5.5 energy changes

• an exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases
• examples of exothermic reactions

  combustion, oxidisation, neutralisation
• everyday examples or exothermic reactions

  hand warmers
• an endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases
• in exothermic reactions, product molecules must have less energy than the reactants
• in endothermic reactions, product molecules must have more energy than the reactants
• examples of endothermic reactions 


  thermal decomposition
• everyday examples of endothermic reactions 


  sports injury packs
• chemical reactions can occur only when reacting particles collide with enough energy
• activation energy is the minimum amount of energy that particles must have to react
• catalysts are substances which speed up chemical reactions without being used up themselves
• the catalyst lowers the activation energy needed for reaction so it speeds up the rate of reaction
• a catalyst lowers the activation energy needed for a reaction to take place
• the higher the activation energy, the slower the reaction will be at room temperature
• in a chemical reaction, energy must be supplied to break bonds in the reactants
• in a chemical reaction, energy is released when bonds in the products are formed
• endothermic reaction = energy taken in to break > energy released when formed
• exothermic reaction = energy taken in to break < energy released when formed