Question

Hess's Law: When phosphorus pentachloride vapor is mixed with water vapor a reaction takes place which forms POCl3 (gas) + HCl according to this reaction: PCl5(g) + H2O(g) = POCl3 (g) + 2 HCl(g) using the follwing 4 helper equations, determine the change in enthalpy for this reaction. P + 3/2Cl2(g) + 1/2O2(g) = POCl3(g) change in enthalpy = -558.5 kJ H2(g) + Cl2(g) = 2HCl(g) change in enthalpy = -184.6 kJ PCl5(g) = P + 5/2Cl2(g) change in enthalpy = +375 kJ H2O(g) = H2(g) + 1/2O2(g) change in enthalpy = +241 kJ
a. -55 kJ
b. -89 kJ
c. -109 kJ
d. -126 kJ
e. -141 kJ

Answer 1

Question: determine the change in enthalpy for this reaction;

Answer:

$$\mathrm{PCl}_{5(\mathrm{g})} + \mathrm{H}_{2}\mathrm{O}_{(\mathrm{g})} = \mathrm{POCl}_{3(\mathrm{g})} + 2 \mathrm{HCl}_{(\mathrm{g})}$$ $$\mathrm{P} + 3/2\mathrm{Cl}_{2(\mathrm{g})} + 1/2\mathrm{O}_{2(\mathrm{g})} = \mathrm{POCl}_{3(\mathrm{g})} \quad \Delta \mathrm{H}_{1} = -558.5\ \mathrm{kJ}$$ $$\mathrm{H}_{2(\mathrm{g})} + \mathrm{Cl}_{2(\mathrm{g})} = 2 \mathrm{HCl}_{(\mathrm{g})} \quad \Delta \mathrm{H}_{2} = -184.6\ \mathrm{kJ}$$ $$\mathrm{PCl}_{5(\mathrm{g})} = \mathrm{P} + 5/2\mathrm{Cl}_{2(\mathrm{g})} \quad \Delta \mathrm{H}_{3} = +375\ \mathrm{kJ}$$ $$\mathrm{H}_{2}\mathrm{O}_{(\mathrm{g})} = \mathrm{H}_{2(\mathrm{g})} + 1/2\mathrm{O}_{2(\mathrm{g})} \quad \Delta \mathrm{H}_{4} = +241\ \mathrm{kJ}$$ $$\Delta \mathrm{H} = \Delta \mathrm{H}_{3} + \Delta \mathrm{H}_{1} + \Delta \mathrm{H}_{2} + \Delta \mathrm{H}_{4} = -126\ \mathrm{kJ}$$