Question

The $\Delta {E^o}$ of combustion of isobutylene is $- X$ $kJmo{l^{ - 1}}$ . The value of $\Delta {H^o}$ is:
A) $= \Delta {E^0}$
B) $> \Delta {E^o}$
C) Zero
D) $< \Delta {E^o}$

Answer 1

Heat of combustion is defined as the amount of energy which is produced when any substance is burned in the presence of oxygen under standard conditions. It is also known as calorific value. In the case of organic compounds complete combustion produces carbon dioxide and water molecules.
Formula used:
$\Delta {H^o} = \Delta {E^o} + \Delta {n_g}RT$
Where,
$\Delta {n_g}$ = number of moles of gases
$R$ = Gas constant
$T$ = Temperature .

Complete Step by Step Solution
Combustion of isobutylene will show the following chemical reaction:
${(C{H_3})_2}C = C{H_{2(g)}} + 6{O_{2(g)}} \to 4C{O_{2(g)}} + 4{H_2}{O_{(l)}}$
As it is an organic compound that is a hydrocarbon which produces water and carbon dioxide as the complete combustion product.
Here,**
$\Delta {n_g}$ will be the difference between the number of moles of carbon dioxide and the number of moles of oxygen which is required for complete combustion.
So,
\Delta {n_g} = 4 - 6 \\\ \Rightarrow \Delta {n_g} = - 2 \\\
Here the value of $\Delta {n_g}$ is negative. As we know the relation between $\Delta {H^o}$ and $\Delta {E^o}$ is:
$\Delta {H^o} = \Delta {E^o} + \Delta {n_g}RT$
As $\Delta {n_g}$ is negative so, $\Delta {H^o} > \Delta {E^o}$
Hence, Option D is correct.

Additional Information
The boiling point of a liquid depends on the surrounding environmental pressure. A liquid in a partial vacuum has a lower boiling point than the liquid which is at atmospheric pressure. A liquid at high pressure has a higher boiling point than the liquid which is at atmospheric pressure. For example, water boils at ${100^o}C$ at sea level, but at ${93.4^o}C$ at $1905metres$ altitude. For a given pressure, each and every liquid will boil at different temperatures.

Note
Chlorine and sulphur are not standardized. They are generally supposed to convert to hydrogen chloride gas and sulphur dioxide or sulphur trioxide gas and also to dilute aqueous hydrochloric acid and sulphuric acids, respectively, when the combustion takes place in a bomb with some water.