A beaker contains 100 mL of 1.0 M HCl at 25°C. A student adds a single 2.0 g piece of zinc to start the reaction that produces hydrogen gas. The student proposes stirring the solution continuously with a glass rod while keeping temperature, concentration, and zinc size the same. Assuming stirring mainly helps bring fresh acid into contact with the zinc surface, what is the best prediction about the reaction rate with stirring compared to without stirring?
- Faster, because stirring increases contact between reactant particles near the zinc surface, increasing collision frequency there. (correct answer)
- Slower, because stirring breaks zinc into fewer pieces, reducing surface area.
- No change, because only temperature can change collision frequency.
- Slower, because stirring lowers the kinetic energy of particles by removing heat.
Explanation: This question tests your ability to predict how changes in reaction conditions (temperature, concentration, surface area) will affect reaction rate using collision theory reasoning. Stirring enhances reaction rates in heterogeneous systems by mixing reactants, effectively increasing the local concentration of fresh acid at the solid surface and removing products, which boosts collision frequency without changing overall concentration or temperature. Continuous stirring brings more HCl molecules into contact with the zinc surface faster, leading to more frequent effective collisions compared to a static setup. Choice A correctly predicts the rate change by properly applying collision theory to explain how the condition change affects collision frequency or effectiveness. Choice B fails because stirring doesn't break the zinc; it mixes the solution, like stirring soup to cook evenly faster! The rate change prediction recipe: (1) Identify what's changing: Is temperature going up or down? Is concentration increasing or decreasing? Is surface area getting larger (smaller pieces) or smaller (bigger chunks)? (2) Connect to particles: Temperature change → particle speed changes. Concentration change → particle density changes. Surface area change → number of exposed particles changes. (3) Connect to collisions: Faster/more particles → more frequent collisions. Higher energy particles → more effective collisions. More exposed particles → more possible collisions. (4) Predict rate: More or more effective collisions → FASTER rate. Fewer or less effective collisions → SLOWER rate. This four-step chain works for any condition change! Quick prediction rules (use collision theory to understand WHY these work): INCREASE to speed up reaction: raise temperature (most powerful!), increase concentration, increase surface area (for solids), add catalyst (if available). DECREASE to slow down reaction: lower temperature (refrigeration!), decrease concentration (dilute), decrease surface area (use larger pieces), remove catalyst. For exam questions asking 'which change would most increase rate,' temperature increase usually wins because it affects BOTH collision frequency AND effectiveness. Concentration and surface area mainly affect frequency only. This is why we cook with heat, not just by adding more ingredients!