Question

The heat of combustion of benzene in a bomb calorimeter (i.e., constant volume) was found to be $3263.9 kJmol^{-1}$ at 25 C. Calculate the heat of combustion of benzene at constant pressure.

Answer 1

Hint: Heat of combustion is defined as the heat released when one mole of the component or the substance which is burnt. The combustion reactions involve the reactions which are of carbon and oxygen thereby producing carbon dioxide and water as products. Heat of combustion is known as enthalpy of combustion.

Complete step by step answer:
A bomb calorimeter is a form of steady-volume calorimeter utilized in measuring the heat of combustion of a specific reaction. Bomb calorimeters ought to resist the big stress in the calorimeter because the reaction is being measured. Electrical strength is used to ignite the fuel; as the gas is burning, it's going to warmth up the encircling air, which expands and escapes through a tube that leads the air out of the calorimeter. When the air is escaping through the copper tube it's going to additionally warm up the water outside the tube. The change in temperature of the water lets in for calculating calorie content material of the gasoline.
The reaction is
$${C_6}{H_6}(l) + 7\dfrac{1}{2}{O_2}(g) \to 6{O_2}(g) + 3{H_2}O(l)$$
In this reaction, $${O_2}$$is the only gaseous reactant and $$C{O_2}$$is the only gaseous product.
$$\Delta {n_g} = {n_p} - {n_r}$$
$$6 - \dfrac{{15}}{2}$$
$$ - \dfrac{3}{2}$$
Also $$\Delta U = - 3263.9kJmo{l^{ - 1}}$$
T= $${25^o}C$$=298 K $$ = \dfrac{{8.314}}{{1000}}kJmo{l^{ - 1}}$$
$$\Delta H = \Delta U + \Delta {n_g}RT = - 3263.9kJmo{l^{ - 1}}$$
=-3267.6 $$kJmo{l^{ - 1}}$$

Additional Information:
In more latest calorimeter designs, the entire bomb, pressurized with excess natural oxygen (normally at 30 atm) and containing a weighed mass of a sample reason ensuring that each one water produced is liquid, and doing away with the want to consist of enthalpy of vaporization in calculations), is submerged under a recognized volume of water (ca. 2000 ml) before the charge is electrically ignited. The bomb, with the known mass of the pattern and oxygen, forms a closed system — no gases break out at some point of the reaction.

Note: Heat of combustion is applicable for determining the heating values of fuels. It is also used for determining the stability of chemical compounds. The heat of combustion is always negative. The heat of combustion can be calculated by multiplying the mass of the water times the specific heat of the water times the change in temperature.