Marble chips, powder and a misleading rate experiment
Dev is studying how the concentration of hydrochloric acid affects the rate of reaction with calcium carbonate. He puts 5.0 g of large marble chips into 50 cm³ of 1.0 mol/dm³ hydrochloric acid and measures the mass lost every 30 seconds. For the second experiment he uses 50 cm³ of 0.5 mol/dm³ acid, but replaces the chips with 5.0 g of powdered calcium carbonate because 'the powder reacts more evenly'. The powder fizzes vigorously and the mass loss in the first 30 s is 0.45 g, compared with 0.25 g for the chips in 1.0 mol/dm³ acid. Dev concludes that decreasing acid concentration increases the rate of reaction. He has not repeated either experiment. His teacher is concerned about the vigorous fizzing and asks him to justify his conclusion.
1. Explain why Dev's conclusion is not valid, identifying the variable he failed to control and its effect on rate.
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Dev changed two things at once: acid concentration and particle size. Powdered calcium carbonate has a much greater surface area than large chips, so more particles are exposed to acid and collisions are more frequent. That alone increases the rate. The faster mass loss in the 0.5 mol/dm³ experiment could therefore be due to surface area, not to lower concentration. The conclusion reverses the expected effect and is invalid. In addition, a single trial cannot establish a rate reliably, and the vigorous reaction may cause loss of liquid spray, making mass loss unreliable.
2. Describe a fair experiment to test concentration, including at least three controlled variables and how rate would be measured from the data.
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Use the same mass (5.0 g), the same particle size (large chips of similar size), the same volume (50 cm³), the same temperature, and the same swirling or agitation. Vary only acid concentration, for example 0.5, 1.0, 1.5 and 2.0 mol/dm³, with enough acid so that it is not limiting. Measure mass loss at fixed intervals, or collect gas in a syringe. Plot mass loss against time; rate is the gradient. Use the initial linear gradient over the first 30 s, not total mass loss, because total mass loss depends on how much acid reacted. Repeat each concentration at least three times and compare mean initial rates.
3. Explain why the teacher's safety concern is justified and state one precaution.
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Powder reacts very fast because of its large surface area. The rapid production of carbon dioxide can froth over, and acid spray can reach eyes or skin. The teacher is right to be concerned. Precautions include wearing goggles, using a smaller mass of powder, using a larger beaker to contain froth, adding the powder in small portions, and not stoppering the vessel. If gas volume is collected, allow for the rapid initial burst. Good safety is part of a valid method, not an optional extra.