`Cr_2O_7^(2-)//H^+` is an oxidising agent for alcohol. Stage of completion is indicated by

Options

A. green colour
B. blue colour
C. orange colour
D. yellow colour

Answer

To solve the question regarding the oxidation of alcohol by the dichromate ion \((Cr_2O_7^{2-})\) in an acidic medium, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reactants**:
We are given that \((Cr_2O_7^{2-})\) is an oxidizing agent for alcohol. Let's consider methanol \((CH_3OH)\) as our alcohol for this reaction.

2. **Understand the Role of \((Cr_2O_7^{2-})\)**:
The dichromate ion \((Cr_2O_7^{2-})\) acts as a strong oxidizing agent in acidic medium. In this case, it will oxidize the alcohol (methanol) to a carboxylic acid (methanoic acid).

3. **Determine the Oxidation States**:
In \((Cr_2O_7^{2-})\), chromium is in the +6 oxidation state. The oxidation state can be calculated as follows:
\[ 2X + 7(-2) = -2 \quad \Rightarrow \quad 2X - 14 = -2 \quad \Rightarrow \quad 2X = 12 \quad \Rightarrow \quad X = +6 \]
Thus, chromium is in the +6 oxidation state.

4. **Reaction Process**:
When methanol is oxidized, it forms methanoic acid:
\[ CH_3OH \rightarrow HCOOH \]
During this reaction, the chromium in \((Cr_2O_7^{2-})\) is reduced from +6 to +3.

5. **Color Change Observation**:
The initial color of \((Cr_2O_7^{2-})\) in solution is orange. After the reaction, as chromium is reduced to +3, the solution changes color:
- Chromium in the +3 state is green.

6. **Conclusion**:
Therefore, the stage of completion of the reaction can be indicated by the color change from orange to green. This signifies that the reaction has gone to completion, and \((Cr^{3+})\) is now present in the solution.

### Final Answer:
The stage of completion is indicated by a **green color**.

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