Magnesium oxide of very high energy content from decomposition of magnesium hydroxide

In an effort to determine the reason that the H/sub 2/O produced by decomposition of Mg(OH)/sub 2/ in effusion cells approaches apparent equilibrium pressures only approx.10/sup -4/ times the expected equilibrium pressures, the heat of solution of MgO in HCl solutions was measured as a function of the area of the surface accessible to N/sub 2/ gas. Excess heats of formation of MgO as great as 73 kJ (17 kcal) per mol of MgO were measured for particles of N/sub 2/-accessible surface areas of approx.300 m/sup 2//g, but of probable total surface areas of approx.600 m/sup 2//g or more. The activities calculated for the MgO from the excess heats are high enough to account for the low equilibrium H/sub 2/O pressures. 37 references, 3 figures, 2 tables.