Question

Which of the following has the lowest calorific value?
A.${C_2}{H_6}$
B.${C_2}{H_5}OH$
C.${C_2}{H_4}$
D.${H_2}$

Answer 1

To answer this question, we need to find the lowest calorific value and thus, we can compare the calorific values of individual component elements. Hydrogen has very high calorific value followed by carbon while oxygen has the lowest of the three elements.

Complete step by step solution:
We know that calorific value can be defined as the amount of energy that is released or evolved when $1{\text{kg}}$ of a fuel is burnt in presence of oxygen. The calorific value is measured in ${\text{kJ/kg}}$.
Calorific value can be either higher calorific value, also known as gross calorific value (GCV) or lower calorific value (LCV), also known as net calorific value.
The higher calorific value is the value of the maximum heat energy released on complete combustion of the fuel, neglecting the heat used up in order to evaporate the water formed as water vapour. If we subtract the amount of heat required to evaporate water from the higher calorific value, we obtain the lower calorific value of the fuel.
Hydrogen is one of the best fuels and has a very high calorific value. So more the amount (per kilogram) of hydrogen in the compound, more would be the calorific value of the fuel. As a result, among organic fuels, oxygen containing organic compounds like alcohols, ethers, aldehydes and ketones have the lowest calorific values.
Among the given compounds, ${C_2}{H_5}OH$ has the lowest percentage composition of hydrogen compared to the others. So, it has the lowest calorific value.

Thus, the correct answer is B.

Note:
When a fuel is burnt in oxygen, water vapour is produced. Using certain methods, the heat that is contained by the released water vapour can be recovered. This heat gives us the value of calorific value.