Evaluating A2 experiments and explaining unexpected results

Explaining unexpected results

• Autocatalysis: the rate of MnO4- with ethanedioate increases at first, because Mn2+ produced is a catalyst. So a concentration-time graph gets steeper before it levels off, unlike most reactions (January 2022 paper).
• Air oxidation: Fe2+ solutions left standing give smaller manganate(VII) titres; Fe(OH)2 and Mn(OH)2 precipitates darken.
• Water uptake: anhydrous or wet products weigh more than expected.
• Assumptions failing: Ka from pH assumes negligible dissociation; calorimetry assumes no heat loss; 1/t assumes little reaction has occurred.

Evaluating a method

For each problem, say:

1. what it is;

2. which measured quantity it affects and in which direction;

3. how it changes the final result;

4. a specific improvement.

Example: 'The pH meter was not calibrated, so every pH could be off by the same amount; pKa would have a systematic error. Calibrate with pH 4 and pH 7 buffers first.'

Comparing with accepted values

Calculate the percentage error and compare it with the total percentage uncertainty. If the error is much bigger, look for a systematic cause and explain it with chemistry. Do not suggest 'repeat' as the fix for a systematic error.

Worked example

The rate of reaction between MnO4- and C2O4²- at point A (early) is lower than at point B (later), although the concentrations are lower at B. Explain.

1. Normally rate falls as concentrations fall.
2. Here Mn2+ is a product and a catalyst for the reaction.
3. As Mn2+ builds up, the rate increases despite the lower reactant concentrations: autocatalysis.

Answer: Autocatalysis by Mn2+ makes the later rate higher.

Check yourself

1. Which feature of a concentration-time graph shows autocatalysis?
   • The gradient becomes steeper after the start before the curve levels off
   • The gradient is steepest at the start and decreases steadily
   • The line is straight
   • Concentration increases with time
   Show answer

   The gradient becomes steeper after the start before the curve levels off. The reaction speeds up as the catalyst forms.

2. An iron(II) solution was left in an open beaker for a day before titration with manganate(VII). How will the titre compare with a fresh sample?
   • Larger, because Fe3+ also reacts with manganate(VII)
   • The same
   • Smaller, because some Fe2+ has been oxidised by air to Fe3+
   • Zero, because all the iron has precipitated
   Show answer

   Smaller, because some Fe2+ has been oxidised by air to Fe3+. Only Fe2+ reacts with MnO4-.

3. Ka is calculated from the pH of a weak acid solution using Ka = [H+]² / c. Which assumption does this make?
   • The amount of acid that dissociates is negligible compared with c
   • The acid is fully dissociated
   • The solution is a buffer
   • The pH meter reads 7 in water
   Show answer

   The amount of acid that dissociates is negligible compared with c. [HA] at equilibrium is taken as c.

4. A cell's measured emf is lower than the calculated E°cell. Which explanation is chemically reasonable?
   • The voltmeter has too high a resistance
   • The salt bridge contains potassium nitrate
   • The electrodes were cleaned
   • The concentration of ions in the positive half-cell is below 1.00 mol dm^-3
   Show answer

   The concentration of ions in the positive half-cell is below 1.00 mol dm^-3. Lower concentration in the positive half-cell lowers its potential.

5. An activation energy is found to be 48 kJ mol^-1. The accepted value is 53 kJ mol^-1. Calculate the percentage error.
   Show answer

   9.4 %. (53 - 48) / 53 x 100 = 9.4%.

6. A percentage error is much larger than the total apparatus uncertainty. Which response is best?
   • Repeat the experiment more times
   • Use a burette with finer divisions
   • Round the answer to fewer significant figures
   • Identify and remove the systematic error in the method
   Show answer

   Identify and remove the systematic error in the method. Repeats do not fix systematic errors.

7. Suggest a specific improvement for a kinetics experiment in which the room temperature changed during the session.
   Show answer

   Carry out each run in a thermostatically controlled water bath at a fixed temperature (and record the temperature of the mixture).. Temperature affects rate, so it must be controlled.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson IAL Chemistry teacher practical guide
• Pearson examiners' report, WCH16 October 2024
• Pearson WCH16 January 2022 question paper (unused)

Last updated 8 November 2026. Spotted a mistake? Tell us.