4. Calculate AH for the reaction Ca(OH)2(aq) + 2HCl(aq) → CaCl2(aq) + 2H2O(1)
delta H = ?
Given:
CaO(s) + 2HCl(aq) + CaCl2(aq) + H2O(1) delta H = -186 kJ
CaO(s) + H2O) + Ca(OH)(s) delta H = -62.3 kJ
Ca(OH)2(s) + Ca(OH)2(aq)
delta H = - 12.6 kJ

ΔH = -111.1kJ

Explanation:

ΔH of any reaction could be obtained from similar reaction doing algebraic operations. Using the reactions:

(1) CaO(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) ΔH = -186 kJ

(2) CaO(s) + H₂O(l) → Ca(OH)₂(s) ΔH = -62.3 kJ

(3) Ca(OH)₂(s) → Ca(OH)₂(aq)  ΔH = - 12.6 kJ

(1) - (2):

Ca(OH)₂(s) + 2HCl(aq) → CaCl₂(aq) + 2H₂O(l)

ΔH = -186kJ - (-62.3kJ) = -123.7kJ

This reaction -(3):

Ca(OH)₂(aq) + 2HCl(aq) → CaCl₂(aq) + 2H₂O(l)  

ΔH = -123.7kJ - (-12.6kJ) = -111.1kJ