calculating & explaining energy changes

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  • Calculating and explaining energy changes
    • Measuring energy of chemical reactions
      • measuring initial & final temp in a neutralisation reaction in an insulated container lets you calc the energy produced
        • also used in solution reactions with solids
    • Making and breaking bonds
      • breaking chemical bonds = endothermic process
      • forming bonds = exothermic
      • If more energy is required to break old bonds than is released when new bonds are formed, the reaction is endothermic
      • If more energy is released when the new bonds are formed than is needed to break old bonds, the reaction must be exothermic
    • Measuring energy by calorimetry
      • 4.2 joules to heat 1 gram of water by 1 degree c (calorie)
        • 4.3 j = 1 calorie
      • to measure temp change when fuel burns:
        • 1Place 100g water in a glass/metal container
          • find mass (g) of he fuel to be burned
            • Burn fuel under water for a few minutes
              • Record new temp and calc temp change
                • Weigh fuel & calc mass of fuel used
                  • Energy released (Q) = mc delta T
                    • m = mass, c = 4.2 (a constant) and delta T = temp change
    • Energy level diagrams
      • exothermic: energy is given out (lost), so products have less energy than reactants
      • endothermic: energy is taken in (gained), so products have more energy than reactants
      • activation energy is needed to start reaction (to break old bonds)
      • catalysts reduce activation energy needed (faster reaction)
    • Hydrogen as a fuel
      • can be used in combustion engines
      • in fuel cells which produce electricity to power vehicles
      • hydrogen + oxygen = water + heat

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