Which of the following is a disproporationation reaction ?

Options

A. `Cu_(2)O+2H^(o+)rarrCu+Cu^(2+)+H_(2)O`
B. `2CrO_(4)^(2-)+2H^(o+)rarrCr_(2)O_(7)^(2-)+H_(2)O`
C. `CaCO_(3)+2H^(o+)rarrCa^(2)+H_(2)O+CO_(2)`
D. `Cr_(2)O_(7)^(2-)rarr2overset(ө)OHrarr2CrO_(4)^(2-)+H_(2)O`

Explanation

To determine which of the given reactions is a disproportionation reaction, we need to identify if a single element in a compound is both oxidized and reduced in the same reaction. Let's analyze the options step by step.

### Step-by-Step Solution:

1. **Understanding Disproportionation Reaction:**
- A disproportionation reaction is a type of redox reaction where an element in a single oxidation state is simultaneously oxidized and reduced to form two different products with different oxidation states.

2. **Analyzing the First Reaction:**
- Let's say we have a reaction involving copper (Cu).
- Oxidation states:
- Cu changes from +1 to 0 (reduction).
- Cu changes from +1 to +2 (oxidation).
- Since the same element (Cu) is both oxidized and reduced, this is a disproportionation reaction.

3. **Analyzing the Second Reaction:**
- In this case, we have chromium (Cr).
- Oxidation states:
- Cr remains +6 in both instances.
- Since there is no change in oxidation state, this is not a disproportionation reaction.

4. **Analyzing the Third Reaction:**
- Here, we look at calcium (Ca).
- Oxidation states:
- Ca remains +2 in both instances.
- Again, there is no change in oxidation state, so this is not a disproportionation reaction.

5. **Analyzing the Fourth Reaction:**
- This involves another chromium compound (Cr2O7^2-).
- Oxidation states:
- Cr remains +6 in both instances.
- There is no change in oxidation state, indicating that this is not a disproportionation reaction.

6. **Conclusion:**
- From the analysis, only the first reaction involving copper (Cu) shows a change in oxidation states where Cu is both oxidized and reduced. Therefore, the correct answer is option A.

### Final Answer:
The correct option for the disproportionation reaction is **A**.