Chapter 20.2, Problem 12SP

(a)

Interpretation Introduction

Interpretation: The oxidized and reduced species in the reaction, $2{\text{H}}_2\left(g\right)+{\text{O}}_{\text{2}}\left(g\right)\to 2{\text{H}}_{\text{2}}\text{O}\left(l\right)$ is to be determined by using the change in oxidation number.

Concept Introduction: The zero, positive or negative numbers that are assigned to an atom for indicating its degree of oxidation and reduction is the oxidation number. This number indicates the number of electrons that are either gained or lost during the bond formation in a chemical compound.

Answer to Problem 12SP

The chemical species that undergo oxidation is ${\text{H}}_2$ and that undergo reduction is ${\text{O}}_{\text{2}}$ in the given reaction.

Explanation of Solution

The completion of chemical reactions results in a change in the oxidation number of chemical compounds. From the change in oxidation number values, the chemical species that have undergone oxidation and reduction can be easily found.

The one in which the oxidation number is found to be decreased is said to be reduced. Since reduction involves the gain of electrons. While the one in which the oxidation number is found to be increased is said to be oxidized. Since oxidation involves the loss of electrons.

In the reaction,

  $2{\text{H}}_2\left(g\right)+{\text{O}}_{\text{2}}\left(g\right)\to 2{\text{H}}_{\text{2}}\text{O}\left(l\right)$

The oxidation number of ${\text{H}}_2$ is increased to +2 from zero by losing electrons. While the oxidation number of ${\text{O}}_{\text{2}}$ is decreased to -2 from zero by gaining electrons. So, the chemical species that is oxidized is ${\text{H}}_2$ and the reduced is ${\text{O}}_{\text{2}}$ .

(b)

Interpretation Introduction

Interpretation: The oxidized and reduced species in the reaction, $2{\text{KNO}}_3\left(s\right)\to {\text{ 2KNO}}_{\text{2}}\left(s\right){\text{+O}}_{\text{2}}\left(g\right)$ is to be determined by using the change in oxidation number.

Concept Introduction: The zero, positive or negative numbers that are assigned to an atom for indicating its degree of oxidation and reduction is the oxidation number. This number indicates the number of electrons that are either gained or lost during the bond formation in a chemical compound.

Answer to Problem 12SP

The chemical species that oxidized is ${\text{O}}_{\text{2}}$ and that reduced is nitrogen in the given reaction.

Explanation of Solution

The completion of chemical reactions results in a change in the oxidation number of chemical compounds. From the change in oxidation number values, the chemical species that have undergone oxidation and reduction can be easily found.

The one in which the oxidation number is found to be decreased is said to be reduced. Since reduction involves the gain of electrons. While the one in which the oxidation number is found to be increased is said to be oxidized. Since oxidation involves the loss of electrons.

In the reaction,

  $2{\text{KNO}}_3\left(s\right)\to {\text{ 2KNO}}_{\text{2}}\left(s\right){\text{+O}}_{\text{2}}\left(g\right)$

The oxidation number of nitrogen in the reactant is,

  $\begin{array}{c}x-6+1=0\\ x=5\end{array}$

The oxidation number of nitrogen in the product is,

  $\begin{array}{c}x-4+1=0\\ x=3\end{array}$

The oxidation number of nitrogen is decreased from +5 to +3. So, nitrogen is said to be reduced.

The oxidation number of oxygen, ${\text{O}}_{\text{2}}$ is increased from $-6$ to 0. Since elemental compounds have an oxidation number of zero. So, oxygen is said to be oxidized.