Chapter 6, Problem 6.32QP

Shown below is a diagram depicting the enthalpy change of a chemical reaction run at constant pressure.

1. a Is the reaction exothermic or endothermic?
2. b What is the sign of ΔH?
3. c What is the sign of q?
4. d If the reaction does no work, what is the sign of ΔE for this process?

Interpretation Introduction

Interpretation:

The nature of the given reaction has to be identified.

Concept introduction:

Exothermic reaction:

If heat is released during a chemical or physical change those reactions are called as exothermic reactions.

$\begin{array}{l}\text{Example:}\\ {\text{CH}}_{\text{4}}{}_{\left(\text{g}\right)}\text{+}{\text{2O}}_{\text{2}}{}_{\left(\text{g}\right)}\stackrel{}{\to }{\text{CO}}_{\text{2}}{}_{\left(\text{g}\right)}\text{+}{\text{2H}}_{\text{2}}{\text{O}}_{\left(\text{l}\right)}\text{+}\text{890}\text{kJ}\\ \text{In}\text{this}\text{reaction}\text{890}\text{kJ}\text{of}\text{heat}\text{is}\text{released}\text{.}\end{array}$

Endothermic reaction:

If heat is absorbed during a chemical or physical change those reactions are called as endothermic reactions.

$\begin{array}{l}\text{Example:}\\ {\text{Ba(OH)}}_{\text{2}}{}_{\left(\text{aq}\right)}\text{+}{\text{2NH}}_{\text{4}}{\text{NO}}_{\text{3}}{}_{\left(\text{aq}\right)}\stackrel{}{\to }{\text{2NH}}_{\text{3}}{}_{\left(\text{g}\right)}\text{+}{\text{2H}}_{\text{2}}{\text{O}}_{\left(\text{l}\right)}\text{+}\text{Ba}{\left({\text{NO}}_{\text{3}}\right)}_{\text{2}}\\ \text{In}\text{this}\text{reaction}\text{170}\text{.8}\text{kJ}\text{of}\text{heat}\text{is}\text{absorbed}\text{.}\end{array}$

Answer to Problem 6.32QP

Given reaction is endothermic reaction because the enthalpy increases moving from reactants to products.

Explanation of Solution

Given information,

Figure 1

From the diagram we understand that the enthalpy of the reaction increases from reactants to products.  Hence the given reaction is endothermic reaction.

Interpretation Introduction

Interpretation:

The sign of the enthalpy change $\left(\text{ΔH}\right)$ has to be given.

Concept introduction:

Exothermic reaction:

If heat is released during a chemical or physical change those reactions are called as exothermic reactions.

$\begin{array}{l}\text{Example:}\\ {\text{CH}}_{\text{4}}{}_{\left(\text{g}\right)}\text{+}{\text{2O}}_{\text{2}}{}_{\left(\text{g}\right)}\stackrel{}{\to }{\text{CO}}_{\text{2}}{}_{\left(\text{g}\right)}\text{+}{\text{2H}}_{\text{2}}{\text{O}}_{\left(\text{l}\right)}\text{+}\text{890}\text{kJ}\\ \text{In}\text{this}\text{reaction}\text{890}\text{kJ}\text{of}\text{heat}\text{is}\text{released}\text{.}\end{array}$

Endothermic reaction:

If heat is absorbed during a chemical or physical change those reactions are called as endothermic reactions.

$\begin{array}{l}\text{Example:}\\ {\text{Ba(OH)}}_{\text{2}}{}_{\left(\text{aq}\right)}\text{+}{\text{2NH}}_{\text{4}}{\text{NO}}_{\text{3}}{}_{\left(\text{aq}\right)}\stackrel{}{\to }{\text{2NH}}_{\text{3}}{}_{\left(\text{g}\right)}\text{+}{\text{2H}}_{\text{2}}{\text{O}}_{\left(\text{l}\right)}\text{+}\text{Ba}{\left({\text{NO}}_{\text{3}}\right)}_{\text{2}}\\ \text{In}\text{this}\text{reaction}\text{170}\text{.8}\text{kJ}\text{of}\text{heat}\text{is}\text{absorbed}\text{.}\end{array}$

Answer to Problem 6.32QP

The sign of enthalpy change $\left(\text{ΔH}\right)$ is positive $\left(\text{ΔH}\text{>}\text{0}\right)\text{.}$

Explanation of Solution

Given information,

Figure 1

The enthalpy is increased from reactants to products so the sign of enthalpy is positive.

