Given two processes:<br>(i) <br>(ii) <br>The value of of is — Thermodynamics and Thermochemistry Chemistry Question
Question
Given two processes:<br>(i) $1/2 \text{P}_4\text{(s)} + 3\text{Cl}_2\text{(g)} \to 2\text{PCl}_3\text{(l)}; \Delta H = -635\text{ kJ}$<br>(ii) $\text{PCl}_3\text{(l)} + \text{Cl}_2\text{(g)} \to \text{PCl}_5\text{(s)}; \Delta H = -137\text{ kJ}$<br>The value of $\Delta_f H$ of $\text{PCl}_5\text{(s)}$ is
Answer: B
💡 Solution & Explanation
The standard enthalpy of formation involves forming 1 mole of product. Multiply eq (i) by 0.5: $1/4 \text{P}_4 + 1.5 \text{Cl}_2 \to \text{PCl}_3$ ($\Delta H = -317.5$). Add eq (ii): $\text{PCl}_3 + \text{Cl}_2 \to \text{PCl}_5$ ($\Delta H = -137$). Total $\Delta H = -317.5 - 137 = -454.5\text{ kJ/mol}$.
💬Ask on WhatsApp →
Still have doubts about this question?
Send it to our AI chemistry tutor on WhatsApp — gets answered in minutes