Question

The heat evolved in the combustion of benzene is given by
${{C}_{6}}{{H}_{6}}(l)+7\frac{1}{2}{{O}_{2}}(g)\to 3{{H}_{2}}O(l)+6C{{O}_{2}}(g),\Delta H=- 781.0Kcalmo{{l}^{- 1}}$
When 156g of ${{C}_{6}}{{H}_{6}}$is burnt in an open container, the amount of heat energy released will be
(A) $150.2kcalmo{{l}^{- 1}}$
(B) $1562.0kcalmo{{l}^{- 1}}$
(C) $6528.2kJmo{{l}^{- 1}}$
(D) $2448.5kJmo{{l}^{- 1}}$

Answer 1

Hint: Combustion or burning is a high- temperature exothermic redox chemical reaction between a fuel (which is a reductant) and an oxidant which is usually atmospheric oxygen, that produces oxidized which is often a gaseous product, in a mixture termed as smoke.

Complete Step by step answer:
- Combustion does not always result in fire, but when a fire is produced a flame is a characteristic indicator of the reaction.
- The original substance in combustion is called the fuel and it can be a solid, liquid, or gas. Likewise, the oxidizer which is the source of oxygen can be a solid, liquid, or gas.
- Let us begin solving this question by calculating the heat energy released in the combustion of benzene:
${{C}_{6}}{{H}_{6}}(l)+7\frac{1}{2}{{O}_{2}}(g)\to 3{{H}_{2}}O(l)+6C{{O}_{2}}(g),\Delta H=- 781.0Kcalmo{{l}^{- 1}}$
The heat energy released when 1 mol of benzene undergoes combustion$=- \Delta H=- (- 781kcal)=+781kcal$
- According to the question,
Weight of benzene which is undergoing combustion = 156 g
- Molecular weight of benzene = $(12\times 6)+(1\times 6)=78g$
- Calculating the number of moles from the mass of benzene given,
Number of moles of benzene burnt $\frac{\text{Given mass}}{\text{Molar mass}}=\frac{156}{78}=2mol$
- Since, we know that,
Amount of heat released when 1 mol of benzene undergoes combustion = +781kcal
Therefore, the amount of heat released when 2 mol of benzene undergoes combustion = $781kcal\times 2=1562kcalmo{{l}^{- 1}}$
So, the correct answer is option B.

Note: Combustion of hydrocarbons requires a plentiful supply of oxygen to give carbon dioxide and water. Likewise, the combustion of benzene also produces carbon dioxide and water.
$2{{C}_{6}}{{H}_{6}}+15{{O}_{2}}\to 12C{{O}_{2}}+6{{H}_{2}}O$ .
However, the combustion of hydrocarbons hardly ever completes, especially if they are burnt in air. The high proportion of carbon in the molecules mans they need a very high proportion of oxygen to get completely combusted.