CalorimetryEdexcel International A Level Chemistry: Revision notes
Section 1
Energy transferred: q = mcΔT
In calorimetry the heat released or absorbed by a reaction is measured by the temperature change of a known mass of water or solution.
q = m × c × ΔT
- q: energy transferred, in J
- m: mass in g of water or solution (for dilute solutions, 1.00 cm³ ≈ 1.00 g)
- c: specific heat capacity, 4.18 J g⁻¹ °C⁻¹ for water
- ΔT: temperature change in °C
Divide by 1000 to convert J to kJ.
Using only the volume of one reagent for m. m is the mass of all the solution being heated, e.g. 50 + 50 = 100 g.
Section 2
From energy to enthalpy change
ΔH = q ÷ n, in kJ mol⁻¹, where n is the amount (in mol) of the limiting reagent, or the substance whose enthalpy change is wanted.
The sign comes from the temperature change: temperature rises → exothermic → ΔH negative; temperature falls → endothermic → ΔH positive.
Worked example: 100.0 g of solution rises 6.8 °C; n(H₂O) = 0.0500 mol. q = 100.0 × 4.18 × 6.8 = 2842 J = 2.842 kJ. ΔH = –2.842 ÷ 0.0500 = –56.8 kJ mol⁻¹.
Forgetting to convert J to kJ before dividing by moles, or leaving off the negative sign.
Section 3
Experiments in an insulated container
For reactions in solution (e.g. neutralisation, displacement, dissolving) the reagents are mixed in an insulated container, a polystyrene cup with a lid, which is a poor conductor so less heat is lost. The mixture is stirred and the temperature is measured with a thermometer.
In a temperature–time experiment, the temperature is measured at regular intervals before and after mixing.
Section 4
Graphs and cooling-curve corrections
Heat is lost to the surroundings while the reaction happens, so the highest recorded temperature is too low.
- Record the temperature for a few minutes before mixing.
- Mix at a recorded time (without a reading at that moment) and keep recording.
- Plot temperature against time and draw a line of best fit through the cooling points.
- Extrapolate the cooling line back to the time of mixing.
- ΔT = extrapolated temperature at the time of mixing – initial temperature.
Take ΔT from the extrapolated line at the mixing time, not the highest point on the graph.
Section 5
Combustion experiments
To find an enthalpy change of combustion, a spirit burner containing the fuel heats a known mass of water in a copper can.
- Weigh the burner before and after: mass of fuel burned = difference.
- n = mass burned ÷ Mr.
- q = mass of water × 4.18 × ΔT.
- ΔcH = –q ÷ n.
Results are usually much less exothermic than data-book values because of heat loss, incomplete combustion and evaporation of fuel. A draught shield and a closer flame reduce losses.
Section 6
Evaluating: errors, uncertainty and assumptions
Systematic errors (consistently the same direction), e.g. heat loss, incomplete combustion, the heat capacity of the can ignored.
Assumptions: density of solution is 1.00 g cm⁻³, c = 4.18 J g⁻¹ °C⁻¹ as for water, no heat is lost, the reaction goes to completion.
Uncertainty: % uncertainty = (absolute uncertainty ÷ measurement) × 100. A temperature change uses two readings, so the uncertainties add: for a thermometer with ±0.5 °C, ΔT has ±1.0 °C. Add percentage uncertainties for the total.
Compare the total percentage uncertainty with the percentage difference from the data-book value: if the difference is far larger, systematic error is the cause.
Larger temperature rises and larger masses of fuel burned reduce the percentage uncertainty.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Calorimetry
- A student measures the enthalpy change of neutralisation. She mixes 50.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid with 50.0 cm³ of 1.00 mol dm⁻³ sodium hydroxide solution in a polystyrene cup with a lid, stirring gently. Both solutions start at the same temperature and the highest temperature reached is 6.8 °C higher. Assume that the solution has density 1.00 g cm⁻³ and specific heat capacity 4.18 J g⁻¹ °C⁻¹.Calculate the energy transferred to the solution, in J, and state one assumption made in your calculation.2 marks
- A student determines the standard enthalpy change of combustion of ethanol, C₂H₅OH (Mr = 46.0). She heats 200.0 g of water in a copper can using a spirit burner and measures a rise in temperature of 25.0 °C. The mass of the burner falls by 0.92 g. The specific heat capacity of water is 4.18 J g⁻¹ °C⁻¹. The data-book value for ethanol is –1367 kJ mol⁻¹.The experimental value is less exothermic than the data-book value. Suggest two reasons for this.2 marks
- A student measures the enthalpy change for the reaction Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). She puts 50.0 cm³ of 0.200 mol dm⁻³ copper(II) sulfate solution in a polystyrene cup and records its temperature every 30 seconds for 2 minutes. At 2.5 minutes she adds an excess of zinc powder (without taking a reading) and carries on recording the temperature every 30 seconds until 8 minutes, stirring throughout. She plots temperature against time.Explain why a cooling-curve correction is needed and describe how the student uses her graph to find the correct temperature change.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).