Chemistry and Chemical Reactivity (9th Edition)

a) $P_4(g)+6\ Cl_2(g)\rightarrow 4\ PCl_3(g)$ b) From the data given: $4×-287.0-1×+58.9=-1206.9\ kJ/mol$ c) Bonds broken: 6 P-P, 6 Cl-Cl Bonds formed: 12 P-Cl Enthalpy of reaction: $-1206.9\ kJ/mol=6×201+6×242-12×x$ $x=322\ kJ/mol$ Table 8.8 says it's 326 kJ/mol, so it's a reasonable estimate.