Chapter 5, Problem 5.140AP

(a)

Interpretation Introduction

Interpretation: The conversion of glucose into formaldehyde is to be predicted as oxidation or reduction process.

The heat of the reaction for the conversion of glucose into formaldehyde is to be calculated for the given data.

Concept introduction: The chemical processes in which addition of hydrogen or removal of oxygen takes place, are termed as reduction and it’s vice versa are termed as oxidation.

The heat of reaction is also termed as change in enthalpy of reaction and calculated by the formula,

$\Delta H_{\text{Heat of rxn}}^{\text{ο}}={\displaystyle \sum n\times \Delta }{H}_{\text{f,HCHO}\left(g\right)}^{\text{ο}}-{\displaystyle \sum m\times \Delta }{H}_{{\text{f,C}}_6{\text{H}}_{12}{\text{O}}_6\left(s\right)}^{\text{ο}}$

To determine: The conversion of glucose into formaldehyde as oxidation or reduction process.

Answer to Problem 5.140AP

Solution

The conversion of glucose into formaldehyde is an oxidation process.

Explanation of Solution

Explanation

The given conversion of glucose into formaldehyde is stated as,

${\text{C}}_6{\text{H}}_{12}{\text{O}}_6\to 6\text{HCHO}$

The given conversion in open form is shown in Figure $1$.

Figure $1$

The chemical processes in which addition of hydrogen or removal of oxygen takes place, are termed as reduction and it’s vice versa are termed as oxidation.

The Figure $1$ clearly shows that all of the $-{\text{CH}}_2\text{OH}$ groups get converted into $-\text{CHO}$ groups by removal of hydrogen. Hence, the conversion of glucose into formaldehyde is an example of oxidation process.

(b)

Interpretation Introduction

To determine: The heat of the reaction for the conversion of glucose into formaldehyde for the given data.

Answer to Problem 5.140AP

Solution

The heat of the reaction for the conversion of glucose into formaldehyde is $\underset{\_}{900kJ/mol}$.

Explanation of Solution

Explanation

Given

The $\Delta H_{\text{comb}}^{\text{ο}}$ of formaldehyde gas $\left(\Delta H_{\text{comb, HCHO}\left(g\right)}^{\text{ο}}\right)$ is $-572.9\text{kJ}/\text{mol}$.

The $\Delta H_{\text{f}}^{\text{ο}}$ of solid glucose is $\left(\Delta H_{{\text{f,C}}_6{\text{H}}_{12}{\text{O}}_6\left(s\right)}^{\text{ο}}\right)$ $-1274.4\text{kJ}/\text{mol}$.

The $\Delta H_{\text{comb}}^{\text{ο}}$ of formaldehyde gas is the change in enthalpy when one mole of formaldehyde gas combusted in the presence of excess air.

The balanced chemical reaction for the combustion of formaldehyde gas is,

$\text{HCHO}\left(g\right)+{\text{O}}_{\text{2}}\left(g\right)\to {\text{CO}}_2\left(g\right)+{\text{H}}_{\text{2}}\text{O}\left(g\right)$ (1)

In equation (1),

• The stoichiometry coefficients of both products and reactants are one.
• The change in enthalpy for the formation of oxygen gas $\left(\Delta H_{{\text{f,O}}_2\left(g\right)}^{\text{ο}}\right)$ is $0\text{kJ}/\text{mol}$.
• The change in enthalpy for the formation of carbon dioxide gas $\left(\Delta H_{{\text{f,CO}}_2\left(g\right)}^{\text{ο}}\right)$ is $-\text{393}\text{.5}\text{kJ}/\text{mol}$.
• The change in enthalpy for the formation of water vapor $\left(\Delta H_{{\text{f,H}}_2\text{O}\left(g\right)}^{\text{ο}}\right)$ is $-241.820\text{kJ}/\text{mol}$.

The standard formula for calculation of change in enthalpy is,

$\Delta H_{\text{rxn}}^{\text{ο}}={\displaystyle \sum n\times \Delta }{H}_{\text{f}}^{\text{ο}}\left(\text{Products}\right)-{\displaystyle \sum m\times \Delta }{H}_{\text{f}}^{\text{ο}}\left(\text{Reactants}\right)$ (2)

Where,

• $\Delta H_{\text{rxn}}^{\text{ο}}$ is change in enthalpy for a given reaction.
• $n$ is stoichiometry coefficient of products in a balanced chemical equation.
• $m$ is stoichiometry coefficient of reactants in a balanced chemical equation.
• $\Delta H_{\text{f}}^{\text{ο}}\left(\text{Products}\right)$ is change in enthalpy for the formation of products.
• $\Delta H_{\text{f}}^{\text{ο}}\left(\text{Reactants}\right)$ is change in enthalpy for the formation of products.

