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Redox Reactions
Calculation of Oxidation Number
2 questions from 2 papers, each with its answer.
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Concepts tested
- Calculation of oxidation number
- Oxidation States
All Questions
2025 1 question
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Consider the following compounds: $\underline{\text{K}}\text{O}_2$, $\text{H}_2\underline{\text{O}}_2$ and $\text{H}_2\underline{\text{S}}\text{O}_4$. The oxidation states of the underlined elements in them are, respectively:
In $\text{KO}_2$, $\text{K} = +1$ (superoxide). In $\text{H}_2\text{O}_2$, $\text{O} = -1$ (peroxide). In $\text{H}_2\text{SO}_4$, $\text{S} = +6$.
1. In potassium superoxide ($\text{KO}_2$), potassium is an alkali metal and always has an oxidation state of $+1$ (the superoxide ion is $\text{O}_2^-$ where each oxygen is $-1/2$). 2. In hydrogen peroxide ($\text{H}_2\text{O}_2$), the peroxide linkage gives oxygen an oxidation state of $-1$. 3. In sulfuric acid ($\text{H}_2\text{SO}_4$), $2(+1) + x + 4(-2) = 0 \implies x = +6$. Hence, the oxidation states are $+1, -1$, and $+6$.
2020 Phase-2 1 question
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The oxidation number of the underlined atom in the following species: (1) $H\underline{Au}Cl_4$ is +3 (2) $\underline{Cu}_2O$ is $-1$ (3) $\underline{Cl}O_3^-$ is +5 (4) $K_2\underline{Cr}_2O_7$ is +6. Identify the incorrect option.In $Cu_2O$, oxygen is $-2$; for charge balance, $2x+(-2)=0\Rightarrow x=+1$. So copper in $Cu_2O$ is $+1$, not $-1$ — the statement in option (2) is incorrect. (Options 1, 3, 4 are correctly assigned oxidation states.)