Studies of heat of solution of calcium and magnesium oxides and hydroxides

Abstract

The heat evolved in the solution of MgO in hydrochloric acid was found to decrease as the tcmperaturc at which the MgO was prepared was increased. This is shown in the following eq uations, where the values given are those of !:lH, the increment in the heat content. The negative sign , therefore, indieate~ the evolution of heat. MgO(e) [prepared by dehydrating Mg(OHh for 2.5 h ours at 450° CJ+ 17,630 g of 2.085 !vI HCI = solution; !:lH29S. 1oK =-36.67 ± 0.07 kcal, and MgO (c) [prepared by dehydrating Mg(OH) 2 for 2.5 hours at 1,425° CJ+ 17,630 g of 2.085 !vI HCI =solution; !:lH298.1°K=-35.72 ± 0.03 kcal. The decrease in the heat evolved is ascribed mainly to an increase in the particle size of the M gO as the temperature of ignition was increased. Since X-ray dif-fraction patterns were similar there appeared to be no change in crystalline form. The increase in particle size was indicated by a decrease in the rate of solu tion in the calorimeter and was verified by sed imentation experiments. Heat values have a lso been obtained for the solution of Mg(OHh, CaO, and Ca(OHh in bydroehloric acid. Heat of hydration of MgO and CaO was calculated from beat-of-solution data. The heat of hydration of MgO to Mg(OH)2 dcpends on the characteristic properties of both the MgO and Mg(OHh. Heat of hydration of CaO to Ca(OHh was also determined by direct hydration of CaO in a solu tion saturated with respect to Ca(OHh. CONTENTS Page I. Introduction _ _ _ _ _ _ __ _ _ _ __ _ __ _ _ _ __ _ _ _ ___ _ _ _ _ _ _ _ __ _ _ _ __ _ _ _ _ _ _ _ 133 II. General procedure____ __ _ __ _ __ __ _ _ __ _ _ __ _ _ _ _ _ _ __ __ _ ___ _ _ __ _ _ _ _ 134 III. Apparatus and materials ______________________________________ 135 IV. Effect of the temperature of ignition on the heat of I:'olution of MgO in hydrochloric aeid_ _ (a) Corrections to common concentrations_ _____________ 138 (b) Correction to a temperature of 25° C_ _____ ______ ___ 142 V. Heat of solution of Mg(OHh in hydrochloric acid_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 143 VI. Heats of solution of CaO and Ca(OHh in hydrochloric acid_ _ _ _ _ _ _ 144 VII. Heats