Transition metal and their compounds are used as catalyst in industry and in biological system. For example, in the Contact Process, vanadium compounds in the +5 state `(V_(2)O_(5) or VO_(3)^(-))` are used to oxidise `SO_(2)` to `SO_(3)`:
`SO_(2)+(1)/(2)O_(2) overset(V_(2)O_(5))(to) SO_(3)`
It is thought thta the actual oxidation process takes place in two stages. In the first step, `V^(5+)` in the presence of oxide ions converts `SO_(2)` to `SO_(3)`. At the same time, `V^(5+)` is reduced to `V^(4+)`.
`2V^(5+)+O^(2-)+SO_(2) to 2V^(4+)+SO_(3)`
In the second step, `V^(5+)` is regenerated from `V^(4+)` by oxygen:
`2V^(4+)+(1)/(2)O_(2) to 2V^(5+)+O^(2-)`
The overall process is, of curse, the sum of these two steps:
`SO_(2) +(1)/(2)O_(2) to SO_(3)`
Q. During the course of the reaction:

Options

A. Catalyst undergoes changes in oxidation state
B. Catalyst increase the rate constant
C. Catalyst is regenerated in its original form when the reactants form the products
D. All are correct

Answer

To solve the question regarding the role of transition metals and their compounds as catalysts in the Contact Process, we can break down the information provided into a systematic analysis.

### Step-by-Step Solution:

1. **Understanding the Contact Process**:
- The Contact Process is used to produce sulfur trioxide (SO3) from sulfur dioxide (SO2) and oxygen (O2) in the presence of a catalyst, vanadium pentoxide (V2O5).
- The overall reaction can be represented as:
\[ SO_2 + \frac{1}{2} O_2 \overset{V_2O_5}{\rightarrow} SO_3 \]

2. **Mechanism of the Reaction**:
- The oxidation of SO2 to SO3 occurs in two stages.
- **First Stage**:
- Vanadium in the +5 oxidation state (V^5+) reacts with oxide ions (O^2-) and SO2.
- The reaction can be written as:
\[ 2V^{5+} + O^{2-} + SO_2 \rightarrow 2V^{4+} + SO_3 \]
- Here, V^5+ is reduced to V^4+ while oxidizing SO2 to SO3.

3. **Regeneration of the Catalyst**:
- **Second Stage**:
- V^4+ is then oxidized back to V^5+ by reacting with oxygen:
\[ 2V^{4+} + \frac{1}{2} O_2 \rightarrow 2V^{5+} + O^{2-} \]
- This step regenerates the catalyst (V^5+) for further reactions.

4. **Overall Reaction**:
- The overall process can be summarized as:
\[ SO_2 + \frac{1}{2} O_2 \rightarrow SO_3 \]
- The catalyst (V2O5) is not consumed in the reaction but undergoes changes in oxidation state during the process.

5. **Analysis of Statements**:
- **Statement 1**: "Catalyst undergoes changes in oxidation state."
- This is **True**. V^5+ changes to V^4+ and is then regenerated back to V^5+.
- **Statement 2**: "Catalyst increases the rate constant."
- This is **True**. Catalysts increase the rate of reaction by lowering the activation energy.
- **Statement 3**: "Catalyst is regenerated in its original form when the reactants form the products."
- This is **True**. The catalyst returns to its original state (V^5+) after the reaction.

6. **Conclusion**:
- All statements regarding the role of the catalyst in the Contact Process are correct.
- Therefore, the answer to the question is that all options are correct.