Calorimetry (Edexcel IGCSE Chemistry)

• We can experimentally determine the relative amounts of energy released by a fuel 
• We do this using simple calorimetry
• There are two types of calorimetry experiments you need to know:
  • Enthalpy changes of reactions in solution
  • Enthalpy changes of combustion 

Reactions in solution

• To calculate the amount of energy produced by a chemical reaction in solution we measure the temperature change when the solutions are mixed together
• The solutions need to be mixed together in an insulated container to prevent heat loss
• This method can be used for:
  • Neutralisation reactions
  • Dissolving solids in water
  • Displacement reactions

• For the purposes of the calculations, some assumptions are made about the experiment:
  • That the specific heat capacity of the solution is the same as pure water, i.e. 4.18 J/g/°C
  • That the density of the solution is the same as pure water, i.e. 1 g/cm³
  • The specific heat capacity of the container is ignored
  • The reaction is complete
  • There are negligible heat losses

• A calorimeter can be made up of a polystyrene drinking cup, a vacuum flask or metal can

A simple calorimeter 

A polystyrene cup can act as a calorimeter to find enthalpy changes in a chemical reaction

• Method:
  1. A fixed volume of one reagent is added to the calorimeter and the initial temperature taken with a thermometer
  2. An excess amount of the second reagent is added and the solution is stirred continuously
  3. The maximum temperature is recorded and the temperature rise calculated
     
     
• The energy released would be calculated using:

Q = m x c x ΔT

• • Q = the heat energy change, J
  • m = the mass of the substance being heated, g
  • c = the specific heat capacity, J/g/°C
  • ΔT = the temperature change, °C

Enthalpy of combustion experiments

• The principle here is to use the heat released by a combustion reaction to increase the heat content of water
• A typical simple calorimeter is used to measure the temperature changes to the water

Diagram to show the set up of calorimetry equipment

A lid is used to prevent heat loss

• The steps are: 
  1. Measure a fixed volume of water into a copper can
  2. Weigh the spirit burner containing  a fuel using a balance
  3. Measure the initial temperature of the water
  4. Burn the fuel and stir the water 
  5. Wait until the temperature has risen by approximately 20 ^oC and extinguish the flame
  6. Record the final temperature of the water and re-weigh the spirit burner
     
     
• To calculate the energy released by the fuel we can use the data obtained from the experiment above and the specific heat capacity of water 
• The specific heat capacity, c, is the energy needed to raise the temperature of 1 g of a substance by 1 °C
  • The specific heat capacity of water is 4.18 J/g/°C
• The heat energy change is calculated using:

Q = m x c x ΔT

• • Q = the heat energy change, J
  • m = the mass of the substance being heated, g
  • c = the specific heat capacity, J/g/°C
  • ΔT = the temperature change, °C

Sources of error

• Not all the heat produced by the combustion reaction is transferred to the water
  • Some heat is lost to the surroundings
  • Some heat is absorbed by the calorimeter

• To minimise the heat losses the copper calorimeter should not be placed too far above the flame and a lid placed over the calorimeter
• Shielding can be used to reduce draughts
• In this experiment the main sources of error are
  • Heat losses
  • Incomplete combustion