energy changes

• energy is moved around in chemical reactions
• energy is conserved in reactions
• in an exothermic reaction, heat is given out to the surroundings
• neutralisation acids are an exothermic reaction
• uses of exothermic reactions:

  -hand warmers

  -self heating cans
• in an endothermic reaction, heat is taken in from the surroundings
• endothermic reactions:

  -reaction between citric acid and sodium hydrogencarbonate

  -thermal decomposition
• endothermic reaction use:

  -sports injury packs
• energy must always be supplied to break bonds
• during a chemical reaction, old bonds are broken down and new bonds are formed.
• bond breaking is an endothermic reaction
• bond formation is an exothermic reaction
• the overall energy change is the sum of all energies needed to break bonds minus the energy released when bonds are formed
• Br-Br bonds are weaker than Cl-Cl bonds
• H-Br bonds are weaker than H-Cl bonds so less energy is needed to break them down
• chemical reactions in a cell produces electricity
• the voltage of a cell depends on:

  -type of metal

  -difference in the reactivity series

  -type of electrolyte
• in non-rechargeable batteries, the reactants get used up
• in a rechargeable cell, the reaction can be reversed by putting it in a circuit with a battery
• fuel cells use fuel and oxygen to produce electrical energy
• when the fuel enters the cell, it becomes oxidised and sets up a potential difference within the cell
• hydrogen-oxygen fuel cells involve a redox reaction