Chapter 21, Problem 106A

(a)

Interpretation Introduction

Interpretation: The given redox reaction needs to be balanced.

  ${\text{Br}}_{\text{2}}\left(g\right)+\text{NaOH}\left(aq\right)\to {\text{NaBrO}}_{\text{3}}\left(aq\right)+\text{NaBr}\left(aq\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

Concept Introduction: In a redox reaction, oxidation and reduction take place simultaneously. Here, the oxidation number change method can be used to balance the chemical equation. In oxidation, an increase in oxidation number takes place and in reduction, a decrease in oxidation number takes place.

Explanation of Solution

To balance a redox reaction, the oxidation number change method can be used. Give the oxidation number of atoms as follows:

  ${\stackrel{0}{\text{Br}}}_{\text{2}}\left(g\right)+\text{NaOH}\left(aq\right)\to \stackrel{5}{{\text{NaBrO}}_{\text{3}}}\left(aq\right)+\text{Na}\stackrel{-1}{\text{Br}}\left(aq\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

Here, Br undergoes reduction as well as oxidation. The increase in oxidation number is from 0 to +5 and the decrease in oxidation number is from 0 to -1.

To make the increase in oxidation number and decrease in oxidation number equal, give coefficient 5 to $\text{NaBr}\left(aq\right)$ .

  ${\text{Br}}_{\text{2}}\left(g\right)+\text{NaOH}\left(aq\right)\to {\text{NaBrO}}_{\text{3}}\left(aq\right)+5\text{NaBr}\left(aq\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

Now to balance the number of Br atoms, gives coefficient 3 to ${\text{Br}}_{\text{2}}\left(g\right)$ and to balance number of Na atoms give coefficient 6 to $\text{NaOH}\left(aq\right)$ .

  ${\text{3Br}}_{\text{2}}\left(g\right)+6\text{NaOH}\left(aq\right)\to {\text{NaBrO}}_{\text{3}}\left(aq\right)+5\text{NaBr}\left(aq\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

In the last step, give coefficient 3 to ${\text{H}}_{\text{2}}\text{O}\left(l\right)$ to balance the number of oxygen and hydrogen atoms.

  ${\text{3Br}}_{\text{2}}\left(g\right)+6\text{NaOH}\left(aq\right)\to {\text{NaBrO}}_{\text{3}}\left(aq\right)+5\text{NaBr}\left(aq\right)+3{\text{H}}_{\text{2}}\text{O}\left(l\right)$

The above equation is a balanced chemical equation.

(b)

Interpretation Introduction

Interpretation: The given redox reaction needs to be balanced.

  ${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}\left(s\right)+{\text{H}}_{\text{2}}\left(g\right)\to \text{Fe}\left(s\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

Concept Introduction: In a redox reaction, oxidation and reduction take place simultaneously. Here, the oxidation number change method can be used to balance the chemical equation. In oxidation, an increase in oxidation number takes place and in reduction, a decrease in oxidation number takes place.

Explanation of Solution

To balance a redox reaction, the oxidation number change method can be used. Give the oxidation number of atoms as follows:

  ${\stackrel{+3}{\text{Fe}}}_{\text{2}}{\text{O}}_{\text{3}}\left(s\right)+{\stackrel{0}{\text{H}}}_{\text{2}}\left(g\right)\to \stackrel{0}{\text{Fe}}\left(s\right)+{\stackrel{+1}{\text{H}}}_{\text{2}}\text{O}\left(l\right)$

In the above redox reaction, oxidation of H and reduction of Fe takes place. The change in oxidation state takes place from +3 to 0 for Fe and 0 to +1 for H.

To make the change in oxidation state equal, give coefficient 3 to both ${\text{H}}_{\text{2}}\left(g\right)$ and ${\text{H}}_{\text{2}}\text{O}\left(l\right)$ .

  ${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}\left(s\right)+3{\text{H}}_{\text{2}}\left(g\right)\to \text{Fe}\left(s\right)+3{\text{H}}_{\text{2}}\text{O}\left(l\right)$

Now, give coefficient 2 to $\text{Fe}\left(s\right)$ to balance number of Fe atoms.

  ${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}\left(s\right)+3{\text{H}}_{\text{2}}\left(g\right)\to 2\text{Fe}\left(s\right)+3{\text{H}}_{\text{2}}\text{O}\left(l\right)$

Thus, the above reaction is balanced.