An exothermic change releases heat to its surroundings, and an endothermic change absorbs it. The test is the direction of heat flow, which you read from what happens to the temperature of the surroundings.
This lesson is part of SPM Chemistry thermochemistry. After it, continue with reading energy-level diagrams.
What rule do you use?
Follow the heat. Decide where the heat goes, then read the surroundings.
| Observation | Heat flow | Type | Sign of ΔH |
|---|---|---|---|
| Temperature of mixture rises | Reaction to surroundings | Exothermic | Negative |
| Temperature of mixture falls | Surroundings to reaction | Endothermic | Positive |
Worked examples with readings
Four original situations. Each gives the change in temperature of the mixture.
Sodium hydroxide dissolving in water: 27.0 °C to 35.5 °C. The temperature rose, so heat was released. Exothermic, ΔH negative.
Ammonium nitrate dissolving in water: 28.0 °C to 21.5 °C. The temperature fell, so heat was absorbed. Endothermic, ΔH positive.
Calcium oxide added to water: the beaker becomes hot. Heat is released, so the change is exothermic.
Heating calcium carbonate strongly until it decomposes: heat must be supplied continuously. Heat is absorbed, so the change is endothermic.
The mistake that costs marks
The common slip is to reason from the reaction mixture, not the surroundings. A student sees the flask turn cold and writes “the reaction loses heat, so it is exothermic”.
The flask turns cold because the reaction takes heat from the water and the flask. The reaction gained heat, so it is endothermic.
| Statement | Wrong reasoning | Right reasoning |
|---|---|---|
| Flask feels cold | Reaction loses heat, exothermic | Reaction absorbs heat from the surroundings, endothermic |
Always ask whether the reaction is taking heat in or giving it out, then link it to the thermometer.
How does this link to bonds?
Bond breaking absorbs energy, and bond forming releases it. If forming the products’ bonds releases more than breaking the reactants’ bonds absorbed, the net result is heat released, which is exothermic.
Check yourself
A student mixes two solutions and the thermometer reading changes from 26.0 °C to 31.5 °C. State whether the reaction is exothermic or endothermic, the sign of ΔH, and where the products sit compared with the reactants on an energy-level diagram.
Answer
The temperature rose, so heat was released to the surroundings: the reaction is exothermic.
ΔH is negative.
On an energy-level diagram, the products are at a lower energy level than the reactants, because energy has left the system.
What to study next
Next, see how the same idea is drawn in reading energy-level diagrams. Then put numbers to it in calculating heat change from experimental data.
The mole and stoichiometry steps tool supports later calculations. If you want a teacher to test your reasoning on unseen readings, see online one-to-one Chemistry tuition.