Question

Determine the change in oxidation number of sulfur in ${H_2}S$ and $S{O_2}$ respectively in the following reaction.
$2{H_2}S + S{O_2} \to 2{H_2}O + 3S$
A. \[0, + 2\]
B. $ + 2, - 4$
C. $ - 2, + 2$
D. $ + 4,0$

Answer 1

Hint: The change in oxidation number means the change of oxidation state of an element in the reactant state to the oxidation state of that element in the product state. In the above reaction, the sulfur is present in both reactants ${H_2}S$ and $S{O_2}$ but only in the product as $S$ which means one needs to compare reactant values with only product value.

Complete step by step answer:
- First of all we will learn about the concept of change in oxidation number where it means a loss of negatively-charged electrons which results in an increase in oxidation number and in case of a gain of electrons results in a decrease in the oxidation number of that element.
-Now we will calculate the oxidation number of sulfur present in the ${H_2}S$ where the two hydrogen atoms have an oxidation number of $ + 1$ for each hydrogen atom which stands a total of $ + 2$ two hydrogen atoms. Therefore, the oxidation number for the sulfur in the compound is $ - 2$ and the total sum of the oxidation numbers of all elements in the compound is zero. As the oxidation number of sulfur is $ - 2$
-Now let us calculate the oxidation number for the sulfur in $S{O_2}$ a molecule where the two oxygen atoms have an oxidation number of $ - 2$ for each oxygen atom which stands a total of $ + 4$ two oxygen atoms. Therefore, the oxidation number for the sulfur in the compound is $ + 4$ and the total sum of the oxidation numbers of all elements in the compound is zero.
- The oxidation number of sulfur in the product $S$ is zero as it is a single element.
Now let us calculate the change in oxidation number for ${H_2}S$ where it changes from $ - 2$ to zero which means change is $ + 2$ and in $S{O_2}$ molecule it changes from $ + 4$ to zero which means change is $ - 4$. Hence, the change in oxidation number of sulfur in ${H_2}S$ and $S{O_2}$ respectively $ + 2, - 4$ which shows option B as the correct choice.

Note: The element that is oxidized undergoes an increase in oxidation number and if it gets reduced there is a decrease in oxidation number. The total sum of all the oxidation numbers present in a compound must be equal to zero. The oxidation number of a single element is always zero.