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9476 / 2027
Data and Experimental Skills

Lesson 6 of 8

Plan an investigation

Specify a fair comparison and how it will answer the question.

A-Level 9476 (2026-2027)

  • I can plan an investigation that answers a defined question.Syllabus

    Syllabus 9476, P. Plan an investigation that answers a defined question

  • I can explain gas collection principles.Syllabus

    Syllabus 9476, T3. Gas collection principles

  • I can explain chemical kinetics.Syllabus

    Syllabus 9476, T5. Chemical kinetics

Plan a comparison that can answer the question

Specify variables, measurements, analysis and relevant precautions.

A plan should allow another student to carry out the comparison. Define the independent variable and a sensible range; define the dependent measurement; name important controls and how you will maintain them. Then describe apparatus, quantities, the order of operations, repeat measurements and the intended graph or calculation. A diagram helps where arrangement matters, but does not replace operational detail.

A starting dilution series: keep the total volume at 20.0 cm3
1.00 mol dm-3 HCl / cm3Water / cm3HCl concentration / mol dm-3
4.016.00.20
8.012.00.40
12.08.00.60
16.04.00.80
20.00.01.00

Worked exampleTurn a rate question into a usable plan

Compare the initial rate of Mg + 2HCl -> MgCl2 + H2 as acid concentration changes. Use the dilution series above and pieces cut from the same cleaned magnesium ribbon, each of mass about 0.024 g and the same dimensions.

Try it first. Then show the worked stepsHide the worked steps
  1. Check feasibility first. n(Mg) = 0.024 / 24.3 = 9.88 × 10-4 mol. It needs about 0.00198 mol HCl, while even the most dilute run contains 0.20 × 0.0200 = 0.00400 mol. Acid is in excess in every run. At room conditions the expected final hydrogen volume is about 23.7 cm3, which fits a 50 cm3 gas syringe.
  2. Measure stock acid and water with separate graduated apparatus into the same reaction flask. Use a water bath at 25 °C, allowing each solution to reach that temperature. Keep the magnesium surface preparation, exposed area, mixing and collection apparatus the same.
  3. Put the ribbon in a small dry inner tube so it is initially separate from the acid. Connect the stoppered flask to a freely moving gas syringe, check for leaks and note its initial reading. Tilt to mix without removing the stopper and start the timer at mixing.
  4. In a preliminary run, check that readings every 2 s resolve the early curve. If reaction is too fast, choose a consistently lower concentration range or use a suitable data logger, then keep the chosen method for all runs. Record gas volume against time through the initial region and towards a plateau.
  5. Repeat each concentration three times with fresh reactants. Draw volume-time curves and compare initial tangent gradients in cm3 s-1; plot mean initial rate against initial HCl concentration and show the spread. Do not use the final gas volume as the rate.
  6. Wear eye protection, keep hydrogen away from ignition sources and ensure the syringe can move freely. An airtight connection prevents gas loss; a blocked outlet creates pressure and invalidates the experiment.
Answer

The plan states quantities, controls, how the reaction starts, what is measured and how the evidence answers the question. The preliminary run tests whether the proposed range and recording interval are practical.

H2 students can reason about gas collection, but the syllabus says gas collection by displacement of water or gas syringe is not required as a hands-on examination task. Initial-rate methods can instead use other supplied signals, such as colour or concentration. Match your proposed method to the question rather than treating one collection device as compulsory.

For a fixed-endpoint timing method, 1/t can compare rates only when each experiment reaches the same small extent of reaction and the other relevant conditions are controlled. Doubling the endpoint amount while also changing concentration would confound the comparison.

Worked exampleA clock time needs a fixed chemical endpoint

In a supplied H2 iodine-clock method, the blue colour appears after 1.0 × 10-5 mol of iodine has formed. Runs of the same total volume reach this endpoint in 80 s and 40 s. Can their initial rates be compared?

