Calorimetry (Oxford AQA International A Level Chemistry)

Calorimetry

• The process of calorimetry is used to measure enthalpy changes 

Measuring enthalpy changes of combustion

• The apparatus below is used to find the enthalpy change when a fuel burns

• 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:

• The symbols / letters are:

  • 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

• We can compare the amount of energy released per gram and per mole for different fuels

• In both cases, the energy released (q) is calculated first

• To calculate the energy released per gram of fuel:

energy released per gram= energy released / mass of fuel burned 

• To calculate the energy released per mole of fuel:

energy released per mole= energy released / number of moles

Enthalpy changes of reactions in solutions

• 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

  • Displacement reaction

Equipment Set up

• 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

• Again, the energy released would be calculated using:

• We assume that the density of the solution is the same as pure water, i.e. 1 g/cm³

• To calculate the energy released per mole, kJ mol^-1:

Cooling curves

• For reactions which are not instantaneous there may be a delay before the maximum temperature is reached

• During that delay the substances themselves may be losing heat to the surroundings and would lead to inaccurately calculated enthalpy changes

• To overcome this problem we can produce a cooling curve:

  1. Place one reactant in the polystyrene cup

  2. Take the temperature of the reactant every minute for approximately 4 minutes

  3. Add the second reactant and continue recording the temperature every minute for approximately 6 minutes

  4. Plot the graph of temperature against time

  5. Extrapolate the cooling part of the graph until you intersect the time at which the second reactant was added