In which of the following reactions the underlined substance is oxidised?

Options

A. `3Mg+ ul(N_(2)) to Mg_(3)N_(2)`
B. `2Kl + ul(Br_(2)) to 2KBr + I_(2)`
C. `ul(Cu)O + H_(2) to Cu +H_(2)O`
D. `ul(C)O + Cl_(2) to COCl_(2)`

Answer

To determine whether the underlined substance is oxidized in the given reaction (K2Cr2O7 to Cr2(SO4)3), we need to analyze the oxidation states of chromium in both compounds.

### Step 1: Determine the oxidation state of chromium in K2Cr2O7
- In K2Cr2O7, potassium (K) has an oxidation state of +1.
- There are 2 potassium atoms, contributing a total of +2.
- Oxygen (O) has an oxidation state of -2. There are 7 oxygen atoms, contributing a total of -14.
- Let the oxidation state of chromium (Cr) be x. The equation for the oxidation states is:
\[ 2(+1) + 2x + 7(-2) = 0 \]
Simplifying this:
\[ 2 + 2x - 14 = 0 \]
\[ 2x - 12 = 0 \]
\[ 2x = 12 \implies x = 6 \]
- Therefore, the oxidation state of chromium in K2Cr2O7 is +6.

### Step 2: Determine the oxidation state of chromium in Cr2(SO4)3
- In Cr2(SO4)3, the sulfate ion (SO4) has a charge of -2. Since there are 3 sulfate ions, the total contribution from sulfate is:
\[ 3(-2) = -6 \]
- Let the oxidation state of chromium in Cr2(SO4)3 be y. The equation for the oxidation states is:
\[ 2y + 3(-2) = 0 \]
Simplifying this:
\[ 2y - 6 = 0 \]
\[ 2y = 6 \implies y = 3 \]
- Therefore, the oxidation state of chromium in Cr2(SO4)3 is +3.

### Step 3: Compare the oxidation states
- In K2Cr2O7, the oxidation state of chromium is +6.
- In Cr2(SO4)3, the oxidation state of chromium is +3.
- The oxidation state of chromium decreases from +6 to +3, indicating that chromium is being reduced.

### Conclusion
Since the oxidation state of chromium decreases, it means that the substance is reduced, not oxidized. Therefore, the underlined substance (K2Cr2O7) is not oxidized in this reaction.

### Final Answer
The correct answer is that the underlined substance is not oxidized; it is reduced.

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