Question

When ethyne is passed through a red hot tube, then benzene formation takes place?
Given, ${H^o}_{f({C_2}{H_{2)(g)}}} = 230kJ{\text{ mo}}{{\text{l}}^{ - 1}}$ ,
$\Delta {H^o}_{f({C_6}{H_{6)(g)}}} = 85kJ{\text{ mo}}{{\text{l}}^{ - 1}}$
Calculate the standard heat of trimerisation of ethyne to benzene?
$3{C_2}{H_2}(g) \to {C_6}{H_6}(g)$
(A) ${\text{205 kJ mo}}{{\text{l}}^{ - 1}}$
(B) ${\text{605 kJ mo}}{{\text{l}}^{ - 1}}$
(C) ${\text{ - 605 kJ mo}}{{\text{l}}^{ - 1}}$
(D) ${\text{ - 205 kJ mo}}{{\text{l}}^{ - 1}}$

Answer 1

Hint : As in the question, standard heat of formation of benzene from ethyne is asked, so to solve this question, first we need to understand the enthalpy of formation or the enthalpy change during the formation of products from reactants.

Complete Step By Step Answer:
So, the standard enthalpy of formation is the enthalpy change when $1mol$ of a product is formed at standard state (or in standard conditions) from the pure reactant at the same conditions.
Now, the standard conditions are - $1atm$ pressure and $298.15K$ temperature.
Also we know the formula to calculate the standard heat of formation-
$\Delta {H^o}_{f(reaction)} = \sum {\Delta {H^o}_f(products) - \sum {\Delta {H^o}_f({\text{reactants)}}} }$
In our case, equation given is
$3{C_2}{H_2}(g) \to {C_6}{H_6}(g)$
So, benzene is our product and ethyne is the reactant.
Hence, putting values in the formula to calculate standard heat of formation, we get –
$\Delta {H^o}_{f(reaction)} = \sum {\Delta {H^o}_f(products) - \sum {\Delta {H^o}_f({\text{reactants)}}} }$
$\Delta {H^o}_{f(3{C_2}{H_2}(g) \to {C_6}{H_6}(g))} = \sum {\Delta {H^o}_f{(_{{C_6}{H_{6)(g)}}}}) - \sum {\Delta {H^o}_f{(_{({C_2}{H_{2)(g)}}}}{\text{)}}} }$
$\Delta {H^o}_{trimerisation} = 85 - (3 \times 230)$
${\text{ = - 605 kJ mo}}{{\text{l}}^{ - 1}}$
Thus, by solving the equation, we get our answer ${\text{ = - 605 kJ mo}}{{\text{l}}^{ - 1}}$
Thus option C is the correct answer.

Note :
In other words, we can also say that the enthalpy change is the amount of heat absorbed or released during a reaction of conversion of reactants into its products at the standard pressure and temperature. The change in enthalpy is denoted by the symbol ${H^o}_f$ .
There is one other formula through which our change in enthalpy can be calculated and that is $\Delta H = ms\Delta T$ , where m is the mass of the substance and s is the specific heat and $\Delta T$ is change in temperature.