Chapter 20.3, Problem 21SP

a)

Interpretation Introduction

Interpretation : Whether the given chemical reaction is redox or not is to be identified and if it is redox then oxidized and reduced atoms in the chemical reaction are to be named.

Concept Introduction: The chemical reaction in which oxidation and reduction take place simultaneously is known as a redox reaction.

Answer to Problem 21SP

The reaction is a redox reaction in which the magnesium atom is getting oxidized and the bromine atom is getting reduced.

Explanation of Solution

Oxidation is the removal of an electron and reduction is the addition of an electron or oxygen atom.

Rules to calculate the oxidation number are as follows:

• All elements of the alkali group in a combined form with other atoms always show $\left(+1\right)$ oxidation state.
• All elements of alkaline earth metal groups in a combined form with other atoms always show $\left(+2\right)$ oxidation state.
• The charge of the oxygen atom is generally $\left(-2\right)$ .
• All elements of halogen groups in a combined form with other atoms always show $\left(-1\right)$ oxidation state.
• The charge of the hydrogen atom is always $\left(+1\right)$ .
• Atoms in their elemental state have zero oxidation state.

The given reaction,

  $Mg(s)+B{r}_2(l)\to MgB{r}_2(s)$

The oxidation number of atoms in their elemental state is zero. So, the oxidation number of magnesium on the reactant side is zero.

The oxidation number of bromine in $B{r}_2$

is calculated as follows:

Let the oxidation number of the Bromine atom of $B{r}_2$ be $x$ .

  $\begin{array}{c}2x=0\\ x=\frac{0}{2}\approx (notdefined)\\ x=0\end{array}$

So, let the oxidation number of the Magnesium atom of $MgB{r}_2$ be $x$ and the charge of the bromine atom in a combined state with another atom is always $\left(-1\right)$ .

  $\begin{array}{c}x+\left(2\times \left(-1\right)\right)=0\\ x-2=0\\ x=+2\end{array}$

Hence,

  

Hence, the reaction is a redox reaction in which the magnesium atom is getting oxidized and the bromine atom is getting reduced.

b)

Interpretation Introduction

Interpretation : Whether the given chemical reaction is redox or not is to be identified and if it is redox then oxidized and reduced atoms in the chemical reaction are to be named.

Concept Introduction: The chemical reaction in which oxidation and reduction take place simultaneously is known as a redox reaction.

Answer to Problem 21SP

The chemical reaction $H_{2}C{O}_3(aq)\to C{O}_2(g)+2H_{2}O(l)$ is not a redox reaction.

Explanation of Solution

Oxidation is the removal of an electron and reduction is the addition of an electron or oxygen atom.

The given reaction,

  $H_{2}C{O}_3(aq)\to C{O}_2(g)+2H_{2}O(l)$

The oxidation number of carbon in $H_{2}C{O}_3$

is calculated as follows:

Let the oxidation number of the carbon atom be x and the charge of oxygen and hydrogen atoms is always $\left(-2\right)$ and $\left(+1\right)$ .

  $\begin{array}{c}x+\left(3\times \left(-2\right)\right)+\left(2\times 1\right)=0\\ x-6+2=0\\ x=+4\end{array}$

So, let the oxidation number of the carbon atom of $C{O}_2$ be $x$ and the charge of the oxygen atom is always $\left(-2\right)$ .

  $\begin{array}{c}x+\left(2\times \left(-2\right)\right)=0\\ x-4=0\\ x=+4\end{array}$

In water molecules, the charge of the hydrogen atom is $\left(+1\right)$ and that of the oxygen atom is $\left(-2\right)$ .

  

Thus, the reaction $H_{2}C{O}_3(aq)\to C{O}_2(g)+2H_{2}O(l)$ is not a redox reaction.