Two acids, two learners, one unfair comparison
Two learners compare hydrochloric acid and ethanoic acid, each 0.50 mol/dm³, reacting with marble chips (calcium carbonate). Each uses 20 cm³ of acid at 22 °C with excess marble, collecting gas in a 250 cm³ syringe. Learner A uses chips of about 5 mm; Learner B uses powdered marble. B's syringe reaches 60 cm³ after 15 s; A's reaches 60 cm³ after 55 s. Both stop at 120 cm³. B concludes that ethanoic acid reacts faster than hydrochloric acid. Neither learner repeats the experiment, and the room warms by 3 °C during B's run.
1. Identify two variables that were not controlled and explain, for each, the direction in which it changes the result.
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Surface area: powder exposes far more carbonate to the acid than 5 mm chips, so collisions between acid particles and the solid are much more frequent and gas is produced faster. A time of 15 s is expected on particle size alone, whichever acid is used.
Temperature: B's run was 3 °C warmer, and reaction rate rises with temperature because a larger fraction of particles have enough energy to react on collision. This again favours B.
Both uncontrolled variables push the result in the same direction as B's conclusion, so the experiment cannot separate the effect of the acid from the effects of particle size and temperature. The 55 s versus 15 s difference is at least partly, and possibly entirely, caused by the method rather than by the identity of the acid.
2. Both syringes stopped at 120 cm³. Explain why, showing the calculation.
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The acid is the limiting reactant. The marble is in excess, so the reaction stops when the acid is used up.
n(acid) = c × V = 0.50 mol/dm³ × 0.020 dm³ = 0.010 mol
Both reactions use two moles of acid per mole of carbon dioxide: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂ 2CH₃COOH + CaCO₃ → (CH₃COO)₂Ca + H₂O + CO₂
So n(CO₂) = 0.010 / 2 = 0.005 mol V = 0.005 mol × 24 000 cm³/mol = 120 cm³
The identical final volume proves only that equal amounts of acid were used; it says nothing about speed. It is a useful check that the two runs are comparable in quantity, which makes a rate comparison meaningful rather than pointless.
3. Describe how you would redo the comparison so that the conclusion is trustworthy.
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Use the same mass of marble of the same particle size for both acids - sieve chips to 4–5 mm and weigh 5.0 g for each. Stand both flasks in a water bath at 25 °C for ten minutes so the acids start at the same temperature, and record the temperature at the start and the end.
Use the same volume, 20 cm³, of acid at the same concentration, 0.50 mol/dm³. Measure the time to collect a fixed small volume, say 20 cm³, long before the acid runs out; this gives the initial rate rather than an average over the whole reaction.
Repeat each acid at least three times and compare mean times. If the ranges overlap, the difference is not real. Only then can a difference be attributed to the acid itself: the stronger acid should give the shorter mean time because it is fully ionised and so has a higher concentration of hydrogen ions.