WAEC Chemistry 2022 Paper

Concept:The rate of a chemical reaction depends on the surface area of the solid reactant and the concentration of the acid. Larger surface area and higher concentration increase the frequency of effective collisions.Explanation:Marble is mainly calcium carbonate, $\mathrm{CaCO}_3$.It reacts with hydrochloric acid, $\mathrm{HCl}$, to produce carbon dioxide gas.The rate of this reaction increases when more reactant particles collide successfully per second.Powdered marble has a much larger total surface area than marble chips of the same mass.Therefore, powdered marble provides more contact area for $\mathrm{HCl}$ molecules to collide with $\mathrm{CaCO}_3$.A higher concentration of $\mathrm{HCl}$, such as $2.5\ \mathrm{mol\ dm^{-3}}$, contains more acid particles per unit volume than $2.0\ \mathrm{mol\ dm^{-3}}$.This further increases the collision frequency between the reactants.Thus, the fastest reaction occurs when the powdered form is combined with the more concentrated acid.Answer:C. Powdered marble and $2.5\ \mathrm{mol\ dm^{-3}}\ \mathrm{HCl}_{(aq)}$