Energy changes — AQA GCSE Chemistry

Energy changes in chemistry focus on the energy transfers that occur during chemical reactions. Every reaction involves a change in energy, usually in the form of heat. This topic is divided into two main areas: exothermic reactions, which release heat energy to the surroundings, and endothermic reactions, which absorb heat energy from the surroundings. Understanding these energy changes is crucial for explaining why reactions happen and for designing processes like hand warmers or cold packs.

The key concept is the idea of bond breaking and bond making. Breaking chemical bonds requires energy (endothermic), while forming new bonds releases energy (exothermic). The overall energy change of a reaction (ΔH) is the difference between the energy needed to break bonds and the energy released when new bonds form. If more energy is released than absorbed, the reaction is exothermic; if more energy is absorbed than released, it is endothermic.

This topic connects to many other areas of chemistry, such as rates of reaction, equilibrium, and the Haber process. It also has real-world applications, including combustion, respiration, and the use of fuels. Mastering energy changes helps students understand why some reactions are spontaneous and how energy is conserved in chemical systems.

• →Exothermic reactions release heat energy to the surroundings, causing a temperature increase. Examples include combustion, neutralisation, and respiration.
• →Endothermic reactions absorb heat energy from the surroundings, causing a temperature decrease. Examples include thermal decomposition, photosynthesis, and dissolving ammonium nitrate.
• →Activation energy is the minimum energy required for a reaction to occur. It is represented by the 'hump' on an energy level diagram.
• →Bond breaking is endothermic (requires energy), and bond making is exothermic (releases energy). The overall energy change (ΔH) is calculated as: ΔH = total energy absorbed (bonds broken) – total energy released (bonds made).
• →Energy level diagrams show the relative energies of reactants and products, with the difference being ΔH. Exothermic reactions have products at a lower energy than reactants; endothermic reactions have products at a higher energy.

• Exothermic reactions transfer energy to the surroundings, increasing temperature.
• Endothermic reactions take in energy from the surroundings, decreasing temperature.
• Activation energy is the minimum energy required for particles to react.
• Reaction profiles show relative energies of reactants and products, activation energy, and overall energy change.
• Bond breaking requires energy (endothermic); bond forming releases energy (exothermic).
• Overall energy change is the difference between energy needed to break bonds and energy released forming bonds.
• Cells and batteries produce electricity from chemical reactions.
• Hydrogen fuel cells produce a potential difference by oxidizing hydrogen to water.

• 💡Always draw a curved line for reaction profiles to show the energy change as the reaction proceeds.
• 💡When calculating energy changes, ensure you sum all bond energies for reactants and products separately before finding the difference.
• 💡Remember that in an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.
• 💡Be prepared to evaluate the use of hydrogen fuel cells compared to rechargeable batteries using provided data.
• 💡Always use the correct sign convention: ΔH is negative for exothermic reactions and positive for endothermic reactions. In calculations, show your working clearly, including the bond energies and the formula ΔH = Σ(bonds broken) – Σ(bonds made).
• 💡When drawing energy level diagrams, label the activation energy (the 'hump') and the overall energy change (ΔH). For exothermic reactions, the products are lower than the reactants; for endothermic, they are higher. Include the arrow showing the activation energy pointing from reactants to the top of the curve.
• 💡In multiple-choice questions, look for keywords like 'temperature increases' (exothermic) or 'temperature decreases' (endothermic). Also, remember that combustion and neutralisation are always exothermic, while thermal decomposition is usually endothermic.

• Confusing the direction of energy transfer in exothermic and endothermic reactions.
• Misinterpreting reaction profiles, particularly the position of the activation energy.
• Failing to recognize that bond breaking is endothermic and bond forming is exothermic.
• Incorrectly calculating the overall energy change by swapping the reactants and products in bond energy sums.
• Assuming all cells are rechargeable.
• Misconception: Exothermic reactions always feel hot. Correction: While many exothermic reactions release heat, some may not feel hot if the heat is released slowly or if the reaction is very small. The key is that heat is transferred to the surroundings, not necessarily that the reaction vessel becomes hot.
• Misconception: Bond breaking releases energy. Correction: Bond breaking always requires energy (endothermic). Energy is released only when new bonds form (exothermic). This is a common confusion because students often think of 'breaking' as releasing stored energy.
• Misconception: The activation energy is the same as the overall energy change. Correction: Activation energy is the energy needed to start the reaction, while ΔH is the net energy change from reactants to products. They are different values on an energy level diagram.