Reaction Rate Factors Steps:

• Recall that reaction rates depend on concentration, surface area, temperature, and catalysts via collision theory.
• Identify ways to slow the rate: lower concentration or reduce particle collisions.
• Evaluate options: A adds catalyst (speeds up), B dilutes acid (lowers concentration), C stirs (increases mixing), D uses powder (increases surface area).
• Select B as it directly reduces reactant concentration to slow the reaction.

Why B is correct:

• Diluting the acid decreases its concentration, reducing the frequency of effective collisions between magnesium and H+ ions, as per the rate law where rate ∝ [H2SO4].

Why the others are wrong:

• A: Catalysts lower activation energy, increasing reaction rate.
• C: Stirring enhances mixing and collisions, speeding up the reaction.
• D: Powder has higher surface area than a strip, increasing collision frequency and rate.

Final answer: B