Pearson Edexcel International GCSE Science Single Award 8: Chemistry: Physical chemistry

Topic 3 asks how chemical reactions transfer heat and why their rates change. These notes follow only the Science Single Award points in Pearson 4SS0 Issue 4. They exclude activation-energy profiles, bond-energy calculations, reversible reactions and equilibrium content found in broader Pearson routes.

The specification names calorimetry and rate investigations. This theory note explains the chemistry and calculation logic; the separate practical-skills hub develops planning, measurement and evaluation.

Exothermic and endothermic reactions

An exothermic reaction transfers heat energy to the surroundings, so the surroundings usually warm and measured temperature rises. An endothermic reaction takes in heat energy from the surroundings, so the surroundings usually cool.

“Exothermic” does not mean a reaction is fast, and “endothermic” does not mean it cannot occur. These words describe the direction of heat transfer. Combustion and many neutralisation reactions are exothermic. Some dissolving processes are endothermic, while others are exothermic, so the type must be determined from evidence rather than the word “dissolving.”

Simple calorimetry

Calorimetry estimates heat transfer from a measured temperature change. A known mass of water or solution is placed in an insulated container, an initial temperature is recorded, reactants are mixed or a fuel is burned, and the maximum or minimum temperature is measured.

The heat-energy magnitude is:

$$Q=mc\Delta T$$

Here $Q$