CalorimetryAQA A-Level Chemistry: Revision notes

When a reaction happens in solution, the heat released or absorbed changes the temperature of the water around it. The heat change is calculated using

q = mcΔT

where q is the heat energy change in joules (J), m is the mass of the substance whose temperature changes in grams (g), c is its specific heat capacity in J g⁻¹ K⁻¹ and ΔT is the temperature change in kelvin (K). A change of 1 °C is the same as a change of 1 K. The specific heat capacity of water, 4.18 J g⁻¹ K⁻¹, is given in questions and need not be recalled.

The mass used is the mass of the solution or water that is heated, not the mass of the reactants added. For dilute solutions, assume the density is 1.00 g cm⁻³, so 50.0 cm³ has a mass of 50.0 g.

The molar enthalpy change ΔH is the heat change per mole of reaction, in kJ mol⁻¹. Follow these steps:

1. Calculate q = mcΔT in joules.
2. Convert to kJ by dividing by 1000.
3. Calculate the moles of the reactant that is not in excess (the limiting reagent), using n = c × V (V in dm³) or n = mass ÷ Mr.
4. Divide: ΔH = q ÷ n.
5. Add the sign. If the temperature rises the reaction is exothermic and ΔH is negative. If the temperature falls it is endothermic and ΔH is positive.

Worked example: 25.0 cm³ of 1.00 mol dm⁻³ HCl and 25.0 cm³ of 1.00 mol dm⁻³ NaOH mix and the temperature rises by 6.8 K. q = 50.0 × 4.18 × 6.8 = 1421 J. n(HCl) = 0.0250 mol. ΔH = −1.421 ÷ 0.0250 = −56.8 kJ mol⁻¹.

To find the enthalpy of combustion of a liquid fuel such as ethanol, the fuel is burned in a spirit burner below a metal (copper) calorimeter containing a known mass of water. The mass of the burner is measured before and after to find the mass of fuel burned, and the temperature rise of the water gives q.

The moles of fuel are n = mass burned ÷ Mr, and ΔcH = −q ÷ n.

Results are usually less exothermic than data book values because of:

• heat lost to the surroundings and absorbed by the calorimeter
• incomplete combustion (soot, carbon monoxide)
• evaporation of fuel from the wick
• non-standard conditions, as the data book value refers to water forming as a liquid under standard conditions

Neutralisation, displacement and dissolving reactions are carried out in a polystyrene cup with a lid, which is a good insulator and has a very low heat capacity. The reactants are measured with a pipette, burette or measuring cylinder and the solution is stirred.

The calculation assumes that:

• the solution has the same density (1.00 g cm⁻³) and specific heat capacity as water
• no heat is lost to the surroundings or absorbed by the cup
• the mass of the solid is negligible, or is included in m if stated

Because these assumptions are not fully true, the result is only approximate. Using a larger quantity of reactant gives a bigger temperature change, so the percentage uncertainty in ΔT is smaller.

In the required practical, you measure an enthalpy change by a temperature measurement, for example neutralisation, displacement of copper by zinc, or dissolving a salt.

1. Measure the volume of solution accurately and put it in the cup.
2. Record the temperature every 30 s for 2–3 minutes.
3. Add the second reactant at a known time, stir and keep recording the temperature.
4. Plot temperature against time. Draw lines of best fit through the points before and after mixing and extrapolate back to the time of mixing. ΔT is the difference between the lines at that time. This corrects for heat exchange with the surroundings.

The uncertainty in ΔT is double the reading uncertainty of the thermometer, since two readings are taken: two readings of ±0.1 °C give ±0.2 K. Percentage uncertainty = (0.2 ÷ ΔT) × 100.