Energy Flow: Exo vs Endo
Track heat absorption and release in reactions.
What this lesson covers
Why it matters
Hold a thermometer near quicklime as water hits it and watch the temperature leap — the slaking of lime pours out heat.
Predict first
Quicklime added to water makes the beaker hot. Where did that energy come from?
Exothermic reactions release energy (the surroundings warm up); endothermic reactions absorb energy (the surroundings cool, or heat must be supplied). (Melting ice and dissolving salts are physical changes, but they still absorb or release energy — the energy rule applies to every change.)
- It was released as new bonds formed — an exothermic reaction — correct
- It was absorbed from the surroundings
- The water was already hot
What you do
Sort each change: does it release heat (exothermic) or absorb heat (endothermic)?
Check yourself
Which of these is an endothermic change?
Decomposing limestone needs a constant supply of heat; combustion and neutralisation give heat out.
In an exothermic reaction, the products have…
The 'lost' chemical energy leaves as heat, so products sit lower in energy than reactants.
- Decomposition of limestone on heating — correct
- Burning of magnesium
- Neutralising an acid with an alkali
- less stored energy than the reactants — correct
- more stored energy than the reactants
- exactly the same energy