Question

: 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

Answer 1

Hint:
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’.

Note:

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.