Chapter 20, Problem 1TS

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) ${\text{H}}_2{\text{+ Cl}}_2\to \text{2 HCl}$

(b) $2\text{HC}\equiv \text{CH}\text{+}\text{5}{\text{O}}_2\to \text{4}{\text{CO}}_2\text{+2}{\text{H}}_2\text{O}$

To determine

Whether the given reactions are exothermic or endothermic.

Answer to Problem 1TS

The given reactions are exothermic reactions.

Explanation of Solution

(a) The given reaction,

${\text{H}}_2\text{+}{\text{Cl}}_2\to \text{2}\text{HCl}$

There is one $\text{H}-\text{H}$ bond and the bond energy absorbed to break this bond from the table is $+436\text{ kJ/mol}$ .

There is one $\text{Cl}-\text{Cl}$ bond and the bond energy absorbed to break this bond from the table is $+243\text{ kJ/mol}$ .

Thus, the total energy absorbed to break the bonds in the reactant is $\left(+436+(+243)\right)\text{ kJ/mol}$ this gives out to be $+679\text{ kJ/mol}$ .

There are two $\text{H}-\text{Cl}$ bonds in the product side. Thus the total energy released will be $2\times \left(-431\right)\text{ kJ/mol}$ this comes out to be $-862\text{ kJ/mol}$ .

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

$\begin{array}{c}\text{Net energy of reaction}=\text{energy absorbed}+\text{energy released}\\ =+679+\left(-862\right)\text{ kJ/mol}\\ =-183\text{ kJ/mol}\end{array}$ .

Thus, it is an exothermic reaction.

(b) The given reaction,

$2\text{HC}\equiv \text{CH}\text{+}\text{5}{\text{O}}_2\to \text{4}{\text{CO}}_2\text{+2}{\text{H}}_2\text{O}$

There is one $\text{C}\equiv \text{C}$ bond and two $\text{C}-\text{H}$ bonds and the bond energy absorbed to break these bonds from the table is $\left(+837\right)+\left(+2\times 414\right)\text{ kJ/mol}$ this comes out to be $+1665\text{ kJ/mol}$ .

There are five oxygen molecules having $\text{O}=\text{O}$ bond and the bond energy absorbed to break these bonds from the table is $5\times \left(+498\right)\text{ kJ/mol}$ this comes out to be $+2490\text{ kJ/mol}$ Thus, the total energy absorbed to break the bonds in the reactant is $\left(+1665+(+2490)\right)\text{ kJ/mol}$ this gives out to be $+4155\text{ kJ/mol}$ .

There are two $\text{C}=\text{O}$ bonds in carbon dioxide at the product side and there are four moles of carbon dioxide present, therefore a total of eight $\text{C}=\text{O}$ bonds present at the product side. Thus the total energy released will be $8\times \left(-803\right)\text{ kJ/mol}$ this comes out to be $-6424\text{ kJ/mol}$ .

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,

$\begin{array}{c}\text{Net energy of reaction}=\text{energy absorbed}+\text{energy released}\\ =+4155+\left(-4568\right)\text{ kJ/mol}\\ =-413\text{ kJ/mol}\end{array}$ .

Thus, it is an exothermic reaction.

Conclusion:

Therefore, the given reactions are exothermic reactions.