Which of the following factors will speed up the rate of evolution of carbon (iv) oxide in the reaction below? 2HCl + CaCO$_3$ → CaCl$_2$ + H$_2$O + CO$_2$

To solve this problem, we need to look at the factors that influence the rate of a chemical reaction.

1. Analyze the Question

The reaction provided is: 2HCl + CaCO$_3$ → CaCl$_2$ + H$_2$O + CO$_2$

We are looking for factors that will increase the speed (rate) at which carbon (IV) oxide (CO$_2$) is produced.

2. Explain the Concept

The rate of a chemical reaction depends on the frequency of effective collisions between reactant particles. Several factors can increase this frequency:

• Concentration: Increasing the concentration of a reactant (like HCl) increases the number of particles per unit volume. This leads to more frequent collisions between the acid and the calcium carbonate.
• Surface Area: When a solid reactant (like CaCO$_3$) is in a powdered form rather than large lumps, more of its particles are exposed to the acid. This increases the number of active sites for collisions, thereby speeding up the reaction.
• Temperature: Increasing the temperature gives particles more kinetic energy, causing them to move faster and collide more often with sufficient energy to react.

3. Show the Solution

• Option A (Increasing the concentration of the acid): By increasing the concentration of HCl, we provide more H$^+$ ions to react with the CaCO$_3$. This will definitely increase the rate of CO$_2$ evolution.
• Option B (Increasing the surface area of CaCO$_3$): By breaking the CaCO$_3$ into smaller pieces or using a powder, we increase the surface area available for the acid to attack. This also increases the rate of CO$_2$ evolution.
• Option C (A and B): Since both A and B are scientifically proven ways to increase the reaction rate, this option covers both correct factors.
• Option D (Reducing the concentration of the acid): Reducing the concentration would decrease the number of collisions, thereby slowing down the reaction. This is incorrect.

4. Identify the Answer

Since both increasing the concentration of the acid and increasing the surface area of the solid reactant will speed up the reaction, Option C is the correct choice.

5. Exam Tip

When you see a question about the "rate of reaction," always think of the Collision Theory. Ask yourself: "Does this change make the particles hit each other more often or with more energy?" If the answer is yes, the rate will increase!