Exothermic Change

An exothermic change is a chemical reaction or physical process that transfers thermal energy to the surroundings, causing the temperature of the surroundings to increase ($\Delta H < 0$) ¹.

Reaction Energy Pathway

In an exothermic process, products have lower chemical energy than reactants because surplus energy is dissipated as heat ¹:

Energy ^
       |       Activation Energy (Ea)
       |             .-'-.
       |   Reactants      \
       |  [========]       \
       |             |      \   Delta H (negative, heat released)
       |             |       \  vvv
       |                     [========] Products
       +---------------------------------------------> Reaction Progress

Bond Energy Balance

In chemical reactions:

• Bond Breaking: Endothermic (absorbs energy).
• Bond Making: Exothermic (releases energy).

A reaction is exothermic when the energy released forming new bonds in products is greater than the energy required to break bonds in reactants ¹.

Common IGCSE Examples

1. Chemical Reactions:
   • Combustion: Burning fuels (e.g. $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{heat}$).
   • Neutralisation: Reaction between acids and bases ¹.
   • Respiration: Cellular breakdown of glucose releasing energy.
2. Physical Phase Changes:
   • Freezing: Liquid turning to solid releases latent heat as intermolecular bonds form ².
   • Condensation: Gas turning to liquid releases latent heat as intermolecular forces bring particles together ².

Related

• Endothermic Change - Opposite thermodynamic process absorbing heat from the surroundings.
• Freezing - Exothermic phase transition into a solid lattice.
• Condensation - Exothermic phase transition into a liquid.
• Melting - Endothermic phase transition absorbing thermal energy.
• Intermolecular Force - Attractive forces formed during exothermic phase transitions.
• Chemistry - Science area covering chemical energetics.