In which of the following reactions, the underlined element has decreased its oxidation number during the reaction?

Options

A. `Fe + CuSO_(4) to Cu + ul(Fe)SO_(4)`
B. `ul(H_(2)) + Cl_(2) to 2ul(H)Cl`
C. `ul(C) + H_(2)O to ul(CO)+ H_(2)`
D. `ul(Mn)O_(2) + 4HCl to ul(Mn)Cl_(2) + Cl_(2) + 2H_(2)O`

Answer

To determine in which of the given reactions the underlined element has decreased its oxidation number, we will analyze each reaction step by step.

### Step 1: Analyze the first reaction (Fe + CuSO4 → Cu + FeSO4)
- The underlined element is Fe.
- In elemental form (Fe), the oxidation state is 0.
- In FeSO4, we can calculate the oxidation state of Fe:
- The sulfate ion (SO4) has a charge of -2.
- Let the oxidation state of Fe be x. Therefore, x + (-2) = 0 → x = +2.
- The oxidation state of Fe changes from 0 to +2, which is an increase, not a decrease.

### Step 2: Analyze the second reaction (H2 + Cl2 → 2 HCl)
- The underlined element is H.
- In elemental form (H2), the oxidation state is 0.
- In HCl, the oxidation state of H is +1 (since Cl has -1).
- The oxidation state of H changes from 0 to +1, which is also an increase.

### Step 3: Analyze the third reaction (C + H2O → CO + H2)
- The underlined element is C.
- In elemental form (C), the oxidation state is 0.
- In carbon monoxide (CO), we know that the oxidation state of O is -2.
- Let the oxidation state of C be x. Therefore, x + (-2) = 0 → x = +2.
- The oxidation state of C changes from 0 to +2, which is again an increase.

### Step 4: Analyze the fourth reaction (MnO2 + 4 HCl → MnCl2 + Cl2 + 2 H2O)
- The underlined element is Mn.
- In MnO2, let the oxidation state of Mn be x. The oxidation state of O is -2.
- Therefore, x + 2(-2) = 0 → x - 4 = 0 → x = +4.
- In MnCl2, let the oxidation state of Mn be y. The oxidation state of Cl is -1.
- Therefore, y + 2(-1) = 0 → y - 2 = 0 → y = +2.
- The oxidation state of Mn changes from +4 to +2, which is a decrease.

### Conclusion
The only reaction where the underlined element (Mn) has decreased its oxidation number is the fourth reaction. Therefore, the correct answer is:

**Option 4: MnO2 + 4 HCl → MnCl2 + Cl2 + 2 H2O**

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