Question

The equilibrium constant $K_p$ for the reaction $PC{l}_5\left(g\right)\rightleftharpoons PC{l}_3\left(g\right)+C{l}_2\left(g\right)$ is $3.81\times {10}^2$ at 600 K and $2.69\times {10}^3$ at 700 K. Calculate ${\Delta }_{r}H$.

• A. 68.248 kJ
• B. 69.04 kJ
• C. 71.89 kJ
• D. 80.09 kJ

Answer 1

Answer: (A)

68.248 kJ

Given $K_p$ for $PC{l}_5\rightleftharpoons PC{l}_3+C{l}_2$

At $600K$ ,it is $3.81\times {10}^2$

At $700K$ ,it is $2.69\times {10}^3$

Using Van-Hoff equation equation:

$\ln \frac{k_2}{k_1}=\frac{-\Delta H}{R}(\frac{1}{T_2}-\frac{1}{T_1})\ln \frac{2.69\times {10}^3}{3.81\times {10}^2}=\frac{-\Delta H}{8.314}(\frac{1}{700}-\frac{1}{600})\Delta H=16.212\left(\frac{700\times 600}{700-600}\right)\Delta H=68248.1J\Delta H=68.248kJ$