|Application of Hess's Law
||[Oct. 12th, 2010|07:49 pm]
Sorry if this posted twice or something weird happened, my internet crashed at the most inopportune moment!|
One of my homework questions is:
"Calcium metal will react in water to form calcium hydroxide, Ca(OH)2. Use the thermochemical data that follow and Hess's Law to calculate the value of ΔH° for the reaction
Ca(s) + 2H2O(l) = Ca(OH)2(s) + H2 (g)
a. H2(g) + ½O2(g)= H2O(l) ΔH°= -286kJ
b. CaO(s) +H2O (l) = Ca(OH)2(s) ΔH° = -64kJ
c. Ca(s) + ½O2(g)= CaO(s) ΔH°= -635kJ "
I know to flip line a and change the sign to a + 286 kJ. I was wondering why I didn't have to multiply line b by 2 because in the reaction H2O has a coefficient of 2.
Any help will be appreciated! I seem to be incompetent when it comes to Hess's law.