THERMODYNAMIC PROPERTIES OF ZRMO2O8(S) AND HFMO2O8(S)

The standard molar Gibbs energies of formation of AMo(2)O(8)(s) (where A = Zr, Hf) have been determined using a solid oxide galvanic cell. T he dependence of e.m.f. on temperature of the cell Pt/ZrMo2O8(s)+MoO2( s)+ZrO2(s)/CSZ/air/Pt could be represented by E(V)+/-0.001=0.7276-2.74 9x10(-4)T(K) (870.5 less than or equal to T(K)less than or equal to 11 59). Similarly the e.m.f. of the cell Pt/HfMo2O8(s)+MoO2(s)+HfO2(s)/CS Z/air/Pt could be represented by E(V)+/-0.001=0.7375-2.801x10(-4)T(K) (906 less than or equal to T(K)less than or equal to 1148). In these c ells CSZ represents 15 mol% calcia stabilized zirconia solid solution. The calculated Delta(f)G(m) degrees(T) values for ZrMo2O8(s) and HfMo 2O8(s) can be given by Delta(f)G(m) degrees(ZrMo2O8,s,T)(kJ mol(-1))+/ -4.8 = -2535.4+0.6247T(K) and Delta(f)G(m) degrees(HfMo2O8,s,T)(kJ mol (-1))+5.2=-2551.4+0.6128T(K). Delta(f)H(m) degrees(298.15 K) of HfMo2O 8(s) and ZrMo2O8(s) are calculated by the second-law method using the heat capacity values and can be given respectively as -(2568.3+/-2.0) and -(2588.6+/-2.2) kJ mol(-1). The standard molar Gibbs energy of for mation of MoO,(s) required for the calculations has also been determin ed using the e.m.f. technique.