In an experiment, an excess quantity of calcium carbonate granules (CaCO_{3(s)}) is reacte… — JAMB UTME

In an experiment, an excess quantity of calcium carbonate granules ( $CaCO_{3(s)}$ ) is reacted with $50\text{ cm}^3$ of $0.20\text{ mol dm}^{-3}$ hydrochloric acid ( $HCl_{(aq)}$ ). Which of the following modifications will increase the initial rate of carbon(IV) oxide gas evolution without altering the total volume of gas collected upon completion of the reaction?

Replacing the calcium carbonate granules with an equal mass of finely powdered calcium carbonateAnswer
B
Adding a catalyst to shift the position of chemical equilibrium toward the products
C
Doubling the volume of the hydrochloric acid solution to $100\text{ cm}^3$ while keeping its concentration at $0.20\text{ mol dm}^{-3}$
D
Decreasing the temperature of the reaction mixture while doubling the mass of calcium carbonate granules

Answer

Replacing the calcium carbonate granules with an equal mass of finely powdered calcium carbonate.

Grinding a solid reactant into a fine powder significantly increases its total exposed surface area. According to collision theory, a larger surface area allows more reactant particles to collide per unit time, resulting in a higher frequency of effective collisions and a faster initial rate of gas evolution. Because the mass of calcium carbonate and the amount of hydrochloric acid are kept constant, the stoichiometric yield of carbon(IV) oxide gas remains identical.

Step-by-Step Solution

Analyze the factors influencing the initial rate of reaction.

The initial reaction rate depends on collision frequency, which increases with higher surface area, higher concentration, higher temperature, or the addition of a catalyst.

Collision theory dictates that rate is directly proportional to the frequency of effective collisions per unit time.

Evaluate the effect of surface area modification on product yield.

Powdered

CaCO_3

has a larger total surface area exposed to

HCl_{(aq)}

than granules, increasing effective collision frequency.

Because the mass of

CaCO_3

remains identical and

HCl

is the limiting reagent, the total moles of reactant and final gas yield remain unchanged.

Key Concept

Effect of Surface Area and Collision Theory on Reaction Rates

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