Chapter 20, Problem 2TS

Use the bond energies in Table 20.1 and the accounting format shown in Section 20.3 to determine whether these reactions are exothermic or endothermic:

(a) ${\text{H}}_2\text{N}-{\text{NH}}_2\to {\text{H}}_2{\text{+H}}_2{\text{+N}}_2$

(b) $\text{2}{\text{H}}_2{\text{O}}_2\to {\text{O}}_2\text{+2}{\text{H}}_2\text{O}$

To determine

Whether the given reactions are exothermic or endothermic.

Answer to Problem 2TS

The given reactions are exothermic reactions.

Explanation of Solution

(a) The given reaction,

${\text{H}}_2\text{N}-{\text{NH}}_2\to {\text{H}}_2{\text{+H}}_2\text{+}{\text{N}}_2$

There is one $\text{N}-\text{N}$ bond and four $\text{N}-\text{H}$ bonds and the bond energy absorbed to break these bonds from the table is $\left(+159\right)+\left(+4\times 389\right)\text{ kJ/mol}$ this comes out to be $+1715\text{ kJ/mol}$ .

There is two $\text{H}-\text{H}$ bonds and the bond energy released to make this bond from the table is $2\times \left(-436\right)\text{ kJ/mol}$ this comes out to be $-872\text{ kJ/mol}$ .

There is one $\text{N}\equiv \text{N}$ bond and the bond energy absorbed to break this bond from the table is $-946\text{ kJ/mol}$ .

Thus, the total energy released to make the bonds in the product is $\left(-872+(-946)\right)\text{ kJ/mol}$ this gives out to be $-1818\text{ kJ/mol}$ .

Thus, the net energy of the reaction can be given as, $\begin{array}{c}\text{Net energy of reaction}=\text{energy absorbed}+\text{energy released}\\ =+1715+\left(-1818\right)\text{ kJ/mol}\\ =-103\text{ kJ/mol}\end{array}$ .

Thus, it is an exothermic reaction.

(b) The given reaction,

$\text{2}{\text{H}}_2{\text{O}}_2\to {\text{O}}_2\text{+2}{\text{H}}_2\text{O}$

There is one $\text{O}-\text{O}$ bond and two $\text{O}-\text{H}$ bonds and the bond energy absorbed to break these bonds from the table is $\left(+138\right)+\left(+2\times 464\right)\text{ kJ/mol}$ this comes out to be $+1066\text{ kJ/mol}$ .

There is one oxygen molecule having $\text{O}=\text{O}$ bond and the bond energy absorbed to break these bonds from the table is $\text{+498 kJ/mol}$ .

Also, there are two $\text{H}-\text{O}$ bonds in the water molecule at the product side and there are two moles of water molecule present, therefore a total of four $\text{H}-\text{O}$ bonds present at the product side.

Thus the total energy released will be $4\times \left(-464\right)\text{ kJ/mol}$ this comes out to be $-1856\text{ kJ/mol}$ .

Thus, the total energy released to make the bonds in the product is $\left(-6424+(-1856)\right)\text{ kJ/mol}$ this gives out to be $-4568\text{ kJ/mol}$ .

Thus, the net energy of the reaction can be given as,

$\begin{array}{c}\text{Net energy of reaction}=\text{energy absorbed}+\text{energy released}\\ =+1066+\left(-4568\right)\text{ kJ/mol}\\ =-3502\text{ kJ/mol}\end{array}$ .

Thus, it is an exothermic reaction.

Conclusion:

Therefore, the given reactions are exothermic reactions.