Chapter 4, Problem 22Q

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

1. a. 2 H₂(g) + CO(g) → CH₃OH(g)
2. b. H₂(g) + O₂(g) → H₂O₂(g)
3. c. 2 BrCl(g) → Br₂(g) + Cl₂(g)

Interpretation Introduction

Interpretation:

The energy changes have to be calculated, the reaction is exothermic or endothermic has to be labelled.

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 is shown below,

Bonds broken in the reactants are given below:

$\begin{array}{l}\text{1 mol H–H bonds = 436 kJ}\hfill \\ \text{1 mol C}\equiv \text{O}\text{triple bonds = 1073 kJ}\hfill \\ \hfill \\ \text{Therefore,}\hfill \\ \hfill \\ \text{2 mol H–H bonds = 2(436 kJ) =}872\text{ kJ}\hfill \\ \hfill \\ \text{Total energy absorbed in bond breaking = 1945 kJ}\hfill \end{array}$

Bonds formed in the products are given below:

$\begin{array}{l}\begin{array}{l}\text{1 mol C–H single bonds = 416 kJ}\\ \text{1 mol C–O single bonds = 336 kJ}\\ \text{1 mol O–H single bonds = 467 kJ}\end{array}\hfill \\ \text{3 mol C–H single bonds = 3}\left(\text{416 kJ}\right)\text{ = 1248 kJ}\hfill \\ \text{Therefore,}\hfill \\ \hfill \\ \text{Total energy released in forming bonds = 2051 kJ}\hfill \end{array}$

$\begin{array}{l}\text{Net energy change}\text{=}\text{total}\text{energy}\text{absorbed}\text{in}\text{the}\text{bond}\text{breaking}\text{-}\text{total}\text{energy}\text{released in}\text{the}\text{bond}\text{forming}\\ \text{Net energy change is (1945 kJ) + (-2051 kJ) = -106 kJ}\end{array}$

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

Interpretation Introduction

Interpretation:

The energy changes have to be calculated, the reaction is exothermic or endothermic has to be labelled.

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 is shown below,

Bonds broken in the reactants are given below:

$\text{1 mol H–H bonds = 436 kJ}$

$\text{1 mol O=O}\text{double bonds = 498 kJ}$

Therefore,

$\text{Total energy absorbed in bond breaking = 934 kJ}$

Bonds formed in the products are given below:

$\begin{array}{l}\text{1 mol O–H single bonds = 467 kJ}\\ \text{1 mol O–O single bonds = 146 kJ}\end{array}$

Therefore,

$\text{2 mol O–H single bonds = 467 kJ = 2}\left(\text{467 kJ}\right)\text{ = 934 kJ}$

$\text{Total energy absorbed in bond breaking = 1080 kJ}$

$\begin{array}{l}\text{Net energy change}\text{=}\text{total}\text{energy}\text{absorbed}\text{in}\text{the}\text{bond}\text{breaking}\text{-}\text{total}\text{energy}\text{released in}\text{the}\text{bond}\text{forming}\\ \text{Net energy change is (934 kJ) + (-1080 kJ) = -146 kJ}\end{array}$

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

Interpretation Introduction

Interpretation:

The energy changes have to be calculated, the reaction is exothermic or endothermic has to be labelled.

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 is shown below,

Bonds broken in the reactants are given below:

$\text{1 mol Br–Cl bonds = 217 kJ}$

$\text{2 mol Br–Cl bonds = 2(217) kJ = 434 kJ}$

Therefore,

$\text{Total energy absorbed in bond breaking = 434 kJ}$

Bonds formed in the products are given below:

$\begin{array}{l}\text{1 mol Br–Br single bonds = 193 kJ}\\ \text{1 mol Cl–Cl single bonds = 242 kJ}\end{array}$

Therefore,

$\text{Total energy absorbed in bond breaking = 435 kJ}$

$\begin{array}{l}\text{Net energy change}\text{=}\text{total}\text{energy}\text{absorbed}\text{in}\text{the}\text{bond}\text{breaking}\text{-}\text{total}\text{energy}\text{released in}\text{the}\text{bond}\text{forming}\\ \text{Net energy change is (434 kJ) + (-435 kJ) = -1 kJ}\end{array}$

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