Question

Which of the following reactions is not a disproportionation reaction?

• A. \(\mathrm{CaCO_3 \longrightarrow CaO+CO_2}\)
• B. \(\mathrm{P_4+3NaOH+3H_2O \longrightarrow 3NaH_2PO_2+PH_3}\)
• C. \(\mathrm{2H_2O_2 \longrightarrow 2H_2O+O_2}\)
• D. \(\mathrm{2Cu^+ \longrightarrow Cu^{2+}+Cu}\)

Hint:

In disproportionation reactions, the same element must undergo both oxidation and reduction simultaneously.

Answer 1

The Correct Option is A

Step 1: Define disproportionation.
A disproportionation reaction is one in which a single element in one oxidation state simultaneously undergoes both oxidation and reduction, producing two different oxidation states of that same element.
Step 2: Analyze CaCO₃ decomposition.
CaCO₃ $\rightarrow$ CaO + CO₂. In CaCO₃: Ca = +2, C = +4, O = -2. In CaO: Ca = +2, O = -2. In CO₂: C = +4, O = -2. No element changes its oxidation state. This is a simple thermal decomposition, NOT a redox reaction.
Step 3: Confirm no oxidation-state change.
C remains +4 throughout, Ca remains +2, O remains -2. There is no oxidation or reduction occurring, so this cannot be a disproportionation reaction.
Step 4: Check what disproportionation would require.
For disproportionation: the same element would need to appear in two different products with different oxidation states, one higher and one lower than in the reactant. None of this applies to CaCO₃ decomposition.
Step 5: Contrast with true disproportionation examples.
Example: Cl₂ + 2NaOH $\rightarrow$ NaCl + NaOCl + H₂O. Here Cl goes from 0 to -1 (reduction) and 0 to +1 (oxidation). CaCO₃ decomposition has nothing like this.
Step 6: Conclude.
CaCO₃ $\rightarrow$ CaO + CO₂ is not a disproportionation reaction because no element changes its oxidation state.
\[ \boxed{\text{Option 1: } CaCO_3 \rightarrow CaO + CO_2 \text{ is NOT a disproportionation reaction}} \]