Mean endpoint times at 293 K and 323 K are 80.0 s and 21.0 s. Calculate the rate proxy 1/t at 323 K and the factor by which the rate proxy has increased from 293 K.
[3 marks]
Total for this question: 3
A-level Chemistry required practicals
Effect of temperature on reaction rate
AQA 3.1.5.3 · RP3
A-level Chemistry (7405) · Required practical 3 — 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
Investigate how temperature affects reaction rate by timing a fixed, reproducible visual endpoint.
Apparatus
- Thermostatically controlled water bath or beakers used as water baths
- Thermometer or temperature probe
- Conical flask and separate reactant containers
- Volumetric pipettes, measuring cylinders or syringes
- Stopwatch and a fixed visual marker or cross
- Reactants that produce a visible endpoint, such as a suspension
Apparatus & techniques (AT)
AT a · measurement
Measure fixed reactant volumes, temperature and endpoint time consistently across the full temperature range.
AT b · controlled heating
Use a controlled water bath to bring separate reactant portions to the target temperature without direct flame heating.
AT k · safe handling
Handle irritant reactants and warm glassware safely, using small quantities and suitable ventilation.
Safety
Hazard
Warm water and glassware can cause minor burns, and rapid heating can create uneven reactant temperatures.
Control
Use a controlled water bath, keep temperatures within the assessed range and handle warmed vessels carefully.
Hazard
Sodium thiosulfate reacting with hydrochloric acid releases toxic sulfur dioxide, which irritates the airways and can trigger asthma.
Control
Wear eye protection, use small quantities in a well-ventilated room or fume cupboard, and ensure asthma sufferers are not exposed to sulfur dioxide.
Hazard
Acidic or irritant reagents used in other rate contexts can damage eyes and skin.
Control
Wear eye protection, use small measured volumes and rinse any splashes immediately with plenty of water.
Method
- 1Measure fixed portions of the reactants into separate labelled containers.
- 2Place both portions in the same water bath and allow enough time for each to reach the selected temperature; check rather than assume equilibrium.
- 3Combine the portions in the reaction flask, start the stopwatch at the same defined mixing point and mix in the same way each time.
- 4View the flask from a fixed position and stop timing when the same visual endpoint is reached, such as a marker just becoming obscured.
- 5Repeat at that temperature, then collect results at several well-spaced temperatures while keeping concentrations, volumes and viewing geometry unchanged.
CPAC focus (editorial)
This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.
- CPAC 2: Editorial focus: vary only temperature and design a fixed endpoint whose timing can be compared fairly.
- CPAC 3: Editorial focus: control warm-water and reagent hazards while preserving a workable timing method.
- CPAC 4: Editorial focus: record target and measured temperatures, repeat endpoint times and any anomalous observations.
- CPAC 5: Editorial focus: convert time to a rate proxy, choose suitable graph axes and interpret limitations in that proxy.
Variables
Independent
Temperature of the reaction mixture
Dependent
Time to a fixed visual endpoint, processed as a rate proxy 1/t
Control
- Reactant concentrations and volumes
- Total liquid depth and reaction vessel
- Mixing, viewing position, marker and endpoint judgement
Results & processing
- Record the measured reaction temperature and repeat endpoint times, then calculate a mean time after investigating any anomaly.
- Use 1/t as a rate proxy only because every run reaches the same fixed extent of reaction.
- Plot 1/t against temperature; a faster reaction gives a larger rate proxy, not merely a smaller plotted time.
Analysis skills
- Calculate mean endpoint time, 1/t and an appropriate spread such as half the range for repeated timings.
- Plot rate proxy against temperature with sensible scales and identify the expected increase in rate.
- Use the Maxwell–Boltzmann distribution to explain why a small temperature rise causes a large increase in the fraction of molecules with energy at least equal to the activation energy, producing a large rate increase.
- Where the dataset supports it, plot ln(rate) against 1/T in kelvin and use gradient = -Ea/R to obtain activation energy.
Uncertainty
Sources
- Reaction and stopwatch start may not coincide exactly, especially while the mixture is being poured and mixed.
- The visual endpoint is subjective, and the reacting mixture may drift away from the water-bath temperature after mixing.
Calculations
- Estimate percentage timing uncertainty as absolute timing uncertainty divided by the measured time, multiplied by 100.
- For repeat times, use their spread to quantify random variation and propagate the time uncertainty into 1/t where required.
Interpretation
- Short high-temperature runs have the largest percentage timing uncertainty for the same reaction-time delay.
- Repeats reveal random endpoint scatter but do not remove a consistent viewing bias or thermal drift.
Exam angles
- Write a repeatable method that equilibrates both reactants and defines exactly when timing starts and stops.
- Identify valid controls and explain why 1/t represents rate only for a common fixed endpoint.
- Use a Maxwell–Boltzmann distribution to explain why a small temperature rise can produce a large increase in reaction rate.
- Choose graph scales, interpret the sign of a gradient and calculate activation energy from an Arrhenius plot when supplied.
- Suggest an uncertainty reduction that addresses timing or endpoint judgement rather than a variable already controlled.
Where students lose marks
Describing an elaborate water-bath arrangement without stating how both reactants reach a known common temperature.
Fix: Give a repeatable baseline: equilibrate separate measured portions, verify the temperature, then mix and start timing at a defined point.
- Examiner report: C3-19 · PDF p. 4
Plotting time as if it were rate or giving a negative gradient for an increasing rate–temperature graph.
Fix: For a fixed endpoint use rate proportional to 1/t; label the transformed axis and interpret its direction before finding a gradient.
- Examiner report: C2-25 · PDF p. 8
Claiming that repeating the experiment removes the subjective endpoint error.
Fix: Repeats quantify random spread; use a sensor-based endpoint or fixed observer and geometry to reduce systematic judgement differences.
Improve the method
- Use a thermostatically controlled bath and confirm both reactant portions have reached the target temperature before mixing.
- Use the same observer and fixed viewing geometry, or replace the visual judgement with a suitable light sensor when available.
- Choose concentrations that lengthen very short runs without changing temperature as the intended independent variable.
- Repeat each temperature in a varied order to help reveal drift during a long session.
Source references
- Specification: CSpec 3.1.5.3 · PDF p. 24
Try it — exam-style
Medium
ORIGINAL
Hard
ORIGINAL
A graph of ln(rate) against 1/T has gradient -5.20 x 103 K. Use gradient = -Ea/R and R = 8.31 J mol−1 K−1 to calculate Ea in kJ mol−1.
[3 marks]
Total for this question: 3
Questions are written in the style of past AQA papers — never copied from them.
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