Which of the following statement is incorrect ?

Options

A. Standard Gibbs free energy change is always zero at equilibrium
B. Addition of solid does not affect equilibrium
C. On addition of catalyst the value of equilibrium constant is not affected
D. Equilibrium constant for a reaction with negative `DeltaH` value decreases as the temperature increases

Answer

To determine which statement is incorrect, we will analyze each statement one by one.

### Step 1: Analyze the first statement
**Statement 1:** Standard Gibbs free energy change is always zero at equilibrium.

- **Explanation:** The standard Gibbs free energy change (ΔG°) is not equal to zero at equilibrium. Instead, the Gibbs free energy change (ΔG) is zero at equilibrium. The relationship is given by the equation:
\[ \Delta G° = -2.303 RT \log K \]
where K is the equilibrium constant. Therefore, this statement is incorrect.

### Step 2: Analyze the second statement
**Statement 2:** Addition of solid does not affect equilibrium.

- **Explanation:** This statement is true. The activity of solids and pure liquids is considered to be constant and does not affect the position of equilibrium. Therefore, adding a solid does not change the equilibrium position.

### Step 3: Analyze the third statement
**Statement 3:** On addition of catalyst, the value of equilibrium constant is not affected.

- **Explanation:** This statement is true. A catalyst speeds up the rate of both the forward and reverse reactions equally, thereby reaching equilibrium faster, but it does not change the equilibrium constant (K). Therefore, this statement is correct.

### Step 4: Analyze the fourth statement
**Statement 4:** Equilibrium constant for a reaction with negative ΔH value decreases as the temperature increases.

- **Explanation:** This statement is true. According to Le Chatelier's Principle, for an exothermic reaction (negative ΔH), increasing the temperature shifts the equilibrium position to the left, which decreases the equilibrium constant (K). Therefore, this statement is also correct.

### Conclusion
After analyzing all the statements, we find that the first statement is the only incorrect one.

**Final Answer:** The incorrect statement is: "Standard Gibbs free energy change is always zero at equilibrium."

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