热力学第一定律 Group Contributions Sometimes it is difficult to exactly thermodynamically break enthalpies of formation down into contributions from individual atoms and bonds mean bond enthalpies,△H(A-B)平均键焓 enthalpy change associated with breaking of a specific bond, A-B A-B(g) A(g)+ b(g) △H(A-B) This is an unreliable method, since AH(A-B )are average values for series of unrelated compounds(different geometries, isomers, etc. thermochemical groups atom or physical group of atoms bound to at least two other atoms enthalpy of formation is associated with sum of contributions associated with all of the thermochemical groups into which the molecule can be divided-sometimes called Benson thermochemical groups平均键焓 热力学第一定律