Chemistry - Energy Changes

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  • Energy Changes
    • Exothermic Reactions
      • A reaction that transfers energy to the surroundings
      • E.g. Combustion and Neutralisation reactions
      • The energy released from breaking bonds is greater than the energy used in forming them
      • To calculate  the overall energy change, first find energy required to break the original bonds
        • Take that away from the sum of the energy released when forming new bonds
    • Endothermic Reactions
      • A reaction that takes in energy from its surroundings
      • The energy used to break bonds is greater than the energy released from forming them
      • To calculate  the overall energy change, first find energy required to break the original bonds
        • Take that away from the sum of the energy released when forming new bonds
    • Cells
      • A system made of two different electrodes (metals) in contact with an electrolyte whilst connected by a wire
        • The ions in the electrolyte react with the electrodes, causing a charge difference between the electrodes
          • That charge is able to flow, producing electricity
      • The greater the difference   in reactivity, the bigger the voltage
      • The Electrolyte also affects the voltage
      • A battery is formed when multiple cells are connected in series. This combines the voltages of all the cells
    • Fuel Cells
      • An electrical cell that  is supplied with fuel and oxygen, and used the energy from their reaction to produce electricity
        • When fuel enters the cell, it becomes oxidised and sets up a potential difference
      • Hydrogen Fuel Cells
        • Electrolyte: often potassium hydroxide
        • Electrodes: porous carbon with a catalyst
        • Hydrogen goes into the anode compartment and oxygen into the cathode compartment
          • Hydrogen  loses electrons to make H+ ions, which then move to the cathode.
            • The oxygen  gains electrons and reacts with H+ ions to make water
              • O2 + 4H+  +4e- = 2H2O
              • Overall: 2H2 + O2 = 2H2O
            • The electrons flow through an external circuit from anode to cathode, an electric current
            • H2  = 2H+  +  2e-
      • In a fuel cell, the anode is negative and the cathode is positive.

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