C5

• chemicals store a certain amount of energy - and different chemicals store different amounts of energy
• if the products of a reaction store more energy than the original reactants, then they must have taken in the difference in energy between the products and the reactants from the surroundings during the reaction. but if they store less, then excess energy was transferred to surroundings during reaction
• the overall amount of energy doesnt change. this is because energy is conserved in reactions - it cannot be created or destroyed only moved around
• an exothermic reaction is one which transfers energy to the surroundings, usually by heating. this is shown by a rise in temperature
• the best example of an exothermic reaction is combustion (burning fuels)
• an endothermic reaction is one which takes in energy from the surroundings. this is shown by a fall in temperature