Confounded Reaction-Rate Data with a Sticking Gas Syringe
During a laboratory investigation of how concentration affects reaction rate, three student groups react 2.0 cm strips of magnesium ribbon with hydrochloric acid and collect hydrogen gas in a gas syringe. Group A uses 25.0 mL of 1.00 M HCl; Group B uses 25.0 mL of 2.00 M HCl; Group C uses 25.0 mL of 2.00 M HCl but cuts its magnesium strip into four pieces. All groups record gas volume every 10 s for 2 min at room temperature. Group A obtains a smooth curve with an initial rate of 0.42 mL/s. Group B’s initial rate is 0.91 mL/s, but its syringe plunger sticks at 18 mL. Group C’s rate is 0.60 mL/s. The teacher asks the class to decide whether Group B’s data support the claim that doubling acid concentration doubles the rate and to revise the procedure for the next class.
1. Using the rate data, evaluate the claim that doubling the HCl concentration doubles the reaction rate. Identify which group’s result is confounded and explain what must be held constant before the claim can be tested.
Read the worked answer
The only intended difference between A and B is acid concentration: same 25.0 mL volume, same 2.0 cm ribbon, same temperature, and same collection method. The initial-rate ratio is 0.91/0.42 = 2.17, which is close to but not exactly 2.00. A single trial and a ratio above 2 do not prove direct proportionality. More importantly, B’s plunger sticks at 18 mL; any rate calculated from data after sticking is invalid, and if sticking began before the initial slope was measured, B’s initial rate is also suspect. Group C cannot test concentration because cutting the ribbon into four pieces changes surface area as well as concentration. The claim is therefore only tentatively supported by A and B, and only if B’s initial data are unaffected by the syringe problem. Replicates and several concentrations are needed.
2. Propose a revised procedure that would produce valid class data, including at least three controlled variables, one safety precaution, and a way to handle the sticking syringe.
Read the worked answer
Use identical ribbons of the same length and mass, cleaned with emery cloth to remove oxide, and keep the same total liquid volume (25.0 mL) and temperature (water bath at 25.0 °C). Vary only HCl concentration, for example 0.50, 1.00, 1.50, and 2.00 M, with at least three trials each. Add acid last and start timing at mixing. Replace or lubricate the gas syringe; if it sticks, discard affected points and record only the smooth initial portion before sticking. Keep the Mg strip fully immersed and use a stopper with no leaks. Safety: wear goggles and gloves because HCl is corrosive; keep hydrogen away from flames because it is flammable. Calculate initial rate from the slope of the first 10–30 s, average trials, and plot rate versus concentration rather than relying on one ratio.