In a chemical reaction, surface area of reactants can affect

1. Analyze the Question

The question asks which of the given chemical reactions is affected by the surface area of the reactants. In chemistry, surface area is a factor that influences the rate of reaction, specifically in heterogeneous reactions—reactions where the reactants exist in different phases (e.g., a solid reacting with a liquid or a gas).

2. Explain the Concept

• Collision Theory: For a reaction to occur, particles must collide with sufficient energy (activation energy) and the correct orientation.
• Surface Area: When a solid reactant is involved, increasing the surface area (by grinding it into a powder) exposes more particles to the other reactant. This increases the frequency of collisions, thereby increasing the rate of reaction.
• Application: Surface area only significantly affects reactions involving at least one solid reactant. If all reactants are in the same phase (e.g., all aqueous or all gaseous), changing the surface area has no effect on the reaction rate.

3. Show the Solution

Let’s examine the states of the reactants in each option:

• Option A: $\text{CaCO}_3(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)$ Here, Calcium carbonate ($\text{CaCO}_3$) is a solid, and Hydrochloric acid ($\text{HCl}$) is an aqueous solution. Because this is a solid-liquid (heterogeneous) reaction, increasing the surface area of the solid $\text{CaCO}_3$ will increase the rate of the reaction.

• Option B: $\text{MgCl}_2(aq) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{MgSO}_4(aq) + 2\text{HCl}(aq)$ Both reactants are in the aqueous phase. Surface area does not apply here.

• Option C: $\text{H}_2(g) + \text{Cl}_2(g) \rightarrow 2\text{HCl}(g)$ Both reactants are in the gaseous phase. Surface area does not apply here.

• Option D: $\text{NaOH}(aq) + \text{HCl}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)$ Both reactants are in the aqueous phase. Surface area does not apply here.

4. Identify the Answer

Since Option A is the only reaction involving a solid reactant, it is the only one where the surface area of the reactant will affect the reaction rate.

5. Exam Tip

Whenever you see a question about "surface area" affecting reaction rates, look for the solid reactant in the equation. If there is no solid, surface area is not the factor you are looking for!