JEE Main & Advanced Chemistry Thermodynamics / रासायनिक उष्मागतिकी Question Bank Topic Test - Thermodynamics and Thermochemistry (8-5-21)

The heat of combustion of \[C{{H}_{4(g)}},\,{{C}_{(\text{graphite})}}\] and \[{{H}_{2(g)}}\] are \[20\,kcal,\,-40\,kcal\] and \[-10\,kcal\] respectively. The heat of formation of methane is

A) \[-4.0\,kcal\]

B) \[+40\,kcal\]

C) \[-80\,kcal\]

D) \[+80\,kcal\]

Correct Answer: C

Solution :

[c] Given,

(i) \[C{{H}_{4}}+2{{O}_{2}}\to C{{O}_{2}}+2{{H}_{2}}O\], \[\Delta H=+20\]kcal

(ii) \[C+{{O}_{2}}\to C{{O}_{2}}\] , \[\Delta H=-40\,\,kcal.\]

(iii) \[{{H}_{2}}+\frac{1}{2}{{O}_{2}}\to {{H}_{2}}O\], \[\Delta H=-10\,\,kcal.\]

Aim: \[C+2{{H}_{2}}\to C{{H}_{4}}\]

(ii) + 2 × (iii) - (i) gives.

\[\Delta H=-40+2\,(-10)-(+\,20)=-80\, & \,kcal.\]