The enthalpy of mixing (J/mol) of Au-Pt solid solutions can be approximately giv

Question

The enthalpy of mixing (J/mol) of Au-Pt solid solutions can be approximately given by the following relation: Hm= 22,500(XPt XP2t) . Since the enthalpy of mixing is positive, solid solutions of Au and Pt will exhibit phase separation at low temperatures. Compute the following quantities.(total of 5 points)

(a) The T above which single-phase solutions of Au and Pt can exist for any composi- tion. For this solution use the Gibbs energy of mixing relation Gm = RT(XAulnXAu + XPtlnXPt) + Hm , where Hm is the heat of mixing.

(b) At what composition of Au and Pt does the free energy of mixing given by the form in part (a) have its maximum magnitude for any given temperature.

Explanation / Answer

a.

T above which single phase solutions can exist{critical temperature} is 22500/8.314*2 = 1353.13 K

b.

d/dx(x[1-x]) = 0

:x=0.5 which means to say equal composition that is 50% each of pt and Au .

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