If 5.0 g of marble reacts with 25.0 cm3 hydrochloric acid, which of the following combinations has the fastest reaction rate?

Analyze the question: We need the combination that gives the fastest rate for CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. The mass of marble (5.0 g) and the volume of acid (25.0 cm³) are the same in every option. Only the form of the marble (chips or powder) and the acid concentration change.

Concept: Two factors from collision theory matter here.

1. Surface area: Powder has a much larger exposed surface than chips, so more particles can collide with the acid at once. This increases the rate.
2. Concentration: A higher concentration of HCl means more acid particles per unit volume. Collisions are then more frequent, so the rate increases.

Solution:

• Powdered marble beats marble chips, which rules out A and B.
• Between C and D, both use powder, so concentration decides. 2.5 mol dm⁻³ is higher than 2.0 mol dm⁻³.
• Option C therefore combines the larger surface area with the higher concentration.

Answer: C. Powdered marble and 2.5 mol dm⁻³ HCl

Exam tip: For "fastest rate" questions, pick the option that makes every factor favourable: powder (not lumps), higher concentration, and higher temperature if it is listed. Remember "Powder + Power (high concentration) = Faster."