Equal masses of calcium trioxocarbonate(iv) were added to dilute hydrochloric acid at the temperature…

Analyze the question: Equal masses of CaCO₃ react with dilute HCl under different conditions of particle size (chips vs powder) and temperature (20°C vs 40°C). We need the condition that gives the slowest rate.

Concept: Two factors affect the rate here:

1. Surface area: Powder has a much larger surface area exposed to the acid than chips, so more collisions happen per second and the reaction is faster. Chips have less surface area, so the reaction is slower.
2. Temperature: A higher temperature gives particles more kinetic energy. More collisions have enough energy to overcome the activation energy, so the reaction is faster. A lower temperature makes it slower.

Equation: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Comparing the options:

• A. Chips, 20°C: small surface area and low temperature, so slowest
• B. Chips, 40°C: small surface area but high temperature, so moderate
• C. Powder, 20°C: large surface area but low temperature, so moderate
• D. Powder, 40°C: large surface area and high temperature, so fastest

Option A has both the rate-lowering factors (larger lumps and lower temperature), so the reaction is slowest there.

Answer: A

Tip: For "slowest" or "fastest" questions, check each factor separately. The slowest case has the lowest temperature, lowest concentration and smallest surface area (big lumps). The fastest case has the opposite of each.