Energy changes

Energy changes is a fundamental topic in AQA GCSE Combined Science that explores how energy is transferred during chemical reactions. You'll learn to distinguish between exothermic reactions, which release energy to the surroundings (e.g., combustion, neutralisation), and endothermic reactions, which absorb energy (e.g., thermal decomposition, photosynthesis). This topic also introduces reaction profiles—diagrams showing the energy changes during a reaction—and the concept of activation energy, the minimum energy needed for a reaction to occur. Understanding energy changes is crucial because it explains why some reactions feel hot or cold and underpins real-world applications like hand warmers (exothermic) and cold packs (endothermic).

Beyond qualitative ideas, you'll calculate energy changes using bond energies. In a chemical reaction, bonds in the reactants must be broken (which requires energy) and new bonds form in the products (which releases energy). The overall energy change (ΔH) is the difference between the energy absorbed to break bonds and the energy released when new bonds form. A negative ΔH indicates an exothermic reaction, while a positive ΔH indicates an endothermic reaction. This quantitative skill is essential for predicting whether a reaction will release or absorb energy and for comparing different reactions.

Energy changes connect to other topics in the course, such as rates of reaction (where activation energy affects reaction speed) and chemical cells (which convert chemical energy into electrical energy). Mastering this topic builds a foundation for understanding how energy flows in chemical systems, which is key to topics like electrolysis and fuel cells. In exams, you'll be expected to interpret reaction profiles, calculate energy changes from bond energies, and explain everyday examples of exothermic and endothermic processes.