Question

Oxygen gas reacts with hydrogen to produce water. The reaction is represented by the equation:
${{\text{O}}_{\text{2}}}{\text{(g)}}\,{\text{ + }}{{\text{H}}_{\text{2}}}{\text{(g)}}\, \to {{\text{H}}_{\text{2}}}{\text{O(g)}}$
The above reaction is an example of:
a. oxidation of hydrogen
b. reduction of oxygen
c. reduction of hydrogen
d. redox reaction

A. a, b and c
B. b, c, and d
C. a, c and d
D. a, b and d

Answer 1

We will check which species is taking electrons and which is losing an electron to determine oxidized and reduced species. Then we will determine if both oxidation and reduction reactions are taking place simultaneously or not.

Complete answer
By the loss of electrons, a cation forms and by the gain of electrons, an anion forms.
The loss of electrons is known as oxidation and the gain of electrons is known as reduction.
The species which loses electrons goes under the oxidation process and the atom which gains electrons goes under the reduction process.
We will determine the change in oxidation number as follows:
${{\text{O}}_{\text{2}}}{\text{(g)}}\,{\text{ + }}{{\text{H}}_{\text{2}}}{\text{(g)}}\, \to {{\text{H}}_{\text{2}}}{\text{O(g)}}$
Oxidation number of oxygen in elemental form ${{\text{O}}_{\text{2}}}$is$0$and oxidation number of hydrogen in water is $+ 1$ so, the oxidation number of oxygen in water is,
$\Rightarrow \left( { + 1 \times 2} \right) + \left( {x \times 1} \right)\, = 0$
$\therefore x = - 2$
So, the oxidation number of oxygen in water is $- 2$. The oxidation number of oxygen is changing from $0$ to$- 2$during the reaction. It means oxygen is taking electrons and getting reduced. The reaction is an example of the reduction of oxygen.

Oxidation number of hydrogen in elemental form ${{\text{H}}_{\text{2}}}$is$0$and oxidation number of oxygen in water is $- 2$ so, the oxidation number of hydrogen in water is,
$\Rightarrow \left( {x \times 2} \right) + \left( { - 2 \times 1} \right)\, = 0$
$\Rightarrow 2x\, = + 2$
$\therefore x = + 1$
So, the oxidation number of hydrogen in water is $+ 1$. The oxidation number of hydrogen is changing from $0$ to$+ 1$during the reaction. It means hydrogen is losing an electron and getting oxidised. The reaction is an example of oxidation of hydrogen.

The redox reactions are the reaction in which one species is oxidized and another is reduced. Here, oxygen is reduced and hydrogen is getting oxidized, so this is a redox reaction.

Therefore, option (D) a, b and d, is the answer correct.

Note: The reaction in which electrons are transferred between two reactants is known as the redox reaction. The species which lose electrons cause another species to accept electrons, so the species which lose electrons is known as reducing species. The species which gain electrons cause another species to lose electrons, so the species which gain electrons is known as oxidizing species.