Exothermic and endothermic reactions
Exothermic and endothermic
- Every reaction transfers energy — out or in.
- Exothermic: gives out thermal energy → the surroundings get hotter.
- Endothermic: takes in thermal energy → the surroundings get colder.
- Exothermic examples: combustion, neutralisation. Endothermic example: thermal decomposition.
Practice
An exothermic reaction:
Exothermic reactions release heat to the surroundings, raising their temperature.
Practice
Classify each reaction.
Combustion and neutralisation give out heat; thermal decomposition takes heat in.
Enthalpy change, ΔH
- The energy transferred is the enthalpy change, $\Delta H$.
- $\Delta H$ is negative for exothermic (energy leaves the chemicals).
- $\Delta H$ is positive for endothermic (energy is taken in).
Practice
For an exothermic reaction, the enthalpy change ΔH is:
Energy leaves the chemicals, so ΔH is negative for exothermic reactions (positive for endothermic).
You've got it
Key idea
- exothermic = heat out, surroundings warmer, $\Delta H$ negative
- endothermic = heat in, surroundings cooler, $\Delta H$ positive
- combustion + neutralisation are exothermic; thermal decomposition is endothermic