Question 25 of 31intermediate🔧 ApplyShort Answer2 marks

The Mn³⁺ ion is unstable in solution and undergoes disproportionation to give Mn²⁺, MnO₂, and H⁺ ion. Write a balanced ionic equation for the reaction.

Correct Answer

2Mn3+(aq)+2H2O(l)→Mn2+(aq)+MnO2(s)+4H+(aq)2\text{Mn}^{3+}(\text{aq}) + 2\text{H}_2\text{O}(\text{l}) \rightarrow \text{Mn}^{2+}(\text{aq}) + \text{MnO}_2(\text{s}) + 4\text{H}^+(\text{aq})

Exercise: EXERCISES | Q: 7.21 | (Page Page 20)
For More Understanding

Explanation

The disproportionation reaction involves the same species undergoing both oxidation and reduction. Mn³⁺ is reduced to Mn²⁺ and oxidized to MnO₂. By balancing the atoms and charges using the standard method for acidic medium, we add water to balance oxygen and H⁺ ions to balance hydrogen, resulting in the final balanced ionic equation.

Solution Steps

  1. Step 1: Write the skeletal ionic equation for the disproportionation reaction.

    Mn3+(aq)→Mn2+(aq)+MnO2(s)\text{Mn}^{3+}(\text{aq}) \rightarrow \text{Mn}^{2+}(\text{aq}) + \text{MnO}_2(\text{s})

  2. Step 2: Balance the Mn atoms by multiplying Mn3+\text{Mn}^{3+} by 2.

    2Mn3+(aq)→Mn2+(aq)+MnO2(s)2\text{Mn}^{3+}(\text{aq}) \rightarrow \text{Mn}^{2+}(\text{aq}) + \text{MnO}_2(\text{s})

  3. Step 3: Balance the O atoms by adding 2H2O2\text{H}_2\text{O} molecules to the left side.

    2Mn3+(aq)+2H2O(l)→Mn2+(aq)+MnO2(s)2\text{Mn}^{3+}(\text{aq}) + 2\text{H}_2\text{O}(\text{l}) \rightarrow \text{Mn}^{2+}(\text{aq}) + \text{MnO}_2(\text{s})

  4. Step 4: Balance the H atoms by adding 4H+4\text{H}^+ ions to the right side.

    2Mn3+(aq)+2H2O(l)→Mn2+(aq)+MnO2(s)+4H+(aq)2\text{Mn}^{3+}(\text{aq}) + 2\text{H}_2\text{O}(\text{l}) \rightarrow \text{Mn}^{2+}(\text{aq}) + \text{MnO}_2(\text{s}) + 4\text{H}^+(\text{aq})

  5. Step 5: Verify the ionic charges on both sides. Left side: 2(+3)=+62(+3) = +6. Right side: +2+4(+1)=+6+2 + 4(+1) = +6. The charges are equal, so the equation is balanced.

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