Skip to content
Oscar Song Tutoring
A-level Chemistry required practicals

Initial-rate and continuous-monitoring rate methods

AQA 3.1.9.2 · RP7

A-level Chemistry (7405) · Required practical 7 — 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 how changing one reactant concentration affects initial rate using both a fixed-endpoint clock method and continuous monitoring.

Apparatus

  • Reactant solutions, measuring cylinders or pipettes, and distilled water
  • Conical flask, bung and delivery tube where gas collection is used
  • Stopwatch and thermometer
  • Gas syringe, balance or suitable concentration probe/data logger for continuous monitoring
  • Eye protection

Apparatus & techniques (AT)

AT a · measurement

Measures reactant volumes, temperature, time and the monitored quantity at suitable precision.

AT k · safe handling

Handles reactive solutions and any evolved gas with suitable eye protection, containment and ventilation.

AT l · initial-rate and continuous-monitoring rate methods

Uses both a fixed-endpoint initial-rate method and continuous monitoring to determine initial rate.

Safety

Hazard

Reactive or irritant solutions may splash during rapid mixing.

Control

Wear eye protection, use small quantities and swirl with the flask directed away from people.

Hazard

A gas-producing reaction can pressurise a sealed apparatus.

Control

Use a freely moving gas syringe, check the delivery path is clear and never clamp the plunger.

Method

  1. 1Initial-rate clock method: prepare mixtures with different concentrations of one reactant while keeping total volume constant, equilibrate them to the same temperature, mix and start the stopwatch.
  2. 2Use the same clearly defined visual endpoint each time, stop the clock at that endpoint, repeat each concentration and use 1/time as the rate proxy.
  3. 3Continuous-monitoring method: repeat the concentration series while recording gas volume, mass or concentration at short, regular intervals from the moment the reactants mix.
  4. 4Use the same apparatus geometry and mixing procedure throughout; when monitoring mass loss, use a conical flask to limit spray and droplet loss while still allowing gas to escape.
  5. 5Repeat the full time series, identify anomalous runs and retain the raw readings as well as processed initial rates.

CPAC focus (editorial)

This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.

  • CPAC 2: Editorial focus: choose a rate method and concentration range that produce measurable, discriminating data.
  • CPAC 4: Editorial focus: record repeat clock times or a sufficiently dense continuous time series with units.
  • CPAC 5: Editorial focus: obtain initial rates, infer order and evaluate uncertainty without confusing rate with time.

Variables

Independent

Initial concentration of one reactant

Dependent

Initial rate, obtained from 1/time or the initial gradient of a quantity–time graph

Control

  • Concentrations and amounts of every other reactant
  • Total reaction volume and apparatus geometry
  • Temperature, mixing procedure and endpoint criterion

Results & processing

  • For the clock method, tabulate concentration, repeat times, mean time and 1/mean time with units of s−1.
  • For continuous monitoring, plot the measured quantity against time and draw a tangent at time zero; calculate its gradient with quantity and time units.
  • Plot initial rate against initial concentration and use proportional changes or graph shape to infer reaction order for the reactant varied.

Analysis skills

  • Calculate 1/time for a common endpoint and distinguish this rate proxy from elapsed time.
  • Draw and calculate an initial tangent on a quantity–time graph.
  • Use rate–concentration graphs or rate ratios to infer reaction order.

Uncertainty

Sources

  • Stopwatch resolution and reaction-time scatter
  • Subjective endpoint judgement
  • Choice of tangent and drift in temperature

Calculations

  • Use repeat range or standard spread to quantify timing scatter.
  • For a clock run, percentage timing uncertainty is absolute timing uncertainty divided by time × 100.
  • For continuous monitoring, compare plausible steepest and shallowest initial tangents.

Interpretation

  • A short clock time has a larger percentage timing uncertainty for the same absolute timing error.
  • Repeats expose random scatter but do not remove a biased endpoint or heat drift.

Exam angles

  • Explain why a conical flask reduces loss of liquid droplets during mass monitoring.
  • Complete the chain from concentration–time data to initial rates and then to a rate–concentration comparison.
  • State control variables that are not already fixed by the question and give tangent units correctly.

Where students lose marks

Treating the time to a fixed endpoint as the rate, so a slower reaction is reported as faster.

Fix: For a common endpoint, use 1/time as the rate proxy and state its units.

Using two close points on the initial tangent or omitting the measured-quantity unit from its gradient.

Fix: Choose widely separated points on the tangent, not the curve, and report the full gradient units.

  • Examiner report: C2-23 · PDF p. 3

Listing a variable already fixed by the method when asked how to reduce timing uncertainty.

Fix: Lengthen the measurable event without changing the chosen independent variable, then keep the endpoint rule unchanged.

  • Examiner report: C3-25 · PDF p. 4

Stopping after drawing a concentration–time graph instead of converting its initial gradients into a rate comparison.

Fix: Obtain an initial rate for every concentration, then plot or compare those rates against the corresponding initial concentrations.

  • Examiner report: C3-20 · PDF p. 5

Adding separate reading uncertainties to a value that the question already labels as the total uncertainty.

Fix: Read the uncertainty definition carefully and use a stated total once rather than counting its components again.

  • Examiner report: C3-25 · PDF p. 3

Improve the method

  • Use automated gas-volume, mass or concentration logging to obtain more points near time zero.
  • Use a thermostatically controlled water bath and confirm reactants have reached the set temperature before mixing.
  • Repeat each run and use the spread to judge random variation rather than relying only on a mean.

Source references

  • Specification: CSpec 3.1.9.2 · PDF p. 32

Try it — exam-style

Easy
ORIGINAL

An initial tangent on a gas-volume graph passes through (0.0 s, 0.0 cm3) and (24.0 s, 57.6 cm3). Calculate the initial rate.

[2 marks]

Total for this question: 2

Medium
ORIGINAL

A clock reaction has total volume 50.0 cm3. Using 10.0 cm3 of 0.200 mol dm−3 reactant gives a mean time of 80.0 s; using 20.0 cm3 gives 42.0 s. Water makes each mixture up to the 50.0 cm3 total and all other quantities are unchanged. Determine the two initial concentrations and use 1/time to suggest the order with respect to this reactant.

[4 marks]

Total for this question: 4

Questions are written in the style of past AQA papers — never copied from them.

Drill it properly

AQA 7405 specification coverage

Kinetics, rate equations and every 7405 content point.

Stuck on initial-rate and continuous-monitoring rate methods?

Rate questions reward a clean chain from raw readings to order — I drill every link. Free intro call, then a free first lesson.

Book a free intro call