First slide

Hess law of constant summation

Question

The enthalpy of the reaction, $H_{2} (g) + \frac{1}{2} O_{2} (g) \to H_{2} O (g), ∆ H_{1}$ and that of $H_{2} (g) + \frac{1}{2} O_{2} \to H_{2} O (l), ∆ H_{2}$ . Then the correct between the magnitude of $∆ H_{1}$ and $∆ H_{2}$ is

Moderate

A

$\large \Delta {{H}_{1}}<\Delta {{H}_{2}}$
B

$\Delta {{H}_{1}}+\Delta {{H}_{2}}=0$
C

$\Delta {{H}_{1}}>\Delta {{H}_{2}}$
D

$\Delta {{H}_{1}}=\Delta {{H}_{2}}$

Solution

$H_{2} (g) + \frac{1}{2} O_{2} (g) \overset{I}{\to} H_{2} O (l) \overset{II}{\to} H_{2} O (g); Step II is endothermic because it is vapourization; Thus ∆ H_{1} (∆_{f} H_{H_{2} O (g)}) < ∆ H_{2} (∆_{f} H_{H_{2} O (l)});$

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