GCSE C3 Chapter 3

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Comparing the energy released by fuels

  • The energy released by a fuel can be measured by a simple experiment followed by a calculation - when fuels react with oxygen the reactions are exothermic
  • Measure a specific mass of water and pour into a container or calorimeter
  • Measure the start temperature of the water using a thermometer
  • Burn the substance and hold under the calorimeter
  • Continue to light the substance untill it does not light again
  • Measure the final temperature of the water
  • Calculate the temperature change
  • Energy released(J) = mass of water(g) X specific heat capactiy(J/g) X temperature change(C)
  • Energy released per gram of fuel(KJ/g) = Energy released / mass of fuel
  • Energy released per mole of fuel = KJ/g X Realative formula mass
  • The simplist calorimeters do not give very accurate results because much of the energy released is used to heat the surroundings rather than the water. Insulating the cup or putting a lid on the cup gives more accurate results
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Energy transfers in solution

  • When reactions take place in solution energy is transfered either to or from the solution
  • In calculations of reactions in solution we say that the solution behaves like water:
    • Specific heat capacity is 4.2J/g.C
    • Volume 1cm3 = 1g
  • When a solid is added to water or to an aqueous solution we assume that:
    • The volume of the water stays the same
  • If the moles of the reaction are known then the energy change can be calculated in KJ/moles
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Energy level diagrams

  • Energy changes in a reaction can be shown using energy level diagrams
  • The difference between the energy of the reactants and the products is the energy change for the reaction
  • During chemical reactions bonds are:
    • broken which requires energy so is endothermic
    • made which releases energy so is exothermic
  • Energy level diagrams can also show:
    • activation energy for a chemical reaction
    • how catalysts reduce the activation energy by providing an alternate pathway for the particles with a lower activation energy
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Calculations using bond energies

  • The energy needed to break the bond between two atoms is called the bond energy for that bond
  • An equal amount of energy is released when the bond forms between two atoms
  • Bond energies can be used to work out the overall energy change for a reaction
  • Measured in KJ/moles
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Fuel issues

  • Fossil fuels are non-renewable and cause pollution

Hydrogen can be used as a fuel and has advantages and disadvantages

  • Advantages
    • Burns easily with a clean flame
    • Only product formed when burned is water, no polluting gases
    • Releases large amounts of energy per gram
    • Renewable
  • Disadvantages
    • Storage - gas so has a large volume
    • Safety - risk of an explosion
    • Supply
  • Vehicles that use fuel cells need to match the cost, performance and convinience of petrol and diesel
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