Using the following thermochemical data, what is the change in enthalpy for the following reaction?
ca(oh)2(aq) + 2hcl(aq) → cacl2(aq) + 2h2o(l)
cao(s)+2hcl(aq)→cacl2(aq)+h2o(l), δh= –186kj
cao(s)+h2o(l)→ca(oh)2(aq), δh= –65.1 kj

In calculating the change in enthalpy in a certain reaction, it is important to remember that it is the sum of the enthapy of formation for the substances involved in the reaction. 

CaO(s)+2HCl(aq)→CaCl2(aq)+H2O(l)                          ΔH= –186kJ 
Ca(OH)2(aq) →  CaO(s)+H2O(l)                                     ΔH= 65.1 kJ
Ca(OH)2(aq) + 2HCl(aq) → CaCl2(aq) + 2H2O(l)       ΔH= -120.9 kJ