Chapter U5, Problem C20.4RE

(a)

Interpretation Introduction

Interpretation:

The olecular structure of heptanes must be drawn.

Concept Introduction:

Heptane is a straight chain hydrocarbon with 7 carbons.

Answer to Problem C20.4RE

Molecular structure of heptane is drawn below.

  

Explanation of Solution

In the molecular structure of heptanes only carbon chain is shown. Hydrogens are removed for simplicity. There are total 7 carbon and 16 hydrogen atoms. All the carbon is single bonded with another 4 atoms out of which two C and two hydrogen atoms.

b)

Interpretation Introduction

Interpretation:

Combustion reaction of heptane must be given.

Concept Introduction:

Hydrocarbons undergo combustion reaction to produce carbon dioxide and water as shown below.

  ${\text{C}}_{\text{x}}{\text{H}}_{\text{y}}\text{(g)+(x+}\frac{\text{y}}{\text{4}}{\text{)O}}_{\text{2}}\text{(g)}\to {\text{xCO}}_{\text{2}}\text{(g)+}\frac{\text{y}}{\text{2}}{\text{H}}_{\text{2}}\text{O(g)}\text{.}$

Here x and y represents number of carbon and hydrogen atoms in the hydrocarbon.

Answer to Problem C20.4RE

  ${\text{C}}_7{\text{H}}_{16}{\text{(g)+11O}}_{\text{2}}\text{(g)}\to {\text{7CO}}_{\text{2}}{\text{(g)+8H}}_{\text{2}}\text{O(g)}\text{.}$

Explanation of Solution

  $\begin{array}{l}{\text{C}}_7{\text{H}}_{16}\text{(g)+(7+}\frac{16}{\text{4}}{\text{)O}}_{\text{2}}\text{(g)}\to {\text{7CO}}_{\text{2}}\text{(g)+}\frac{16}{\text{2}}{\text{H}}_{\text{2}}\text{O(g)}\text{.}\\ {\text{Or, C}}_7{\text{H}}_{16}{\text{(g)+11O}}_{\text{2}}\text{(g)}\to {\text{7CO}}_{\text{2}}{\text{(g)+8H}}_{\text{2}}\text{O(g)}\text{.}\end{array}$

There are 7 carbons and 16 hydrogens. So from 1 mole heptanes 7 moles of carbon dioxide and 11 moles of water were obtained after complete combustion.

c)

Interpretation Introduction

Interpretation:

Using average bond energies, the heat of reaction of heptanes must be calculated.

Concept Introduction:

Heat of reaction is the difference of energies required to break the bonds and energy obtained after bonds formation.

Answer to Problem C20.4RE

  ${\text{C}}_7{\text{H}}_{16}{\text{(g)+11O}}_{\text{2}}{\text{(g)=7CO}}_{\text{2}}{\text{(g)+8H}}_{\text{2}}\text{O(g)}\text{.}$

Heat of reaction for the above combustion reaction is -4332 kJ/mol.

Explanation of Solution

  

  $\begin{array}{l}{\text{C}}_7{\text{H}}_{16}\text{(g)+(7+}\frac{16}{\text{4}}{\text{)O}}_{\text{2}}{\text{(g)=7CO}}_{\text{2}}\text{(g)+}\frac{16}{\text{2}}{\text{H}}_{\text{2}}\text{O(g)}\text{.}\\ {\text{Or, C}}_7{\text{H}}_{16}{\text{(g)+11O}}_{\text{2}}{\text{(g)=7CO}}_{\text{2}}{\text{(g)+8H}}_{\text{2}}\text{O(g)}\text{.}\end{array}$

In the combustion reaction of heptanes 6 C-C, 16 C-H and 11 O=O bonds are broken. 14 C=O bonds and 16 O-H bonds are produced.

These bond energies must be considered to calculate the heat of reaction for the above combustion reaction.

Bond energies of C-C, C-H and O=O bonds are 346, 413 and 498 kJ/mol respectively.

Bond energies of C=O and O-H are 799 and 463 kJ/mol respectively.

Energy required to break 6 C-C, 16 C-H and 11 O=O bonds can be calculated as follows: $\begin{array}{l}\text{=[(6×346)+(16×413)+(11×498)] kJ/mol}\\ \text{=(2076+6608+5478) kJ/mol}\\ \text{=14162 kJ/mol}\end{array}$

Energy obtained for the formation of 14 C=O bonds and 16 O-H bonds can be calculated as follows: $\begin{array}{l}\text{=[(14×799)+(16×463)] kJ/mol}\\ \text{=(11186+7408) kJ/mol}\\ \text{=18594 kJ/mol}\end{array}$

Heat of reaction

  $\begin{array}{l}\text{=(14162-18594) kJ/mol}\\ \text{=-4332 kJ/mol}\end{array}$