Measuring Enthalpy Changes
Quick Notes
- Enthalpy changes are measured using calorimetry, which determines the heat energy transferred in a reaction by measuring the temperature change of the reactions surroundings.
- The heat energy change equation: q = mcΔT
where:- q = heat energy (Joules)
- m = mass of the substance heated (g)
- c = specific heat capacity (J g−1 K−1) (for water, c = 4.18 J g−1 K−1)
- ΔT = temperature change (K)
- The enthalpy change per mole is calculated using: ΔH = −q / n
where n = moles of the limiting reactant. - Types of calorimetry experiments include:
- Combustion – burning a fuel to heat water.
- Neutralisation – measuring the temperature rise when acid and alkali react.
- Solution – dissolving a solid in water and recording temperature change.
- Common sources of error include heat loss to surroundings and incomplete combustion.
Full Notes
Calorimetry has been outlined with more background theory
here.
This page is just what you need to know for AQA A-level Chemistry :)
Calorimetry is an experimental technique used to measure enthalpy changes.
The key equation used is:
The enthalpy change per mole of reactant is found using:
ΔH = -q / n
where n = moles of the limiting reactant.
Don't forget that the sign of enthalpy change (ΔH) is opposite to the sign of heat change (q).
For example, if q is positive and temperature of surroundings of increases, this means ΔH must be negative.
This is why -q is used in ΔH = -q / n
Measuring Enthalpy Change of Combustion (ΔHc)
The enthalpy of combustion is the energy change when one mole of a substance burns completely in oxygen.
Method:
- Measure a known volume of water in a calorimeter (beaker or copper can).
- Record the starting temperature of the water.
- Weigh the spirit burner containing the fuel.
- Light the burner and allow it to heat the water.
- Stir and measure the final temperature of the water.
- Reweigh the burner to determine mass of fuel burned.
- Calculate q using q = mcΔT, then use ΔH = -q / n.
where n is moles of fuel used
Sources of Error:
- Heat loss to surroundings (e.g., air, beaker).
- Incomplete combustion (producing CO instead of CO2).
- Evaporation of fuel from the wick.
Measuring Enthalpy Change of Neutralisation (ΔHneut)
The enthalpy of neutralisation is the energy change when one mole of water is formed from an acid-alkali reaction.
Method:
- Use a polystyrene cup (to reduce heat loss).
- Add a known volume of acid and record the starting temperature.
- Add a known volume of alkali, stir, and record the maximum temperature.
- Use q = mcΔT to calculate heat energy change, then use ΔH = -q / n.
where n is moles of water produced
Sources of Error:
- Heat loss to surroundings.
- Assumption that the solution has the same specific heat capacity as water.
Measuring Enthalpy Change of Solution (ΔHsol)
The enthalpy of solution is the energy change when one mole of a substance dissolves in a solvent.
Method:
- Add a known mass of solute to a known volume of water in a polystyrene cup.
- Stir and record the temperature change.
- Use q = mcΔT to calculate the heat energy change, then use ΔH = -q / n
where n is moles of substance dissolved.
Sources of Error:
- Heat loss to surroundings.
- Incomplete dissolution (dissolving) of solute.
4. Example Calculation for Enthalpy of Combustion
Calculate ΔHc of Ethanol (C2H5OH) given the following data:
C2H5OH + 3O2 → 2CO2 + 3H2O
- Calculate q: q = mcΔT = 100 × 4.18 × 20 = 8 360 J = 8.36 kJ
- Find moles of ethanol: Mr(C2H5OH) = 46 g mol−1 → n = 0.5 ÷ 46 = 0.01087 mol
- Calculate ΔHc: ΔH = q ÷ n = 8.36 ÷ 0.01087 = −769 kJ mol−1
A student burns 0.5 g of ethanol and heats 100 g of water by 20°C.
Final Answer: ΔHc = −769 kJ mol−1 (exothermic).
Common Errors and Improvements in Calorimetry
| Error | Cause | How to Improve |
|---|---|---|
| Heat loss to surroundings | Heat escapes before being absorbed | Use insulated calorimeter (polystyrene cup, lid) for solutions; use copper calorimeter and windshield for combustion |
| Incomplete combustion | Not enough oxygen → CO, soot forms | Use pure oxygen instead of air |
| Evaporation of fuel | Some fuel evaporates before burning | Cover the burner when not in use |
| Assumption that c = 4.18 J g−1 K−1 | Some solutions have different heat capacities | Use precise values for each substance |
Summary Table of Measuring Enthalpy Changes
| Type of Enthalpy Change | Experiment Setup | Common Errors |
|---|---|---|
| Combustion (ΔHc) | Burn fuel under a beaker of water | Heat loss, incomplete combustion |
| Neutralisation (ΔHneut) | Mix acid and alkali in a polystyrene cup | Heat loss, inaccurate specific heat capacity |
| Solution (ΔHsol) | Dissolve a solid in water | Heat loss, incomplete dissolution |