Chapter 17, Problem 89A

(a)

Interpretation Introduction

Interpretation:

The enthalpy change for the given chemical reactions is to be calculated.

Concept Introduction :

Hess’s law states that the overall change in enthalpy for the solution is the sum of all changes and is independent of the different phases or steps of a reaction.

An exothermic reaction is a reaction in which heat is released and enthalpy change for the reaction is negative.

Answer to Problem 89A

The enthalpy change for the reaction is $\text{-184}\text{.76 kJ }$

Explanation of Solution

The given chemical reaction is

  ${\text{N}}_{\text{2}}{\text{(g)+3H}}_{\text{2}}\text{(g)}\to {\text{2NH}}_{\text{3}}\text{(g)+92}\text{.38 kJ}$

The heat is released in the reaction; thus, it is an exothermic reaction and $\Delta \text{H}$ for the reaction is $\text{-92}\text{.38 kJ}$ .

The enthalpy change for the reaction ${\text{2N}}_{\text{2}}{\text{(g)+6H}}_{\text{2}}\text{(g)}\to 4{\text{NH}}_{\text{3}}\text{(g)}$ is to be found which is two times the given chemical reaction, thus enthalpy change will also be two times.

  ${\text{2N}}_{\text{2}}{\text{(g)+6H}}_{\text{2}}\text{(g)}\to 4{\text{NH}}_{\text{3}}\text{(g) }\Delta \text{H= 2}\times \text{-92}\text{.38 kJ= -184}\text{.76 kJ}$

(b)

Interpretation Introduction

Interpretation:

The enthalpy change for the given chemical reactions is to be calculated.

Concept Introduction :

Hess’s law states that the overall change in enthalpy for the solution is the sum of all changes and is independent of the different phases or steps of a reaction.

An exothermic reaction is a reaction in which heat is released and enthalpy change for the reaction is negative.

Answer to Problem 89A

The enthalpy change for the reaction is $\text{-138}\text{.57 kJ}$

Explanation of Solution

The given chemical reaction is

  ${\text{N}}_{\text{2}}{\text{(g)+3H}}_{\text{2}}\text{(g)}\to {\text{2NH}}_{\text{3}}\text{(g)+92}\text{.38 kJ}$

The heat is released in the reaction; thus, it is an exothermic reaction and $\Delta \text{H}$ for the reaction is $\text{-92}\text{.38 kJ}$ .

The enthalpy change for the reaction $\frac{3}{2}{\text{N}}_{\text{2}}\text{(g)+}\frac{9}{2}{\text{H}}_{\text{2}}\text{(g)}\to 3{\text{NH}}_{\text{3}}\text{(g)}$ is to be found which is $\frac{3}{2}$ times the given chemical reaction, thus enthalpy change will also be $\frac{3}{2}$ times.

  $\frac{3}{2}{\text{N}}_{\text{2}}\text{(g)+}\frac{9}{2}{\text{H}}_{\text{2}}\text{(g)}\to 3{\text{NH}}_{\text{3}}\text{(g) }\Delta \text{H= }\frac{3}{2}\times \text{-92}\text{.38 kJ= -138}\text{.57 kJ}$

(c)

Interpretation Introduction

Interpretation:

The enthalpy change for the given chemical reactions is to be calculated.

Concept Introduction : Hess’s law states that the overall change in enthalpy for the solution is the sum of all changes and is independent of the different phases or steps of a reaction.

An exothermic reaction is a reaction in which heat is released and enthalpy change for the reaction is negative.

Answer to Problem 89A

The enthalpy change for the reaction is $\text{-46}\text{.19 kJ }$

Explanation of Solution

The given chemical reaction is:

  ${\text{N}}_{\text{2}}{\text{(g)+3H}}_{\text{2}}\text{(g)}\to {\text{2NH}}_{\text{3}}\text{(g)+92}\text{.38 kJ}$

The heat is released in the reaction; thus, it is an exothermic reaction and $\Delta \text{H}$ for the reaction is $\text{-92}\text{.38 kJ}$ .

The enthalpy change for the reaction $\frac{1}{2}{\text{N}}_{\text{2}}\text{(g)+}\frac{3}{2}{\text{H}}_{\text{2}}\text{(g)}\to {\text{NH}}_{\text{3}}\text{(g)}$ is to be found which is $\frac{1}{2}$ times the given chemical reaction, thus enthalpy change will also be $\frac{1}{2}$ times.

  $\frac{1}{2}{\text{N}}_{\text{2}}\text{(g)+}\frac{3}{2}{\text{H}}_{\text{2}}\text{(g)}\to {\text{NH}}_{\text{3}}\text{(g) }\Delta \text{H= }\frac{1}{2}\times \text{-92}\text{.38 kJ= -46}\text{.19 kJ}$