Use Hess’ Law to determine the standard enthalpy change for the following reaction.
Target
C3H8 (g) + 5O2 (g)

3C (s) + 4H2 (g) 
C (s) + O2(g)

H2 (g) + ½ O2 (g) 
3CO2 (g)
+
4H2O (l)
Hrxn=???
C3H8 (g)
CO2 (g)
H2O (l)
Hf= -104kJ
Hf= -394kJ
Hf= -286kJ
What is the standard enthalpy of formation for a substance?
NOTE: The standard enthalpy of formation of any element in its standard state = ______
WHY??
Look closely at the Hess’ Law example above and you will see that
Hrxn = ____________________________ - ______________________________
A new equation:
Hrxn
= ___________________________ - ______________________________
Now use this equation to solve the problem above.