|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.
It's because you ultimately add up all three equations to get the overall equation. When you flipped line a, you put H2O on the left. Line b also has H2O on the left. Line c has no H2O in it, so in total (adding all three equations together) you have 2 H2O on the left - just as in the overall reaction equation.
ah, yes, thank you so much :)!
After flipping line a, you'd have another H20 on the left sift of the equation, so you'd have two total. Thus, no need to multiply. :D