topic 5 energy changes

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  • energy changes (topic 5)
    • exothermic reactions
      • transfers energy to its surroundings
      • rise in temperature
      • combustion (burning fuels)
      • neutralisation reactions are often exothermic
      • oxidation reactions
      • hand warmers and self heating can of hot chocolate rely on exothermic reactions.
      • bond making
    • endothermic reactions
      • releases energy into its surrounding
      • decrease in temperature
      • less common than exothermic reactions
      • thermal decomposition
      • sport injury packs
      • bond breaking
    • cells/batteries
      • hydrogen-oxygen fuel cells
        • electrolyte of potassium hydroxide, electrode of carbon
        • anode
          • hydrogen goes into the anode
          • negative electrode (opposite to usual)
          • hydrogen loses electrons to form H+
          • hydrogen=> two hydrogen ions + two electrons
        • cathode
          • positive electrode
          • oxygen gains electrons and reacts with H+ ions to form water
          • oxygen + four hydrogen ions+four electrons=> two water molecules
        • makes water
        • advantages/ disadvantages
          • not many pollutants
          • don't need recharging
          • cheaper
          • more energy can be stored in them
          • takes up a lot of space
          • hydrogen is explosive so needs to be stored correctly
      • normal cells
        • system of two electrodes in contact with an electrolyte
          • electrodes conduct electricity
          • electrolyte contains ions to conduct electricity
          • a cells voltage depend on difference in reactivity between electrodes
            • magnesium is very reactive but copper isn't so its voltage would be high if used together
            • if iron and zinc were used together, there would be a low voltage as they are similar in reactivity
        • (non)rechargeable batteries
          • the electrolyte gets turned back into reactants in a rechargeable batterie
          • in non rechargeable batteries, the electrolyte gets used up

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