Answer:
D. -120.9 kJ
Explanation:
According to Hess's law ,the total enthalpy change for a reaction is the sum of all changes regardless of the stages or the steps of the reaction.
[tex]CaO + 2HCl \rightarrow CaCl_{2} + H_{2}O\ (\Delta H = -186\ kJ)[/tex]....(1)
[tex]CaO + H_{2}O\rightarrow Ca(OH)_{2}\ (\Delta H = - 65 \ kJ)[/tex]
(this reaction should be reversed in order to reach the required reaction )
On reversing the reaction the sign of [tex]\Delta H[/tex] get reversed.
(In this case change sign from '-' to'+'. Hence [tex]\Delta H[/tex] = + 65 kJ)
[tex]CaO + 2HCl \rightarrow CaCl_{2} + H_{2}O\ (\Delta H = - 186\ kJ)[/tex]....(1)
[tex]Ca(OH)_{2} \rightarrow CaO + H_{2}O\ (\Delta H = + 65 kJ )[/tex]......(2)
Adding equation (1) and (2)
[tex]Ca(OH)_{2} + 2HCl \rightarrow CaCl_{2} + 2H_{2}O[tex]
[tex]Delta H = - 186 + 65 = - 121\kJ[/tex]
[tex]Delta H = - 121\kJ[/tex] (It is nearly equal to -120.9 kJ)
