The occurrence of a reaction is impossible if

Options

A. `Delta H` is +ve , `Delta S` is also + ve but `Delta H lt T Delta S`
B. `Delta H` is - ve , `Delta S` is also - ve but `Delta H gt T Delta S`
C. `Delta H` is - ve , `Delta S` is + ve
D. `Delta H` is + ve , `Delta S` is - ve

Answer

To determine the conditions under which the occurrence of a reaction is impossible, we can analyze the Gibbs free energy change (ΔG) for the reaction. The Gibbs free energy change is given by the equation:

\[ \Delta G = \Delta H - T \Delta S \]

Where:
- ΔG = Gibbs free energy change
- ΔH = Change in enthalpy
- T = Temperature in Kelvin
- ΔS = Change in entropy

### Step-by-Step Solution:

1. **Understanding Gibbs Free Energy**:
- A reaction occurs spontaneously if ΔG is negative.
- If ΔG is zero, the system is at equilibrium.
- If ΔG is positive, the reaction is non-spontaneous and will not occur.

2. **Analyzing the Equation**:
- From the equation ΔG = ΔH - TΔS, we can see that ΔG depends on both ΔH and ΔS.
- To determine when ΔG is positive, we need to consider the signs of ΔH and ΔS.

3. **Identifying Conditions for Non-spontaneity**:
- For ΔG to be positive:
- If ΔH is positive (endothermic reaction) and ΔS is negative (decrease in disorder), then:
\[ \Delta G = \Delta H - T \Delta S \]
Here, ΔH is positive, and TΔS is negative (since ΔS is negative). Therefore, the equation becomes:
\[ \Delta G = \text{positive} - \text{positive} \quad (\text{since T is always positive}) \]
This results in ΔG being positive.

4. **Conclusion**:
- The occurrence of a reaction is impossible if ΔH is positive and ΔS is negative. This means that the reaction is endothermic and leads to a decrease in entropy.

### Final Answer:
The occurrence of a reaction is impossible if ΔH is positive and ΔS is negative.

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