Question

Which of the following is a redox reaction?
A: ${H_2} + B{r_2}\xrightarrow{{}}2HBr$
B: $2NaCl + {H_2}S{O_4}\xrightarrow[{}]{{}}N{a_2}S{O_4} + 2HCl$
C: $HCl + AgN{O_3}\xrightarrow[{}]{{}}AgCl + HN{O_3}$
D: $NaOH + HCl\xrightarrow[{}]{}NaCl + {H_2}O$

Answer 1

Hint: Redox reactions are those reactions are those reactions in which oxidation and reduction both take place. In a chemical reaction a substance that loses electrons is said to be oxidized and the substance that gains electrons is said to be reduced.

Complete step by step answer:
Redox reaction is that reaction in which both oxidation and reduction of a chemical species takes place at a time. A chemical substance is said to be oxidized when it loses electrons or the net positive charge on that species increases and a chemical substance is said to be reduced when it gains electrons or the net negative charge on that species increases. In this question we have to find out which reaction among the given reactions is a redox reaction.
First option is:
${H_2} + B{r_2}\xrightarrow{{}}2HBr$
In this reaction the reactant is hydrogen gas and bromine gas. These gases are electrically neutral, that is there is no charge on the atoms of these gases before reaction but after the reaction these both gases combine together to make a compound. In this compound oxidation state of hydrogen is $ + 1$ and oxidation state of bromine is $ - 1$ (bromine is more electronegative than hydrogen due to which bromine has negative charge). This means hydrogen is oxidized (initial charge on hydrogen was zero and after reaction it is $ + 1$) as net positive charge on hydrogen is increased and bromine is reduced (initial charge on bromine was zero and after reaction it is $ - 1$) as net negative charge on bromine is increased. Therefore in this reaction there is both oxidation and reduction of chemical species. So, this is a redox reaction. But in other reactions given in options, there is no change in oxidation state of the reactants. Therefore, those reactions are not redox reactions. So, the correct answer is option A that is ${H_2} + B{r_2}\xrightarrow{{}}2HBr$.

So, the correct answer is Option A .

Note:
Oxidation can also be defined as the gain of oxygen and reduction is defined as the loss of oxygen from a compound.
Spontaneous redox reactions are usually exothermic in nature and the energy release in these reactions can be used as a source of energy.