Measuring and Calculating Enthalpy Changes

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Measuring Enthalpy Changes - Calorimetry

Calorimetry - Dissolving, Displacement and Neutralisation Reactions:

  • To find out the amount of energy transferred, you simply measure the temperature of the reactants and then measure the temperatre again after they have been mixed. 
  • The reaction is usually done in a polystyrene cup so that the heat does not escape easily.
  • Also, the cup can be put into a beaker of cotton wool to make sure that little heat escapes.

Calorimetry - Combustion:

  • To measure the energy changes in a combustion reaction, the heat from the burning is used to heat water and this is how the temperature change is recorded.
  • Record the weight of the spirit burner before lighting it.
  • Put 50ml of water in a copper container (copper is used as it is a good conductor) and record it's temperature.
  • Heat the water until it reaches 50degrees then weigh the spirit burner again.
  • This information can be used to calculate the enthalpy change.
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Calculating Enthalpy Changes

Calculating the Heat Energy Transferred:

  • This is used for the combustion experiment on the other side.
  • If you measure the amount of fuel that was burnt and the temperature change of the water, you can work out how much heat energy was given off by burning 1 gram of fuel. 
  • It takes 4.2joules of energy to heat 1 gram of water by 1 degree. 

Calculating the Molar Enthalpy Changes:

  • Once you've calculated the amount of energy transferred, you can calculate the molar enthalpy change (the amount of energy given out by one mole of the reactant).

1) Calculate the amount of energy transferred from the reaction.

2) Find out how many moles of fuel produced this heat.

3) Divide the amount of energy by the number of moles to work out how much energy is produced from one mole of fuel.

 

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