Endothermic and exothermic reactions
Breaking bonds takes energy. Forming bonds releases energy. A reaction’s overall heat change depends on which is larger.
- An exothermic reaction releases heat to its surroundings, which get warmer. Burning fuel is exothermic.
- An endothermic reaction absorbs heat from its surroundings, which get cooler. An instant cold pack is endothermic.
The heat change at constant pressure is the enthalpy change, ΔH.
| Reaction | Heat | Sign of ΔH | Products compared with reactants |
|---|---|---|---|
| Exothermic | Released | Negative | Lower in energy |
| Endothermic | Absorbed | Positive | Higher in energy |
ΔH is written for the equation as balanced:
CH4 + 2 O2 → CO2 + 2 H2O, with ΔH = -890 kJ
Burning 1 mol of CH4 releases 890 kJ.
Worked example: how much heat is released by burning 3.00 mol of CH4?
3.00 mol × 890 kJ/mol = 2670 kJ released
Common mistake
Thinking an exothermic reaction feels cold because it “loses” heat. The reaction gives its heat to you and the surroundings, so it feels hot.