactly how the ice keeps it cold, as you worked out last lesson.
Water freezing
a freezer
Exothermic — the surroundings warm.
Exothermic. Freezing is melting run backwards, so the energy runs backwards too: the water gives energy out, and the freezer has to carry it away. That is the heat coming off the back of the appliance.
Ammonium nitrate dissolving
a cold pack
Endothermic — the surroundings cool.
Endothermic, strongly enough to be sold as a first aid device. Not every dissolving is — some warm up — but this one takes in enough energy to bring water close to freezing.
Look at how lopsided that list is.
Most changes give energy out. Endothermic ones are the exceptions, and they tend to be the ones that need something driving them — a Bunsen under a carbonate, sunlight falling on a leaf, or a solid pulling itself apart as it dissolves.
Notice the pairs. Melting is endothermic and freezing is exothermic; photosynthesis is endothermic and respiration is exothermic. Run a change backwards and the energy runs backwards with it, by exactly the same amount.
Key fact
Endothermic changes take energy in from the surroundings, so the temperature falls. Exothermic changes give it out and the temperature rises. Reverse a change and you reverse its energy transfer.
Three judgements
Cold on demand
0 of 3 decided
A sports cold pack is squeezed and goes cold in seconds. Why can it only be used once?
Because the change cannot be undone in the packet. To reset it you would have to evaporate the water off and recover the solid, which needs a heat source and an open container. Compare the reusable hand warmer from last lesson, which resets by boiling — the difference is whether the reverse change is practical, not whether it is possible.
Would an endothermic reaction be any use as a fuel?
No. A fuel is something you burn to get energy out, and an endothermic reaction does the opposite: you would have to keep supplying energy to make it run, and get nothing back. Endothermic reactions are useful for cooling, for storing energy and for taking things apart — not for powering anything.
A student claims an endothermic reaction breaks the law of conservation of energy, because energy disappears.
Wrong, and instructively so. The energy has not disappeared: it has been taken out of the surroundings and stored in the new substances, where a thermometer cannot see it. Run the reaction backwards and it comes out again. Nothing is lost — the accounting simply moved to a place the instrument does not read.
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
“An endothermic reaction produces cold.”
The beaker genuinely does get cold. Commit before you read on.
There is no such substance as cold. Cold is not a thing that can be made, pumped or released — it is simply the absence of energy. The reaction did not produce anything; it took energy away, and what is left behind has less than it did.
The same correction applies to a fridge, which does not make cold but moves energy out of the food and dumps it out of the grille at the back. Every temperature change is energy going somewhere. Naming the direction is the whole of this unit.
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 endothermic change?
Rung 2 · The one that catches people
Melting is endothermic. What does that tell you about freezing?
Rung 3 · Explain
Two powders at room temperature are stirred into water and the temperature falls to 12 °C. Explain what has happened in terms of energy, and why it does not break the law of conservation of energy.
Rung 4 · Take it somewhere new
A designer wants a cold pack that can be reused indefinitely. Explain what the chemistry would have to allow, and why the ammonium nitrate pack cannot do it.
Key note
An endothermic change takes energy in from its surroundings, so the temperature falls. Thermal decomposition, photosynthesis, melting and many dissolvings are endothermic; most other changes are exothermic. Energy is never created or destroyed in either case — only transferred, and reversing a change reverses the transfer.
Going further
Instant cold packs in a first aid kit are a sealed bag of ammonium nitrate with a pouch of water inside it. Squeeze the pack, the pouch bursts, the ammonium nitrate dissolves, and the temperature drops to near 0 °C within seconds — no freezer, no electricity, and it works on a mountainside. It is a single-use device because the dissolving cannot be undone in the bag.
The largest endothermic process on Earth is photosynthesis, and it runs on sunlight. Every joule stored in a leaf, and therefore in every food, every fuel and every living thing, was taken in by an endothermic reaction powered by a star. The exothermic reactions that follow — respiration, combustion, digestion — are all spending what photosynthesis banked. The whole carbon cycle is one endothermic reaction feeding an enormous number of exothermic ones.
Before this lesson
Next in this unit
At GCSE this becomes
- Energy level diagrams for endothermic changes, and bond breaking as the energy-absorbing half of every reaction.
Where to next
Ask Mr Badmus AI
Still not sure why cold is not a substance?
An instant cold pack is a real first aid device and gets cold enough to injure skin it is held against directly. It goes on over clothing or a cloth, for short spells, and a used one is thrown away rather than opened.
Lesson content © MrBadmusAI.