Calorimetry
- 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:
- A fixed volume of one reagent is added to the calorimeter and the initial temperature taken with a thermometer
- An excess amount of the second reagent is added and the solution is stirred continuously
- 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:
- Measure a fixed volume of water into a copper can
- Weigh the spirit burner containing a fuel using a balance
- Measure the initial temperature of the water
- Burn the fuel and stir the water
- Wait until the temperature has risen by approximately 20 oC and extinguish the flame
- 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
Examiner Tip
For both types of calorimetry experiment you should be able to give an outline of the experiment and be able to process experimental data.