Question

When 100.mL of 1.0 M HCl is added to a 2.0g piece of CaCO3, CO2 is produced at a certain rate. Which of the changes below will NOT increase the rate of this reaction?

A. adding 100.mL of 2.0 M HCl in place of 100.ml of 1.0M HCl

B. heating the 100.mL of 1.0 M HCl before adding it to the CaCO3

C. adding 150.ml of 1.0 M HCl in place of 100.mL of 1.0 M HCl

D. adding 100.mL of 1.0 M HCl to 2.0g of powdered CaCO3

The answer is C, but please explain why.

Answer 1

Number of moles of CaCO₃ = 2.0g/100g/mol = 0.02mol

Number of moles of HCl taken = 1.0mol/L × 0.100L = 0.10mol

2 HCl + CaCO₃ (s) CaCl₂ + H₂O + CO₂

Number of moles of HCl consumed = 0.02 × 2 = 0.04 mol

If we add 100ml of 1.0M solution , reaction would be complete and it 150ml of 1.0M HCl is added the extra HCl would remain in solution .

But in case of 2.0M HCl solution , rate would increase because rate of reaction is directly proportional to the concentration of reactants .

Rate of reaction is directly proportional to the temperature of the system, on heating the solution , temperature of solution would be higher hence rate of reaction would be higher .