text: At room temperature and pressure (i.e., STP conditions), the main reaction of interest here is:\n\nCaO(s) + H2O(l) = Ca(OH)2(s)\n\nCaO(s) and H2O combine in a 1:1 ratio to form Ca(OH)2(s)\n\nBoth CaO and Ca(OH)2 are relatively insoluble in water, so we'll ignore the heat (enthalpy) changes of the reactions:\n\nCaO(s) = Ca++(aq) + O-- (aq)\nand\nCa(OH)2 = Ca++(aq) + 2OH-(aq)\n\nThe NIST Chemistry Webbook (see source) lists the standard enthalpies of formation for water, CaO, and Ca(OH)2 as:\n\nH2O(l) delta-Hf = -285.83 kJ/mol\nCaO(s) delta-Hf = -635 kJ/mol\nCa(OH)2(s) delta-Hf = -986.09 kJ/mol\n\n\nThe standard enthalpy change for the first reaction is then given by:\n\ndelta-H = -986.09 kJ/mol -(-635 kJ/mol) -(-285.83 kJ/mol) \ndelta-H = -65.26 kJ/mol\n\ndelta-H for this reaction is negative, so heat is released.\n\nEach mole of CaO that reacts with a mole of H2O at STP will release 65.26 * 10^3 J of energy as heat.  If  X moles of CaO reacts, then X * 65.26 * 10^3 J of heat will be released.