A very important exception to purely physical nondissociative dissolution occurs when diatomic gases such as O2, N2 and H2, enter metals. Many metals form M-O, M-N, and M-H bonds that are sufficiently strong to break the bond of the diatomic molecule and absorb the gas in atomic form.1 Palladium of cold fusion fame is an example of this process. Denoting the diatomic gas molecule by A2, the controlling equilibrium is:
The equilibrium condition for this reaction in terms of chemical potentials is:
There is no difficulty in interpreting the chemical potential of the diatomic gas; it is given by Eq (7.44) with the standard state the molecular gas at 1 atm pressure and the temperature of the system. For A dissolved in the metal, the appropriate expression for the chemical potential is the combination of Eqs (7.29) and (7.30). The choice of the standard state for A atoms dissolved in the metal is problematic. Pure A in the solid form, which would be the state closest to that in
1 The rupture of the A-A bond takes place on the metal surface the metal, does not exist, and so is inappropriate as a standard state. Other choices are A(g) and A2(g). Picking the latter converts the above equation to:
>2§A2(g) + >2RTlnpA2 = >2gA2(g) + RTln(y aXa)
where ya and xa are the activity coefficient and mole fraction of A in the metal and pA is the pressure (or partial pressure) of A2 in the gas. The above equation is rewritten in a more useful form as:
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.