Pearson Science Double Award Chemistry 3: Physical Chemistry

Physical chemistry explains why reactions transfer energy, why their rates change and how some products can reform reactants. This note follows the 4SD0 Double Award boundary. It includes simple calorimetry, collision theory, catalysts and two prescribed reversible reactions. Bond-energy calculations, dynamic equilibrium and Le Chatelier predictions belong to separate Chemistry rather than this topic.

Exothermic and endothermic reactions

An exothermic reaction transfers energy from the reacting system to the surroundings. The surroundings warm, so the measured temperature usually rises. Combustion, neutralisation and many oxidation reactions are exothermic. An endothermic reaction takes in energy from the surroundings, so the measured temperature usually falls. Thermal decomposition and some dissolving processes are endothermic.

An energy-level diagram represents the relative chemical energy of reactants and products. Products lie below reactants for an exothermic reaction because energy has left the system. Products lie above reactants for an endothermic reaction because the system has gained energy. The hump represents the activation energy, the minimum energy required for particles to react successfully.

Temperature change is evidence about transfer under the stated conditions, not a complete definition. A lid, insulation and prompt reading reduce energy exchange with the environment. The sign of the temperature change must be linked to the system: a warmer solution means the reaction released energy to it.

Measuring heat transfer

For a solution heated or cooled in a simple calorimeter, estimate the transferred energy using

$$Q = mc\Delta T$$

where $Q$