The change in enthalpy for combustion process of formaldehyde gas is written as,

$\Delta H_{\text{comb, HCHO}\left(g\right)}^{\text{ο}}={\displaystyle \sum n\times \Delta }{H}_{\text{f}}^{\text{ο}}\left({\text{CO}}_2\left(g\right),{\text{H}}_2\text{O}\left(g\right)\right)-{\displaystyle \sum m\times \Delta }{H}_{\text{f}}^{\text{ο}}\left(\text{HCHO}\left(g\right),{\text{O}}_2\left(g\right)\right)$

Substitute the values of $n$, $m$, $\Delta H_{{\text{f,O}}_2\left(g\right)}^{\text{ο}}$, $\Delta H_{{\text{f,H}}_2\text{O}\left(g\right)}^{\text{ο}}$, $\Delta H_{{\text{f,CO}}_2\left(g\right)}^{\text{ο}}$ and $\Delta H_{\text{comb, HCHO}\left(g\right)}^{\text{ο}}$ in the above expression.

$\begin{array}{c}\Delta H_{\text{comb, HCHO}\left(g\right)}^{\text{ο}}=\left(1\times \Delta H_{{\text{f,CO}}_2\left(g\right)}^{\text{ο}}+1\times \Delta H_{{\text{f,H}}_2\text{O}\left(g\right)}^{\text{ο}}\right)-\left(1\times \Delta H_{\text{f,HCHO}\left(g\right)}^{\text{ο}}+1\times \Delta H_{{\text{f,O}}_2\left(g\right)}^{\text{ο}}\right)\\ -572.9\text{kJ}/\text{mol}=\left(\left(1\times -393.5+1\times -241.820\right)-\left(\Delta H_{\text{f,HCHO}\left(g\right)}^{\text{ο}}+0\right)\right)\text{kJ}/\text{mol}\\ -572.9\text{kJ}/\text{mol}=-393.5\text{kJ}/\text{mol}-241.820\text{kJ}/\text{mol}-\Delta H_{\text{f,HCHO}\left(g\right)}^{\text{ο}}\\ \Delta H_{\text{f,HCHO}\left(g\right)}^{\text{ο}}=-635.32\text{kJ}/\text{mol}+572.9\text{kJ}/\text{mol}\end{array}$

Simplify,

$\Delta H_{\text{f,HCHO}\left(g\right)}^{\text{ο}}=-62.4\text{kJ}/\text{mol}$

The heat of reaction is also termed as change in enthalpy of reaction and calculated by the formula as stated in equation (2). So, the heat of reaction for the conversion of glucose into formaldehyde is calculated by,

$\Delta H_{\text{Heat of rxn}}^{\text{ο}}={\displaystyle \sum n\times \Delta }{H}_{\text{f,HCHO}\left(g\right)}^{\text{ο}}-{\displaystyle \sum m\times \Delta }{H}_{{\text{f,C}}_6{\text{H}}_{12}{\text{O}}_6\left(s\right)}^{\text{ο}}$ . (3)

The conversion of glucose into formaldehyde is stated as,

${\text{C}}_6{\text{H}}_{12}{\text{O}}_6\to 6\text{HCHO}$ . (4)

In equation (4),

• The value of $n$ is $6$.
• The value of $m$ is $1$.

Substitute the value of $n$, $m$, $\Delta H_{{\text{f,C}}_6{\text{H}}_{12}{\text{O}}_6\left(s\right)}^{\text{ο}}$ and $\Delta H_{\text{f,HCHO}\left(g\right)}^{\text{ο}}$ in equation (3)

$\begin{array}{c}\Delta H_{\text{Heat of rxn}}^{\text{ο}}={\displaystyle \sum n\times \Delta }{H}_{\text{f,HCHO}\left(g\right)}^{\text{ο}}-{\displaystyle \sum m\times \Delta }{H}_{{\text{f,C}}_6{\text{H}}_{12}{\text{O}}_6\left(s\right)}^{\text{ο}}\\ =6\times -62.4\text{kJ}/\text{mol}-\left(1\times -1274.4\text{kJ}/\text{mol}\right)\\ =-374.4\text{kJ}/\text{mol}+1274.4\text{kJ}/\text{mol}\\ \Delta H_{\text{Heat of rxn}}^{\text{ο}}=900\text{kJ}/\text{mol}\end{array}$

Hence, the heat of the reaction for the conversion of glucose into formaldehyde is $\underset{\_}{900kJ/mol}$.

Conclusion

1. a. The conversion of glucose into fructose is an oxidation process.
2. b. The heat of the reaction for the conversion of glucose into formaldehyde is $\underset{\_}{900kJ/mol}$