HIGH-TEMPERATURE MASS-SPECTROMETRIC STUDIES OF THE BOND-ENERGIES OF GAS-PHASE ZNO, NIO, AND CUO

The equilibria Ni(g) + 1/2O2(g) reversible NiO(g), Cu(g) +1/2O2(g) reversible CuO(g), CuO(g) + Ni(g) reversible Cu(g) + NiO(g), and Zn(g) + 1/2O2(g) reversible ZnO(g) were studied by high temperature mass spectrometry with a position-sensitive ion detection system. Except for ZnO(g), all gaseous species in these equilibria were observed. The DELTA(f)H-degrees-298(II,III)[NiO(g)] and D-degrees-0(II,III)[NiO(g)] determined reinforce literature values within experimental error. Values obtained for DELTA(f)H-degrees-298[CuO(g)] and D-degrees-0[CuO(g)] have a large uncertainty associated with them, perhaps due to lack of equilibrium. Moreover, second and third law heats are not in agreement. ZnO(g) was not observed; however, by evaluating the sensitivity of the instrument, a lower limit for the standard enthalpy of formation for ZnO(g) was determined: DELTA(f)H-degrees-298[ZnO(g)] greater-than-or-equal-to 151 kJ/mol. From this value an upper limit for the dissociation energy of ZnO is determined: D-degrees-0[ZnO(g)] less-than-or-equal-to 226 kJ/mol (less-than-or-equal-to 2.3 eV). This bond energy is 48 kJ/mol (0.5eV) lower than the upper limit obtained from a previous thermochemical study but is in agreement with a more recent determination based on a study of threshold for an ion-neutral reaction.7