C7 Energy changes

• exothermic reactions give out heat to the surroundings (temperature rises)
• neutralisation, combustion and displacement are all exothermic reactions
• endothermic reactions take in heat from the surroundings (temperature drops)
• endothermic reactions are less common than exothermic
• thermal decomposition, photosynthesis and cracking are endothermic reactions
• in exothermic reactions the products are at a lower energy level than the reactants because energy is released
• the difference between energy levels of reactants and products is the energy change
• in endothermic reaction profiles the reactants are at a lower energy level than the products meaning energy is taken in from the surroundings
• activation energy is the minimum amount of energy needed to start a reaction
• bond breaking requires energy to be takin in by the molecule so is endothermic
• bond making releases energy so is exothermic
• the energy needed to break the bond between two atoms is called the bond energy
• hydrogen fuel cells are electrochemical devices which produce electricity by converting hydrogen and oxygen into water
• in hydrogen fuel cells O2 gas is supplied to the positive electrode and is reduced forming hydrogen ions and H2 gas is supplied as a fuel to the negative electrode and is oxidised by the hydroxide ions producing water
• hydrogen fuel cells do not need to be electrically charges, come in all different sizes for different uses and their only product is water
• hydrogen fuel cells use hydrogen which is highly flammable, hard to store and might not be produced by renewable sources