Calorimetry
- Calorimetry is a technique used to measure changes in enthalpy of chemical reactions
- There are two types of calorimetry experiments you need to know:
- Enthalpy changes of reactions in solution
- Enthalpy changes of combustion
Reactions in solution
- The principle of these calorimetry experiments is to carry out the reaction with an excess of one reagent and measure the temperature change over the course of a few minutes
- This technique is suitable for reaction such as dissolving, displacement and neutralisation
- 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 polystyrene cup can act as a calorimeter to find enthalpy changes in a chemical reaction
- 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
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
A simple combustion calorimeter
- A fixed volume of water is measured and transferred to the copper can
- The spirit burner containing, for example, an alcohol, is weighed on a 2 d.p. balance
- The initial temperature of the water is measured and recorded
- The alcohol is ignited and the water is stirred continuously
- When the temperature has risen by about 20 degrees the flame is extinguished and the spirit burner is re-weighed
- The final temperature of the water is recorded
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
Exam 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.