Chapter 22, Problem 81A

Interpretation Introduction

Interpretation: The balanced equation for the combustion of gaseous alkane needs to be written and alkane needs to be identified.

Concept Introduction: The combustion reaction takes place in the presence of oxygen and gives carbon dioxide and water as products.

The general reaction is represented as follows:

  $\text{Hydrocarbon}+{\text{O}}_{\text{2}}\to {\text{CO}}_{\text{2}}+{\text{O}}_{\text{2}}$

Explanation of Solution

According to the question, complete combustion of gaseous alkane gives carbon dioxide and water. It takes place in the presence of oxygen gas.

A gaseous alkane gives volume of carbon dioxide twice the starting alkane at same temperature and pressure. Thus, number of moles of carbon dioxide produced from combustion of alkane will also be twice.

The reaction can be represented as follows:

  ${\text{C}}_n{\text{H}}_{2n+2}+{\text{O}}_{\text{2}}\to {\text{CO}}_{\text{2}}+{\text{H}}_{\text{2}}\text{O}$

Or,

  ${\text{C}}_n{\text{H}}_{2n+2}+{\text{O}}_{\text{2}}\to 2{\text{CO}}_{\text{2}}+{\text{H}}_{\text{2}}\text{O}$

To balance the above reaction, value of n should be 2. Thus,

  $\begin{array}{l}{\text{C}}_2{\text{H}}_{2\left(2\right)+2}+{\text{O}}_{\text{2}}\to 2{\text{CO}}_{\text{2}}+{\text{H}}_{\text{2}}\text{O}\\ {\text{C}}_2{\text{H}}_6+{\text{O}}_{\text{2}}\to 2{\text{CO}}_{\text{2}}+{\text{H}}_{\text{2}}\text{O}\end{array}$

Thus, the given alkane is ${\text{C}}_2{\text{H}}_6$ .

Now, complete balanced equation can be obtained by checking if all the atoms of same elements have same number of atoms on both sides.

To balance the number of H atoms gives coefficient 3 to ${\text{H}}_{\text{2}}\text{O}$ .

  ${\text{C}}_2{\text{H}}_6+{\text{O}}_{\text{2}}\to 2{\text{CO}}_{\text{2}}+3{\text{H}}_{\text{2}}\text{O}$

Now, give coefficient 7/2 to ${\text{O}}_{\text{2}}$

  ${\text{C}}_2{\text{H}}_6+\frac{7}{2}{\text{O}}_{\text{2}}\to 2{\text{CO}}_{\text{2}}+3{\text{H}}_{\text{2}}\text{O}$

Therefore, the complete balanced chemical equation is as follows:

  ${\text{2C}}_2{\text{H}}_6+7{\text{O}}_{\text{2}}\to 4{\text{CO}}_{\text{2}}+6{\text{H}}_{\text{2}}\text{O}$