Chemical reactions involve important energy changes that can be classified as either exothermic or endothermic reactions. Understanding these reactions and their temperature changes is crucial for GCSE chemistry studies.

In exothermic reactions, energy is released to the surroundings, causing an increase in temperature. Common examples include combustion reactions, neutralization between acids and bases, and oxidation reactions. These reactions release heat energy as chemical bonds are formed, which is stronger than the energy needed to break the original bonds. When conducting practical investigations of temperature changes, it's essential to use proper equipment like insulated containers and thermometers to accurately measure the temperature rise. Key control variables include the concentration of reactants, volume of solutions, and initial temperature.

Endothermic reactions absorb energy from the surroundings, resulting in a temperature decrease. Examples include photosynthesis, thermal decomposition, and dissolving ammonium chloride in water. These reactions require more energy to break bonds than is released when new bonds form. When performing temperature changes required practical experiments, students must carefully monitor temperature changes over time and record data systematically. Bond energy calculations are vital for determining whether a reaction is exothermic or endothermic. These calculations involve finding the difference between energy required to break bonds and energy released when forming new bonds. For GCSE level, students should be able to interpret energy level diagrams, calculate overall energy changes, and explain how bond breaking and bond making relate to temperature changes observed during reactions. Understanding these concepts helps explain real-world applications like instant cold packs used in sports injuries (endothermic) and hand warmers (exothermic).