50.0 cm3 of reacting solution, treated as 50.0 g with c = 4.18 J g−1 K−1, warms by 6.80 K. The limiting amount is 0.0250 mol. Calculate the molar enthalpy change.
[4 marks]
Total for this question: 4
A-level Chemistry required practicals
Measuring an enthalpy change
AQA 3.1.4.2 · RP2
A-level Chemistry (7405) · Required practical 2 — method, techniques, safety, analysis and uncertainty. Includes errors documented in examiner reports.
Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works
Measure the temperature change for a chemical process and use calorimetry to determine its molar enthalpy change.
Apparatus
- Insulated cup calorimeter with lid and support beaker
- Thermometer or temperature probe
- Measuring cylinders or volumetric pipettes
- Balance and weighing container where a solid is used
- Stirring rod and stopwatch
- Reactants for a neutralisation, dissolution or displacement reaction
Apparatus & techniques (AT)
AT a · measurement
Measure reactant quantities, time and temperature precisely enough to calculate energy transferred per mole.
AT d · titration/distillation/reflux/qualitative tests/filtration
Use a burette or pipette to transfer known solution quantities into the calorimeter.
AT k · safe handling
Handle corrosive or harmful reactants safely and keep the calorimeter stable while mixing and measuring.
Safety
Hazard
Acids, alkalis and some metal-ion solutions used in calorimetry may be corrosive or harmful.
Control
Wear eye protection, use small quantities, consult the reagent-specific risk assessment and clean spills promptly.
Hazard
An exothermic mixture can become hot and may splash during rapid addition or stirring.
Control
Support the cup in a beaker, add reactants carefully, replace the loose lid and stir without sealing the vessel.
Method
- 1Place an insulated cup in a support beaker and fit a lid with openings for the thermometer and stirrer.
- 2Measure the reactant quantities; where two solutions are used, check or record both starting temperatures.
- 3Record the temperature at fixed intervals for 3–4 minutes before adding the second reactant to establish the baseline trend.
- 4Combine the reactants, replace the lid promptly, start timing and stir throughout without spilling.
- 5Continue recording temperature at the same intervals through the rise or fall and for long enough afterwards to establish the post-mixing trend.
- 6Repeat using the same quantities and starting conditions when repeatability is being assessed.
CPAC focus (editorial)
This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.
- CPAC 2: Editorial focus: select reactant quantities and insulation that give a measurable temperature change without unsafe heating.
- CPAC 4: Editorial focus: record temperature against time at fixed intervals and preserve the raw maximum or minimum readings.
- CPAC 5: Editorial focus: identify the limiting amount, process q = mcDeltaT and report molar enthalpy with sign and units.
Variables
Independent
Reaction investigated or the selected reactant quantity
Dependent
Temperature change of the calorimeter contents
Control
- Reactant concentrations and total solution volume
- Insulation, lid and stirring method
- Starting temperature and timing intervals
Results & processing
- Tabulate time and temperature, including readings before mixing and after the observed maximum or minimum.
- Use the mass of material actually heated in q = mcDeltaT; for dilute aqueous mixtures, the total solution mass may be estimated from volume and density when justified.
- Divide the energy change by the reacting amount in moles and apply the chemical-system sign: an exothermic reaction has a negative enthalpy change.
Analysis skills
- Plot temperature against time and extrapolate the post-mixing trend back to the mixing time to estimate a corrected temperature change.
- Calculate q = mcDeltaT, convert joules to kilojoules and divide by the moles of the limiting reactant as defined by the equation.
- Distinguish experimental heat loss from a theoretical-model limitation such as assumptions in a lattice-enthalpy cycle.
Uncertainty
Sources
- Heat exchange with the cup, thermometer and surroundings makes the measured temperature change smaller in magnitude.
- Thermometer resolution, unequal starting temperatures, incomplete transfer and imperfect stirring add measurement variation.
Calculations
- A temperature change uses two readings, so a stated single-reading absolute uncertainty contributes twice to the maximum uncertainty in DeltaT.
- Combine percentage uncertainties for measured quantities in q and then include the uncertainty in the reacting amount when reporting DeltaH.
Interpretation
- Heat loss usually makes an exothermic enthalpy value less negative and an endothermic value less positive than the ideal magnitude.
- Repeats estimate random scatter but do not remove heat loss shared by every run.
Exam angles
- Select the correct heated mass and reacting amount for neutralisation, dissolution or displacement calorimetry.
- Use a temperature–time graph to obtain a corrected DeltaT and explain the direction of the heat-loss bias.
- Calculate DeltaH with the correct sign and units, including limiting-reagent stoichiometry.
- Evaluate whether a proposed change genuinely increases DeltaT or only repeats the same systematic limitation.
Where students lose marks
Using only the mass of one reactant when the whole solution is heated.
Fix: Identify the complete heated mass; for mixed dilute solutions this is normally based on their total volume and an explicitly stated density assumption.
- Examiner report: C2-23 · PDF p. 5
Using a one-reading thermometer uncertainty for a temperature difference.
Fix: DeltaT is calculated from two temperature readings, so include both reading uncertainties in the maximum absolute uncertainty.
- Examiner report: C3-18 · PDF p. 7
Reporting a positive value for an exothermic reaction because q for the solution is positive.
Fix: The solution gains heat, so the reacting chemical system loses it: DeltaH for the exothermic reaction is negative.
Improve the method
- Use a secure lid and better insulation, and mix promptly so less energy is exchanged before the temperature change is measured.
- Record a temperature–time series and extrapolate to the mixing time instead of taking only the observed maximum or minimum.
- Choose safe quantities that increase DeltaT, reducing its percentage uncertainty, and keep track of the reacting moles the calculation divides by.
Source references
- Specification: CSpec 3.1.4.2 · PDF p. 22
Try it — exam-style
Medium
ORIGINAL
Hard
ORIGINAL
A reaction is mixed at 120 s. Explain how a temperature–time graph can give a better estimate of the temperature change than simply using the highest recorded temperature.
[3 marks]
Total for this question: 3
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
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