text: how much heat energy is released when x amount of calcium oxide cao is mixed with x amount of water h20.? would like a worked example from first principles please at room temperature and pressure (i.e., stp conditions), the main reaction of interest here is:  cao(s) + h2o(l) = ca(oh)2(s)  cao(s) and h2o combine in a 1:1 ratio to form ca(oh)2(s)  both cao and ca(oh)2 are relatively insoluble in water, so we'll ignore the heat (enthalpy) changes of the reactions:  cao(s) = ca++(aq) + o-- (aq) and ca(oh)2 = ca++(aq) + 2oh-(aq)  the nist chemistry webbook (see source) lists the standard enthalpies of formation for water, cao, and ca(oh)2 as:  h2o(l) delta-hf = -285.83 kj/mol cao(s) delta-hf = -635 kj/mol ca(oh)2(s) delta-hf = -986.09 kj/mol   the standard enthalpy change for the first reaction is then given by:  delta-h = -986.09 kj/mol -(-635 kj/mol) -(-285.83 kj/mol)  delta-h = -65.26 kj/mol  delta-h for this reaction is negative, so heat is released.  each 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.