Phosphorous pentachloride is used in the industrial preparation of many organic phosphorous compounds. equation i shows its preparation from pcl3 and cl2: (i) pcl3 (l) + cl2(g)  pcl5(s) use equation ii and iii to calculate ∆hrxs of equation i: (ii) p4 (s) + 6 cl2 (g)  4 pcl3 (l) ∆h = 1280 kj (iii) p4 (s) + 10 cl2 (g)  4 pcl5 (s) ∆h = 1774 kj