Interpretation Introduction

Interpretation:

The sign of heat $\left(\text{q}\right)$ has to be given.

Concept introduction:

Exothermic reaction:

If heat is released during a chemical or physical change those reactions are called as exothermic reactions.

$\begin{array}{l}\text{Example:}\\ {\text{CH}}_{\text{4}}{}_{\left(\text{g}\right)}\text{+}{\text{2O}}_{\text{2}}{}_{\left(\text{g}\right)}\stackrel{}{\to }{\text{CO}}_{\text{2}}{}_{\left(\text{g}\right)}\text{+}{\text{2H}}_{\text{2}}{\text{O}}_{\left(\text{l}\right)}\text{+}\text{890}\text{kJ}\\ \text{In}\text{this}\text{reaction}\text{890}\text{kJ}\text{of}\text{heat}\text{is}\text{released}\text{.}\end{array}$

Endothermic reaction:

If heat is absorbed during a chemical or physical change those reactions are called as endothermic reactions.

$\begin{array}{l}\text{Example:}\\ {\text{Ba(OH)}}_{\text{2}}{}_{\left(\text{aq}\right)}\text{+}{\text{2NH}}_{\text{4}}{\text{NO}}_{\text{3}}{}_{\left(\text{aq}\right)}\stackrel{}{\to }{\text{2NH}}_{\text{3}}{}_{\left(\text{g}\right)}\text{+}{\text{2H}}_{\text{2}}{\text{O}}_{\left(\text{l}\right)}\text{+}\text{Ba}{\left({\text{NO}}_{\text{3}}\right)}_{\text{2}}\\ \text{In}\text{this}\text{reaction}\text{170}\text{.8}\text{kJ}\text{of}\text{heat}\text{is}\text{absorbed}\text{.}\end{array}$

Answer to Problem 6.32QP

The sign of $\text{heat}\left(\text{q}\right)$ is positive $\left(\text{q}\text{>}\text{0}\right)\text{.}$

Explanation of Solution

Given information,

Figure 1

During the reaction the heat is absorbed so the sign of $\text{q}$ is positive.

Interpretation Introduction

Interpretation:

The sign of internal energy $\left(\text{ΔE}\right)$ change has to be given.

Concept introduction:

Relationship between $HandE:$

The relationship between $\text{H}\text{and}\text{E}$ is given by

$\begin{array}{l}\text{ΔH}\text{=}\text{ΔE}\text{+}\text{PΔV}\\ \text{here,}\\ \text{H}\text{-}\text{enthalpy}\\ \text{E}\text{-}\text{internal}\text{energy}\\ \text{P}\text{-}\text{pressure}\text{and}\text{V}\text{-}\text{volume}\end{array}$

Answer to Problem 6.32QP

The sign of internal energy change $\left(\text{ΔE}\right)$ is positive $\left(\text{ΔE}\text{>}\text{0}\right)\text{.}$

Explanation of Solution

Given information,

Figure 1

Given information,

The reaction does no work.

The relationship between $\text{H}\text{and}\text{E}$ is given by

$\begin{array}{l}\text{ΔH}\text{=}\text{ΔE}\text{+}\text{PΔV}\\ \text{here,}\\ \text{H}\text{-}\text{enthalpy}\\ \text{E}\text{-}\text{internal}\text{energy}\\ \text{P}\text{-}\text{pressure}\text{and}\text{V}\text{-}\text{volume}\end{array}$

Here, there is no pressure and volume work occurs, so the sign of internal energy change is positive $\left(\text{ΔE}\text{>}\text{0}\right)$.