Question

In the complete combustion of ethane the number of oxygen molecules required is:
(A) 2
(B) 7
(C) $\dfrac{5}{2}$
(D) $\dfrac{7}{2}$

Answer 1

Hint: Combustion is a high temperature exothermic chemical reaction involving a hydrocarbon or organic molecule reacting with oxygen to give carbon dioxide, water and evolution of heat. For example, the combustion of carbon can be expressed as:
${\text{C(s) + }}{{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{C}}{{\text{O}}_{\text{2}}}{\text{(g)}}$
The change in enthalpy when one mole of a substance undergoes complete combustion is termed as enthalpy of combustion.

Complete step by step answer:
We need to find out the number of oxygen molecules required for the complete combustion of ethane.
By complete combustion, it is meant that the reactants burn in oxygen and gives a limited number of products. The complete combustion of a hydrocarbon or when a hydrocarbon burns in oxygen, carbon dioxide and water are the main products. When elements are burned, the combustion reaction gives their most common oxides as the primary products.
 Ethane is a hydrocarbon having the formula ${{\text{C}}_{\text{6}}}{{\text{H}}_{\text{6}}}$ and so its combustion will give carbon dioxide and water. The complete combustion reaction for ethane is : ${\text{2}}{{\text{C}}_{\text{6}}}{{\text{H}}_{\text{6}}}{\text{ + 7}}{{\text{O}}_{\text{2}}} \to {\text{4C}}{{\text{O}}_{\text{2}}}{\text{ + 6}}{{\text{H}}_{\text{2}}}{\text{O + energy}}$
Thus, it can be seen that 7 molecules of oxygen are required for the complete combustion of 2 molecules of ethane. Hence, $\dfrac{7}{2}$ molecules of oxygen are required for the complete combustion of 1 molecule of ethane.

So, the correct answer is D.

Note: In incomplete combustion, there is not enough oxygen for complete reaction to take place to give carbon dioxide and water. In case of incomplete combustion, water is produced just like in complete combustion. But instead of carbon dioxide, carbon, carbon monoxide and hydroxide are produced in incomplete combustion.