Try it first. Then show the worked stepsHide the worked steps
  1. The mean iodine-production rates to the endpoint are (1.0 × 10-5)/80 = 1.25 × 10-7 mol s-1 and (1.0 × 10-5)/40 = 2.50 × 10-7 mol s-1.
  2. The second mean rate is twice the first. This approximates an initial-rate comparison only if little reactant is consumed before the endpoint, so concentrations and rate change negligibly during each timing.
  3. When diluting the reactant under investigation, replace its missing volume with water. Keep the amount of clock reagent, the other starting concentrations, temperature, mixing and colour criterion constant. Repeat timings and use the same start event.
  4. If the second run instead required twice as much iodine to reach the colour change, the same times would imply a fourfold mean rate, not a twofold rate. The endpoint condition is part of the calculation.
Answer

Use 1/t as a rate proxy only with the fixed-extent and small-conversion assumptions. A supplied clock method offers an alternative to gas collection.

Check your understanding

Why is "keep the temperature constant" a stronger control when the plan also says how?

  1. Temperature has little effect on rate, so the method matters for safety reasons.
  2. Examiners give marks for length, so a longer statement scores more highly.
  3. Saying how (a water bath, time to reach temperature) makes the control actually happen.
Show the answer

Saying how (a water bath, time to reach temperature) makes the control actually happen.

Temperature affects rate, so it must be controlled. Naming a method, such as a water bath with the solutions left to reach the set temperature, shows the control can be carried out.

Exam-style question

Planning question

FA 1 is sulfuric acid with a concentration of about 1.0 mol dm-3. FA 2 is 2.00 mol dm-3 sodium hydroxide. You are to plan a thermometric titration to find the exact concentration of FA 1. Neutralisation releases heat, so the temperature of the mixture changes as FA 2 is added.

H2SO4(aq) + 2NaOH(aq) -> Na2SO4(aq) + 2H2O(l)

You may use polystyrene cups, a 250 cm3 beaker, a 25.0 cm3 pipette, a 50.00 cm3 burette, a thermometer reading to 0.2 °C and normal laboratory apparatus.

  1. (a) Using 25.0 cm3 of FA 1, calculate the volume of FA 2 you expect to need to neutralise it. [2]
  2. (b) Write a plan for the thermometric titration. Give the apparatus and volumes, how you will reduce heat loss, and the readings you will take. [6]
  3. (c) Sketch the graph you expect and explain its shape. [2]
  4. (d) The two lines on a student's graph meet at 24.40 cm3 of FA 2. Calculate the concentration of FA 1. [2]
 [12]

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Exam-style question

Planning question

In acid, propanone reacts with iodine. The acid is a catalyst and is not used up.

CH3COCH3(aq) + I2(aq) -> CH3COCH2I(aq) + HI(aq)

You are to plan an experiment to find the order of reaction with respect to iodine. You are given 2.0 mol dm-3 propanone, 1.0 mol dm-3 sulfuric acid, 0.020 mol dm-3 iodine (dissolved in aqueous potassium iodide), 0.010 mol dm-3 sodium thiosulfate, solid sodium hydrogencarbonate, starch indicator and normal laboratory apparatus. Sodium hydrogencarbonate removes the acid, so it stops the reaction in a sample.

  1. (a) The reaction mixture is made from 25.0 cm3 of propanone, 25.0 cm3 of sulfuric acid and 50.0 cm3 of iodine. Calculate the initial concentrations of propanone and iodine in the mixture, and explain why these amounts let you find the order with respect to iodine alone. [2]
  2. (b) A 10.0 cm3 sample of the mixture is titrated against the thiosulfate. Calculate the titre at the start of the reaction. I2 + 2S2O32- -> 2I- + S4O62- [1]
  3. (c) Describe how you would carry out the experiment and collect the readings you need. [5]
  4. (d) Explain how you would use your results to find the order of reaction with respect to iodine. [2]
 [10]

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