Experimental evidence for gas-ion reactions

In the first instance we will examine the AHi0n(g) for metal dihalides. The metal dihalides have been chosen since they are an intermediate oxidation state for which stability data in the gas phase are well known. 

2 The bond strength or bond energy is usually defined as the heat of association of the neutral atoms rather than the ions however, the two heats differ only by the ionization potential and electron affinity of the reactants. We have chosen this form of the reaction since it is in the same form as that which has been used to discuss equilibria in aqueous solution, i. e. M( q) -f- nX (aq) = MXn(aqj. 

These data appear in Table 2, in which Q is the ratio of column 6 to column 7. 

3 This is also the case for the heat of association of neutral atoms in the gas phase. 

4 This order would vary slightly if other halide differences were used rather than A(MF2—MI2). (In some other systems, MXn, however, the order can be made to vary greatly depending on which halide differences are taken (see later).) Since the atomic replacement reaction (MI2- -2F = MF2+2I) differs only from the ionic replacement reaction (MI2+2F = MF2+2I ) by a constant which is the difference between the electron affinities of F and I, the order which we obtain here is the same order which would be obtained had we originally used AH of association of neutral atoms. 

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