Question

Heat of combustion of benzene is 718K. Cals. When39 grams of benzene undergoes combustion, the heat is liberated as:
A. 718 K. Cals
B. 359 K. Cals
C. 135 K. Cals
D. 1436 K. Cals

Answer 1

A substance undergoes combustion when it reacts with oxygen gas and gives off heat. The substance which burns is called fuel. Benzene is a type of hydrocarbon and when it undergoes combustion it gives out carbon dioxide, water, and heat. The complete combustion of benzene takes place through the following balanced chemical equation:
${C_6}{H_6}(l) + \dfrac{{15}}{2}{O_2}(g) \to 6C{O_2}(g) + 3{H_2}O(g)$
Now let us define the heat of combustion which is defined as heat given out when one mole of a substance burns completely in excess presence of oxygen.

Complete step by step answer:
First, we will calculate the molecular mass of benzene (${C_6}{H_6}$):
$= (12 \times 6) + (1 \times 6)$
$= 72 + 6$
$= 78grams/mole$
This implies that one mole of benzene weighs 78 grams.
Now it is given in the question that heat of combustion of benzene is 718 K. Cals. This means that when one mole of benzene undergoes combustion it gives out the energy of 718 K. Cals. We will use the unitary method to solve this problem as follows :
$1mole$ ($78grams$) of benzene on combustion gives energy $= 718K.cals$
$1gram$ of benzene on combustion gives energy $= \dfrac{{718}}{{78}}K.cals$
$39grams$ of benzene on combustion gives energy $= \dfrac{{718}}{{78}} \times 39K.cals$
$= 359K.cals$

So, the correct answer is Option B.

Note: Since combustion reactions evolve heat, they are called exothermic reactions.When combustion takes place in presence of less oxygen the carbon monoxide is evolved in place of carbon dioxide.The heat of combustion values is used to compare the quality of fuels. A fuel that gives out more heat on combustion is more economic than others. The apparatus used for measurement of heat of combustion is known as a bomb calorimeter.