exothermic and endothermic reactions

• When chemical reactions occur, energy is conserved.
• The amount of energy in the universe at the beginning is the same as at the end.
• If a reaction transfers energy to the surroundings, the product molecules must have less energy than the reactants, by the amount transferred.
• An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.
• Examples of exothermic reactions include combustion, many oxidisation reactions and neutralisation.
• Everyday examples of exothermic reactions include self-heating cans (e.g for coffee) and hand warmers.
• An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.
• Examples of endothermic reactions are thermal decomposition and the reaction of citric acid and sodium hydrogencarbonate.
• Some sports injury packs are based on endothermic reactions.
• Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
• Activation energy = minimum amount of energy that particles must have to react.
• Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.
• You can tell which reaction is exothermic or endothermic by looking at whether the reactants or products have more energy (higher up energy scale=higher energy).
• During a chemical reaction, energy must be supplied to break bonds in the reactants and energy is released when bonds in the products are formed.
• The energy needed to break bonds and energy released when bonds are formed can both be calculated from bond energies.
• The sum of energy taken in to break bonds minus the sum of energy released to form bonds equals the overall energy change.
• If the sum of energy taken in to break bonds is greater than the sum of energy released when formed, the reaction is endothermic.
• If the sum of energy taken in to break bonds is less than the sum of energy released when formed, the reaction is exothermic.
• endothermic energy profile

  
• exothermic energy profile