C6: Energy changes in chemical reactions

• A reaction is endothermic or exothermic depending on whether it breaks or forms bonds
• Exothermic reactions are reactions where the reactants give off energy and the product ends up with less energy than the reactants
• This is because less energy is used to break bonds than the energy used to gain bonds
• Endothermic reactions have a negative change of energy, meaning that the reactants formed a product with less energy
• Endothermic reactions are reactions where the reactants gain energy and the product ends up with more energy than the reactants
• This is because more energy is used to break bonds than the energy used to gain bonds
• Endothermic reactions have a positive change of energy, meaning that the reactants formed a product with more energy

• Energy level diagrams show the change of energy over time, and can help us see whether a reaction is endothermic or exothermic
• In the graph, the line at the start indicates the energy of the reactants, and the line at the end indicates the energy of the products
• You might notice that there is a different in the heights of the two lines. This indicates the amount of energy lost/gained
• The activation energy is the minimum energy to start a reaction. This means that the reactants need to gain a certain amount of energy to become a product
• If the line at the start is lower than the line at the end, that means that the reaction is endothermic, as the products have gained energy
• If the line at the start is higher than the line at the end, that means that the reaction is endothermic, as the products have lost energy

Graphs of exothermic and endothermic reactions