Every chemical reaction involves a change in energy. This is because chemical bonds in the reactants must be broken (requiring energy) and new bonds in the products must be formed (releasing energy). The net difference between these two processes results in a transfer of energy between the chemical system and its surroundings, typically in the form of heat (thermal energy). The IGCSE 0654 Core syllabus requires a clear distinction between reactions that release heat and those that absorb it.

An exothermic reaction is one that transfers thermal energy to the surroundings.

Key Characteristics

• Temperature Change: Because heat is released, the temperature of the surroundings increases. In a laboratory setting, a thermometer placed in an exothermic reaction mixture will show a rising temperature.
• Chemical Energy: The chemical energy stored in the reactants is higher than that in the products. The "excess" energy is released as heat.
• Examples:
• Combustion: The burning of any fuel (e.g., methane or wood).
• Neutralisation: The reaction between an acid and a base.
• Respiration: The biochemical oxidation of glucose in living cells.

An endothermic reaction is one that takes in thermal energy from the surroundings.

Key Characteristics

• Temperature Change: Because heat is absorbed from the environment, the temperature of the surroundings decreases. A thermometer in an endothermic reaction mixture will show a falling temperature.
• Chemical Energy: The chemical energy stored in the products is higher than that in the reactants. The "missing" energy must be supplied from the surroundings.
• Examples:
• Photosynthesis: Plants absorb light energy from the Sun to drive the reaction between $CO_2$ and $H_2O$.
• Thermal Decomposition: Breaking down a substance by heating it (e.g., heating calcium carbonate).
• Physical Changes: While not chemical reactions, the melting of ice and the evaporation of water are endothermic processes.

Understanding chemical energetics is essential for controlling industrial processes and explaining biological metabolism. By identifying whether a reaction is exothermic or endothermic, scientists can predict the energy requirements of a process and design systems to safely manage heat changes, ensuring that chemical transformations are both efficient and sustainable.