Chapter 21, Problem 90A

(a)

Interpretation Introduction

Interpretation: The given chemical reaction needs to be balanced.

  ${\text{H}}_{\text{2}}\text{S}\left(g\right)+{\text{HNO}}_{\text{3}}\left(aq\right)\to \text{S}\left(s\right)+\text{NO}\left(g\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

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

The given reaction is as follows:

  ${\text{H}}_{\text{2}}\text{S}\left(g\right)+{\text{HNO}}_{\text{3}}\left(aq\right)\to \text{S}\left(s\right)+\text{NO}\left(g\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

The above reaction is a redox reaction thus, it can be balanced by the oxidation number change method. To balance the given reaction, assign oxidation states to each atom.

  ${\text{H}}_{\text{2}}\stackrel{-2}{\text{S}}\left(g\right)+\text{H}\stackrel{+5}{\text{N}}{\text{O}}_{\text{3}}\left(aq\right)\to \stackrel{0}{\text{S}}\left(s\right)+\stackrel{+2}{\text{N}}\text{O}\left(g\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

Here, oxidation of S and reduction of N take place. The change in oxidation number is +2 in the case of S and it is -3 in the case of N.

To make the change in oxidation state equal, the increase in oxidation number must be multiplied by 3, and the decrease in oxidation number is multiplied by 2.

This can be done by giving coefficient 3 to ${\text{H}}_{\text{2}}\text{S}\left(g\right)$ and $\text{S}\left(s\right)$ and coefficient 2 to ${\text{HNO}}_{\text{3}}\left(aq\right)$ and $\text{NO}\left(g\right)$ .

  ${\text{3H}}_{\text{2}}\text{S}\left(g\right)+2{\text{HNO}}_{\text{3}}\left(aq\right)\to 3\text{S}\left(s\right)+2\text{NO}\left(g\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

The number of S and N atoms is balanced.

Now, hydrogen and oxygen atoms can be balanced by giving a coefficient 4 to ${\text{H}}_{\text{2}}\text{O}\left(l\right)$

  ${\text{3H}}_{\text{2}}\text{S}\left(g\right)+2{\text{HNO}}_{\text{3}}\left(aq\right)\to 3\text{S}\left(s\right)+2\text{NO}\left(g\right)+4{\text{H}}_{\text{2}}\text{O}\left(l\right)$

The above reaction is a balanced chemical reaction.

(b)

Interpretation Introduction

Interpretation: The given chemical reaction needs to be balanced.

  ${\text{AgNO}}_3\left(aq\right)+\text{Pb}\left(s\right)\to \text{Pb}{\left({\text{NO}}_3\right)}_2\left(aq\right)+\text{Ag}\left(s\right)$

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

The given reaction is as follows:

  ${\text{AgNO}}_3\left(aq\right)+\text{Pb}\left(s\right)\to \text{Pb}{\left({\text{NO}}_3\right)}_2\left(aq\right)+\text{Ag}\left(s\right)$

The above reaction is a redox reaction thus, it can be balanced by the oxidation number change method. To balance the given reaction, assign oxidation states to each atom.

  $\stackrel{+1}{\text{Ag}}{\text{NO}}_3\left(aq\right)+\stackrel{0}{\text{Pb}}\left(s\right)\to \stackrel{+2}{\text{Pb}}{\left({\text{NO}}_3\right)}_2\left(aq\right)+\stackrel{0}{\text{Ag}}\left(s\right)$

The above reaction shows a decrease in the oxidation number of Ag from +1 to 0 and an increase in the oxidation number of Pb from 0 to +2.

To make the change equal, give coefficient 2 to ${\text{AgNO}}_3\left(aq\right)$ and $\text{Ag}\left(s\right)$ .

  ${\text{2AgNO}}_3\left(aq\right)+\text{Pb}\left(s\right)\to \text{Pb}{\left({\text{NO}}_3\right)}_2\left(aq\right)+2\text{Ag}\left(s\right)$

The above reaction is a balanced chemical reaction.

(c)

Interpretation Introduction

Interpretation: The given chemical reaction needs to be balanced.

  ${\text{Cl}}_2\left(g\right)+\text{NaOH}\left(aq\right)\to \text{NaCl}\left(aq\right)+{\text{NaClO}}_3\left(aq\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

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

The given reaction is as follows:

  ${\text{Cl}}_2\left(g\right)+\text{NaOH}\left(aq\right)\to \text{NaCl}\left(aq\right)+{\text{NaClO}}_3\left(aq\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

The above reaction is a redox reaction thus, it can be balanced by the oxidation number change method. To balance the given reaction, assign oxidation states to each atom.

  $\stackrel{0}{{\text{Cl}}_2}\left(g\right)+\text{NaOH}\left(aq\right)\to \text{Na}\stackrel{-1}{\text{Cl}}\left(aq\right)+\text{Na}\stackrel{+5}{\text{Cl}}{\text{O}}_3\left(aq\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

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

To make the change in oxidation state equal, give coefficient 5 to $\text{NaCl}\left(aq\right)$ and then balance the rest of the atoms.

  ${\text{Cl}}_2\left(g\right)+\text{NaOH}\left(aq\right)\to 5\text{NaCl}\left(aq\right)+{\text{NaClO}}_3\left(aq\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

To balance the number of Na and Cl atoms give coefficients 3 to ${\text{Cl}}_2\left(g\right)$ and 6 to $\text{NaOH}\left(aq\right)$ .

  ${\text{3Cl}}_2\left(g\right)+6\text{NaOH}\left(aq\right)\to 5\text{NaCl}\left(aq\right)+{\text{NaClO}}_3\left(aq\right)+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

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

  ${\text{3Cl}}_2\left(g\right)+6\text{NaOH}\left(aq\right)\to 5\text{NaCl}\left(aq\right)+{\text{NaClO}}_3\left(aq\right)+3{\text{H}}_{\text{2}}\text{O}\left(l\right)$

Thus, the above reaction is a balanced chemical reaction.