At a certain temperature T, the endothermic reaction `A rarr B` proceeds almost to completion.The entropy change is :

Options

A. `Delta S = 0`
B. `Delta S lt 0`
C. `Delta S gt 0`
D. Cannot be predicted

Answer

To solve the problem regarding the entropy change of the endothermic reaction \( A \rightarrow B \) that proceeds almost to completion at a certain temperature \( T \), we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Endothermic Reactions**:
- In an endothermic reaction, heat is absorbed from the surroundings. This means that the system (the reactants and products involved in the reaction) takes in energy in the form of heat.

2. **Entropy Change Formula**:
- The change in entropy (\( \Delta S \)) for a process can be expressed as:
\[ \Delta S = \frac{Q}{T} \]
where \( Q \) is the heat absorbed by the system and \( T \) is the absolute temperature at which the process occurs.

3. **Heat Absorption**:
- Since the reaction is endothermic, the heat \( Q \) absorbed by the system is positive. Therefore, we can conclude that:
\[ Q > 0 \]

4. **Calculating Entropy Change**:
- Substituting the positive value of \( Q \) into the entropy change formula gives:
\[ \Delta S = \frac{Q}{T} > 0 \]
This indicates that the change in entropy is positive.

5. **Conclusion**:
- Since the entropy change \( \Delta S \) is greater than zero, we can conclude that the entropy of the system increases as the reaction proceeds.

### Final Answer:
The entropy change for the endothermic reaction \( A \rightarrow B \) that proceeds almost to completion is positive (\( \Delta S > 0 \)). Therefore, the correct option is **C**.