Chapter 4, Problem 21Q

Use the bond energies in Table 5.1 to calculate the energy changes associated with each of these reactions. Label each reaction as endothermic or exothermic.

1. a. N₂(g) + 3 H₂(g) → 2 NH₃(g)
2. b. H₂(g) + Cl₂(g) → 2 HCl₃(g)

Hint: Draw Lewis structures of the reactants and products to determine the number and kinds of bonds.

Interpretation Introduction

Interpretation:

The energy changes have to be calculated.

Concept introduction:

Endothermic reaction: When the heat energy was absorbed by the system from the surrounding is called endothermic reaction.

Exothermic reaction: When heat energy or light energy was unconfined to the surrounding from the system is called exothermic reaction.

Net energy change is calculated from the difference between the total energy absorbed in breaking bonds and total energy released in forming bonds.

Explanation of Solution

The given reaction is shown below,

${\text{N}}_{\text{2}}{}_{\text{(g)}}{\text{ + 3H}}_{\text{2}}{}_{\text{(g)}}\stackrel{}{\to }{\text{2NH}}_{\text{3}}{}_{\text{(g)}}$

Bonds broken in the reactants are given below:

$\begin{array}{l}\text{1 mol N}\equiv \text{N triple bonds = 946 kJ}\hfill \\ \text{1 mol H–H single bonds = 436 kJ}\hfill \\ \hfill \\ \text{Therefore,}\hfill \\ \hfill \\ \text{1 mol N}\equiv \text{N triple bonds = 1}\left(\text{946 kJ}\right)\text{ = 946 kJ}\hfill \\ \text{3 mol H–H single bonds = 3}\left(\text{436 kJ}\right)\text{ = 1308 kJ}\hfill \\ \text{Total energy absorbed in bond breaking = 2254 kJ}\hfill \end{array}$

Bonds formed in the products are given below:

$\begin{array}{l}\text{1 mol N–H single bonds = 391 kJ}\hfill \\ \text{6 mol N–H single bonds = 6}\left(\text{391 kJ}\right)\text{ = 2346 kJ}\hfill \\ \text{Therefore,}\hfill \\ \hfill \\ \text{Total energy released in forming bonds = 2346 kJ}\hfill \end{array}$

$\begin{array}{l}\text{Net energy change}\text{=}\text{total}\text{energy}\text{absorbed}\text{in}\text{the}\text{bond}\text{breaking}\text{-}\text{total}\text{energy}\text{released in}\text{the}\text{bond}\text{forming}\\ \text{Net energy change is (+2254 kJ) + (-2346 kJ) = -92 kJ}\end{array}$

The overall energy change is negative, therefore, the reaction is an exothermic reaction.

Interpretation Introduction

Interpretation:

The energy changes have to be calculated.

Concept introduction:

Endothermic reaction: When the heat energy was absorbed by the system from the surrounding is called endothermic reaction

Exothermic reaction: When heat energy or light energy was unconfined to the surrounding from the system is called exothermic reaction.

Net energy change is calculated from the difference between the total energy absorbed in breaking bonds and total energy released in forming bonds.

Explanation of Solution

The given reaction is shown below,

${\text{H}}_{\text{2}}{}_{\text{(g)}}{\text{ + Cl}}_{\text{2}}{}_{\text{(g)}}\stackrel{}{\to }{\text{2HCl}}_{\text{(g)}}$

Bonds broken in the reactants are given below:

$\begin{array}{l}\text{1 mol Cl–Cl single bonds = 242 kJ}\hfill \\ \text{1 mol H–H single bonds = 436 kJ}\hfill \\ \hfill \\ \text{Therefore,}\hfill \\ \hfill \\ \text{1 mol Cl–Cl single bonds = 1(242 kJ) = 242 kJ}\hfill \\ \text{1 mol H–H single bonds = 1}\left(\text{436 kJ}\right)\text{ = 436 kJ}\hfill \\ \text{Total energy absorbed in bond breaking = 678 kJ}\hfill \end{array}$

Bonds formed in the products are given below:

$\begin{array}{l}\text{1 mol H–Cl single bonds = 431 kJ}\hfill \\ \text{2 mol H–Cl single bonds = 2}\left(\text{431 kJ}\right)\text{ = 862 kJ}\hfill \\ \text{Therefore,}\hfill \\ \hfill \\ \text{Total energy released in forming bonds = 862 kJ}\hfill \end{array}$

$\begin{array}{l}\text{Net energy change}\text{=}\text{total}\text{energy}\text{absorbed}\text{in}\text{the}\text{bond}\text{breaking}\text{-}\text{total}\text{energy}\text{released in}\text{the}\text{bond}\text{forming}\\ \text{Net energy change is (+678 kJ) + (-862 kJ) = -184 kJ}\end{array}$

The overall energy change is negative, therefore, the reaction is an exothermic reaction.