Phosphorus pentachloride is used in the industrial preparation of organic phosph

Question

Phosphorus pentachloride is used in the industrial preparation of organic phosphorus compounds. Equation 1 shows its preparation from PCl3 and Cl2. (1) PCl3(l) + Cl2(g) PCl5(s) Use equations 2 and 3 to calculate ?H

Explanation / Answer

2 P(s) + 5 Cl2(g) ---> 2 PCl5(s) Use the following thermochemical equations. PCl5(s) ---> PCl3(g) + Cl2(g) ?H = 87.9 kJ 2 P(s) + 3 Cl2(g) ---> 2 PCl3(g) ?H = -574 kJ flip first equation (note change in sign of enthalpy): PCl3(g) + Cl2(g) ---> PCl5(s) ?H = -87.9 kJ 2 P(s) + 3 Cl2(g) ---> 2 PCl3(g) ?H = -574 kJ multiply first equation by two (note that enthalpy is also mult by 2): 2PCl3(g) + 2Cl2(g) ---> 2PCl5(s) ?H = -175.8 kJ 2 P(s) + 3 Cl2(g) ---> 2 PCl3(g) ?H = -574 kJ Add two equations together. Note that 2PCl3 cancels out. Note that there are 5 Cl2. Add the two enthalpies for your final answer. Search on Hess' Law for more discussion of this technique.

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