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: In which of the following reactions does conc. ${{\text{H}}_2}{\text{S}}{{\text{O}}_4}$ act as an oxidizing agent?
A)${\text{ZnO + }}{{\text{H}}_2}{\text{S}}{{\text{O}}_4}$
B)${\text{KBr + }}{{\text{H}}_2}{\text{S}}{{\text{O}}_4}$
C)${\text{NaCl + }}{{\text{H}}_2}{\text{S}}{{\text{O}}_4}$
D) None of these

Last updated date: 15th Jul 2024
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The reaction between salts and sulphuric acid results in a displacement reaction. In such reactions the oxidation number of the elements does not change.

Complete step by step answer:
The oxidizing agent increases the oxidation state of the other substance in the chemical reaction. Conc. Sulphuric acid is a strong oxidizing reagent. But when it reacts with salts it either does not give reaction or shows displacement reaction. The reactions are as follows-
1. When zinc oxide reacts with conc. Sulphuric acid, zinc sulphate (salt) and water is formed as zinc displaces hydrogen due to high reactivity.
 $ \Rightarrow $ ${\text{ZnO + }}{{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {\text{ZnS}}{{\text{O}}_4} + {{\text{H}}_2}{\text{O}}$
The oxidation number of zinc does not change in the above reaction. The oxidation number of zinc in zinc oxide is $ + 2$ as the oxygen atom has $ - 2$ state in oxides so the molecule is neutral. In zinc sulphate the oxidation number of zinc is-
$ \Rightarrow x + \left( { - 2} \right) = 0 \Rightarrow x = + 2$ $ \Rightarrow x + \left( { - 2} \right) = + 2$
2. When potassium bromide reacts with conc. Sulphuric acid, potassium displaces hydrogen to form potassium sulphate as it is more reactive than hydrogen.
$ \Rightarrow 2{\text{KBr + }}{{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {{\text{K}}_2}{\text{S}}{{\text{O}}_4} + 2{\text{HBr}}$
Here also the oxidation number of potassium does not change and remains $ + 1$ after the reaction.
The oxidation state of potassium in potassium bromide is-$ \Rightarrow 2x + 2\left( { - 1} \right) = 0 \Rightarrow x = + 1$
The oxidation state of potassium in potassium sulphate is-$ \Rightarrow 2x + \left( { - 2} \right) = 0 \Rightarrow x = + 1$
3. When sodium chloride is reacted with conc. Sulphuric acid, it gives no reaction as sodium chloride is neutral salt.
$ \Rightarrow {\text{NaCl + }}{{\text{H}}_2}{\text{S}}{{\text{O}}_4} \to {\text{no reaction}}$
Since no reaction happens so the oxidation state of sodium does not change.
Hence , in the above reactions the sulphuric acid does not act as an oxidizing agent.

So the correct option is ‘D’.


Sulphuric acid has many uses besides acting as an oxidizing agent which are-
1. As a dehydrating agent it absorbs water from sugar, starch, paper and wood.
2. In manufacturing of paint.
3. In manufacturing of explosives such as nitroglycerine.
5. In petroleum refining.