Energy Changes

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  • Created by: Shivani M
  • Created on: 29-11-17 17:36
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  • Exothermic and Endothermic Reactions
    • energy changes when a reaction occurs
    • Energy Changes
      • Bond Energies
        • during a chemical reaction, old bonds are broken and new bonds are formed
        • energy is needed to break bonds so bond breaking is endothermic
          • in endothermic reactions, the energy used to break bonds is greater than the energy released by forming them
        • energy is released when new bonds are formed so bond making is endothermic
          • in exothermic reactions, the energy released by making new bonds is greater than the energy used to break them
        • reaction profiles are diagrams that show relative energies of reactants and products in a reaction, and how the energy changes
          • the difference in height represents the overall energy change
            • overall energy is the sum of energies needed to break the bond minus the energy released when new bonds are made
          • the activation energy is minimum amount of energy the reactants need to react
      • Cells
        • chemical reactions in cells produce electricity
        • an electro-chemical cell is a system made up of two electrodes in contact with an electrolyte
          • two electrodes need to conduct electricity so are usually metals
          • the electrolyte is an ionic liquid which reacts with the electrodes
            • the chemical reactions between the electrolyte and electrodes set up a charge difference between the electrodes and if connected by a wire, they can produce electricity
        • the voltage of a cell depends on many factors
          • the type of electrode affects the voltage of the cell
          • the bigger the difference in reactivity between the electrodes, the bigger the voltage will be
          • the electrolyte used can also affect the voltage
          • batteries are formed by connecting cells, this also gives a bigger voltage
            • non rechargeable batteries use irreversible reactions, so once the reactants are used up, no more charge is produced
      • Fuel Cells
        • a fuel cell is an electrical cell that is supplied with fuel and oxygen, and it uses the energy from this reaction to produce efficient electrical energy
          • the fuel becomes oxidised and creates a potential difference in the cell
        • hydrogen-oxygen fuel cells can be used in vehicles
          • they produce less pollutants than other fuels
          • only by-products are water and heat
          • easier to dispose of and cheaper
          • store more energy
          • involves a redox reaction
            • the product of the reaction is water
          • the electrolyte used is often potassium hydroxide and the electrodes are mostly carbon with catalysts
            • at the anode, oxidation occurs as hydrogen loses electrons:  H2-> 2H* + 2e-
            • at the cathode, reduction occurs as oxygen gains electrons and reacts with the H* ions: O2 + 4H* + 4e- -> 2H2O
    • energy is always conserved in a reaction
    • exothermic reactions are those that transfer energy to the surroundings e.g. combustion
    • endothermic reactions are those that takes energy from the surroundings e.g. thermal decomposition

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