Question

For the gas phase reaction,

$PCl_{5}\left(\right.g\left.\right)\rightleftharpoons PCl_{3}\left(\right.g\left.\right)+Cl_{2}\left(\right.g\left.\right)$

Which of the following conditions are correct?

• A. $\Delta H < 0$ and $\Delta S < 0$
• B. $\Delta H>0$ and $\Delta S < 0$
• C. $\Delta H=0$ and $\Delta S < 0$
• D. $\Delta H>0$ and $\Delta S>0$

Answer 1

Answer: (D)

Gas phase reaction

$PCl_{5}\left(\right.g\left.\right)\rightleftharpoons PCl_{3}\left(\right.g\left.\right)+Cl_{2}\left(\right.g\left.\right)$

$\Delta H=\Delta E+\Delta n_{g}RT$

$\Delta n=$ change in number of moles of product and reactant species.

Since $\Delta n_{g}=+ve,$ hence $\Delta H=+ve$

also one mole of $PCl_{5}$ is dissociated into two moles of $PCl_{3}$ and $Cl_{2}$ in the same phase.

Therefore, $\Delta S=S_{p r o d u c t}-S_{r e a c t a n t}$

$\Delta S=+ve$