# GCSE C3 Chapter 3

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• Created by: emma998
• Created on: 16-03-14 10:37

## 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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