A catalyst is a substance which

Options

A. increase the equilibrium concentration of the product.
B. change the equilibrium constant of the reaction.
C. shortens the time to rach equilibrium.
D. supplies energy to the reaction.

Answer

**Step-by-Step Solution:**

1. **Understanding the Definition of a Catalyst:**
A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. It provides an alternative pathway for the reaction with a lower activation energy.

2. **Analyzing the Given Options:**
The question provides four options regarding the role of a catalyst:
- A) Increases the equilibrium concentration of the product
- B) Changes the equilibrium constant of the reaction
- C) Shortens the time to reach equilibrium
- D) Supplies energy to the reaction

3. **Evaluating Each Option:**
- **Option A:** A catalyst does not change the equilibrium concentrations of reactants or products; it only speeds up the rate at which equilibrium is reached.
- **Option B:** A catalyst does not alter the equilibrium constant of a reaction; it only affects the rate of reaching that equilibrium.
- **Option C:** A catalyst does shorten the time to reach equilibrium by lowering the activation energy, thus increasing the rate of both the forward and reverse reactions equally.
- **Option D:** A catalyst does not supply energy; it merely lowers the activation energy barrier for the reaction.

4. **Conclusion:**
Based on the analysis, the correct answer is **C) Shortens the time to reach equilibrium**. A catalyst accelerates the reaction rate, allowing the system to reach equilibrium faster without affecting the position of equilibrium.