Question

Given,${H_2} + C{l_2} \to 2HCl$ ; $\Delta H = - 184KJ$
Then what is the amount of energy absorbed when $0.365Kg$ of $HCl$ is formed?
Options-
$A)$ $ - 920KJ$
$B)$ $ - 1840KJ$
$C)$ $ - 184KJ$
$D)$ $ - 92KJ$

Answer 1

Hint: Enthalpy change accompanying a process may also be defined as the sum of the increase in internal energy of the system and the pressure-volume work done or Enthalpy change of a system is equal to the heat absorbed or evolved by the system at constant pressure.

Complete answer:
The reaction given in the question is;
${H_2} + C{l_2} \to 2HCl$
The enthalpy change of the reaction is given in the equation $\Delta H = - 184KJ$ and we have to find the heat absorbed when $0.365Kg$ of $HCl$ is formed.
On dividing the reaction equation with $\dfrac{1}{2}$ , we will get the enthalpy of formation of $HCl$.
$\dfrac{1}{2}{H_2} + \dfrac{1}{2}C{l_2} \to HCl$
The enthalpy of formation will be;
 ${\Delta _f}H(HCl) = \dfrac{{ - 184}}{2}KJ$
                         $ = - 92KJ$
Enthalpy of formation of $1$ mole of $HCl$ is $ - 92KJ$
But we have to find for $0.365Kg$( $365g$ ) of $HCl$ . We can find the number of moles;
Molar mass of $HCl$ = $36.5g{(mol)^{ - 1}}$
$moles = \dfrac{{mass}}{{mol.mass}}$
$moles = \dfrac{{365g}}{{36.5g{{(mol)}^{ - 1}}}}$ $ = 10$ moles of $HCl$
Enthalpy of formation of $1$ mole of $HCl$$ \to - 92KJ$
Enthalpy of formation of $10$ moles of $HCl$ $ \to $ $ - 92 \times 10$
                                                                              $ \to - 920KJ$
Thus, the energy absorbed when $0.365KJ$ of $HCl$ is formed was found to be $ - 920KJ$

The correct option is A).

Additional Information:
We can also define enthalpy as the energy stored within the substance or the system that is available for conversion into heat. Enthalpy is an extensive property just like internal energy (heat absorbed or evolved at constant volume). Also internal energy as well as enthalpy both are state functions. Enthalpy change is zero in a cyclic process

Note:
Whenever the chemical equation is mathematically treated, the enthalpy change should also be treated mathematically. The absolute value of enthalpy can’t be determined and hence only the enthalpy change can be measured.