energy changes (5)

• exothermic reactions


  transfers energy to its surroundings
• examples of exothermic reactions


  combustion, neutralisation, hand warmers
• endothermic reactions, 


  takes in energy from its surroundings
• examples of endothermic reactions


  sports injury packs, photosynthesis
• activation energy 


  minimum amount of energy that particles must have to react
• exothermic reaction profiles 


  products have less energy than reactants
• endothermic reaction profile 


  products have more energy than reactants
• energy must be supplied to break bonds in the reactants
• energy is released when bonds in the products are formed
• batteries 


  two or more cells connected together in series to provide a greater voltage
• fuel cell


  a device that generates electricity by using a chemical reaction to convert fuel and oxygen into electricity, heat and water
• hydrogen fuel cells are a potential alternative to rechargeable batteries


  hydrogen fuel cells can be constantly provided, only produces water when burnt, hydrogen is a gas so needs to be stored at a high pressure and is harder to transport. however rechargeable cells can be recharged by reversing the action so constant fuel isn't needed, it will eventually stop working and is hard to dispose
• equation at the anode 


  H2-->2e-+2H+
• equation at the cathode

  4H+ + O2 + 4e- --> 2H2O