The Reaction That Won't Go Faster
Students study the reaction of magnesium ribbon with 1.0 M HCl by collecting gas in a syringe. Trial 1 uses a 2 cm ribbon at 22 °C and gives a steady but slow rate. To speed it up, the group runs Trial 2 with 2.0 M HCl at 22 °C and sees a clear increase. For Trial 3 they add a copper wire touching the magnesium, expecting a further increase, and observe a dramatic jump in gas production. Puzzled, they ask whether copper is a catalyst. Their teacher notes that copper does not dissolve and that the gas volume collected in Trial 3, per gram of Mg, matches Trials 1 and 2 once the reaction finishes.
1. Is copper acting as a catalyst in Trial 3? Defend your answer using the observations and the definition of a catalyst.
Read the worked answer
No - copper is not functioning as a catalyst here; it acts as a galvanic couple / electron conduit.
- A catalyst is a substance that increases reaction rate without being consumed and that provides an alternate pathway; it is regenerated. Copper does remain (it doesn't dissolve), which is necessary but not sufficient.
- The key evidence: the final gas volume per gram of Mg is unchanged across trials. That means copper did not change the reaction's stoichiometry or the amount of product - it changed only how fast electrons were transferred. In a Mg–Cu contact in acid, copper's low overpotential for H₂ evolution lets Mg oxidize and H⁺ reduce on separate surfaces, accelerating the same net reaction.
- Contrast with a true catalyst: adding a catalyst can change the mechanism and sometimes the products, but here the products and yield are identical.
So the correct description is a galvanic (bimetallic) acceleration, not catalysis. The distinction matters because "not consumed" is a necessary but not sufficient test for catalysis.
2. The group wants to isolate the effect of temperature alone. Design a controlled comparison and state which variable must be held constant and why.
Read the worked answer
Design: Run the reaction at 22 °C and at, say, 35 °C using identical Mg ribbon length (2 cm), identical HCl concentration (1.0 M), identical volume, and no copper wire, measuring initial gas-collection rate.
Held constant: Mg surface area/length, acid concentration and volume, and absence of the copper couple.
Why: Rate depends on concentration, surface area, and temperature simultaneously. If acid concentration or the Cu couple varied between the temperature trials, any rate change could not be attributed to temperature. Because Trial 3 introduced a second accelerating factor, it must be excluded from a temperature-only comparison.
Expected result: A higher rate at 35 °C; a rough rule-of-thumb is that many reactions roughly double per ~10 °C, so the 13 °C rise should produce a clearly measurable, larger-than-noise increase. Repeating each trial 2–3 times lets the group report a mean rate with uncertainty rather than a single reading.