The combustion of benzene (l) gives $\mathrm{CO}_{2}(\mathrm{~g})$ and $\mathrm{H}_{2} \mathrm{O}(\mathrm{l})$. Given that heat of combustion of benzene at constant volume is $-3263.9 \mathrm{~kJ} \mathrm{~mol}^{-1}$ at $25^{\circ} \mathrm{C}$; heat of combustion (in $\mathrm{kJ} \mathrm{mol}^{-1}$ ) of benzene at constant pressure will be : ( $\mathrm{R}=8.314 \mathrm{JK}^{-1} \mathrm{~mol}^{-1}$ )

JEE Main · 2018 · 08 Apr 2018 · Offline · Chemical Thermodynamics

The combustion of benzene (l) gives

CO_{2} (g)

and

H_{2} O (l)

. Given that heat of combustion of benzene at constant volume is

- 3263.9 kJ mol^{- 1}

2 5^{\circ} C

; heat of combustion (in

kJ mol^{- 1}

) of benzene at constant pressure will be :
(

R = 8.314 JK^{- 1} mol^{- 1}

)

Answer: (A)

The combustion is

C X_{6} H X_{6} (l) + \frac{15}{2} O X_{2} (g) 6 CO X_{2} (g) + 3 H X_{2} O (l)

, in which the gaseous moles change by

Δ n_{g} = 6 - \frac{15}{2} = - 1.5

. The constant-volume heat is

Δ U

and the constant-pressure heat is

Δ H = Δ U + Δ n_{g} R T

, so

Δ H = - 3263.9 + (- 1.5) (8.314 \times 1 0^{- 3}) (298) = - 3263.9 - 3.7 \approx - 3267.6

kJ mol

^{- 1}

JEE Main · 2018 · 08 Apr 2018 · Offline · Single correct option