First slide

Relation between Delta H and Delta U

Question

The heat of combustion of benzene at $27^{0} c$ in a bomb calorimeter i.e. for the reaction $C_{6} H_{6} (l) + \frac{15}{2} O_{2} (g) \to 6 {CO}_{2} (g) + 3 H_{2} O (l),$ is -780 K.Cal/mole. The heat evolved on burning 39g of benzene in an open vessel wil be

Easy

Solution

ΔE = -780 kcal/mole
Δn_(g) = 6-15/2 = -3/2
ΔH = ΔE + Δn_(g) RT
$\large \ = \; - 780 + \left( { - \frac{3}{2}} \right)\left( {2 \times {{10}^{ - 3}}} \right) \times 300\$
= -780.9 Kcal/mole
For 78 gm heat evolved = 780.9
For 39 gm = ?
$\large \frac{{39}}{{78}} \times 780.9 = 394.45Kcal\$

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