Chemistry - C3.3 - Energy Calculations

C3.3.1 - Comparing the Energy Released by Fuels

  • Fuels/food + oxygen: exothermic - amount of energy released depends what's used - energy release measured in J (or calories: 1cal = 4.2J)
  • Use calorimeter to measure energy released when substances burn - simple calorimeter: water in glass beaker/metal can, substance underneath burnt and heats water, temperature rise of water depends on energy released - amount of energy transferred calculated by: Q = mcΔT (Q:amount of energy transferred - J, m:mass - g, c:specific heat capacity - J/g°C, ΔT: temperature change - °C)
  • Simple calorimeters don't give accurate results: heat lost to surroundings - but can be used to compare energy released by different fuels
  • To compare energy released by burning substances, energy change in kJ/g or /mole can be used - calculate /mole by multiplying /g by relative fomula mass
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C3.3.2 - Energy Transfers in Solutions

  • Energy transferred to or from solution when reaction takes place in it - reduce energy transfers to surroundings by doing reaction in container
  • In energy calculations, assume solutions behave like water - 1cm³ = 1g and c = 4.2J/g°C
  • When solid is added to water/aqueos solution, assume volume doesn't change
  • If we know no of moles involved in calculated reaction we can calculate energy change in kJ/mol
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C3.3.3 - Energy Level Diagrams

  • Show energy changes for reaction on energy level diagram - difference between energy levels of reactants and products is energy change of reation - exothermic reaction: goes down
  • During chemical reaction, reactants bonds must be broken for reaction to happen - breaking bonds is endothermic: energy taken in - activation energy: minimum energy needed to break bonds
  • New bonds are formed - energy released: exothermic
  • Catalysts increase rate of reaction by providing different pathway with activation energy that's lower
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C3.3.4 - Calculations Using Bond Energies

  • In chemical reaction, energy needed to break bonds and released when bonds are formed - difference in these changes makes reaction exo or endothermic
  • Energy needed to break and make bonds between atoms is bond energy - use it to calculate overall energy change of reaction - bond energy measured in kJ/mol
  • Balanced equation for reaction is needed to calculate energy change for reaction
  • To calculate energy change, calculate: energy needed to break bonds, energy needed to make bonds and difference between the two
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C3.3.5 - Fuel Issues

  • Fossil fuels - non-renewable, cause pollution - urgent need to develop alternatives
  • Hydrogen as a fuel - burns easily, releases large amount of energy/g, no CO2 produced - just H2O, can be burnt in combustion engines or used in fuel cells to power vehicles, can be produced from renewable resources, difficulties include supply, storage and safety problems
  • Vehicles using fuel cells need to match performance, convenience and costs of petrol/diesel vehicles
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