CH 221 - Hess's Law Tutorial

Questions regarding Hess's Law? Try this quick tutorial and quiz.

Your answers to these questions will not affect your grade in CH 221... relax and good luck! Feel free to contact me if you have any comments or questions regarding this tutorial.


Review of Hess's Law

Some practical rules to follow:


Test Yourself using Hess's Law

For the following problems, you will need these standard enthalpy of formation (ΔH°f) values:
CO2(g) = -393.15 kJ
H2O(g) = -241.8 kJ
H2O(l) = -285.8 kJ
CaCO3(s) = -1206.9 kJ
CaO(s) = -635.1 kJ
NH3(g) = -46.1 kJ
CH4(g) = -74.8 kJ
PCl3(l) = -319.7 kJ
C6H6(l) = +49.0 kJ
NO2(g) = +34.0 kJ
CH3OH(l) = -239.0 kJ
C8H18(l) = -269.0 kJ
Al2O3(s) = -1675.7 kJ
SiO2(s) = -910.9 kJ
CuSO4(s) = -771.4 kJ


1. What is ΔH°f for the reaction C(s) + 2 H2(g) --> CH4(g)?

2. Is this equation correct? P4(s) + 6 Cl2(g) --> 4 PCl3(l) ΔH°f = -319.7 kJ
3. What is ΔH° for this reaction: C6H6(l) + 15/2 O2(g) --> 6 CO2(g) + 3 H2O(l)
4. Find ΔH°f for Ca(OH)2(s) if ΔH°rxn = -65.2 kJ for the reaction CaO(s) + H2O(l) --> Ca(OH)2(s).
5. Calculate ΔH°rxn for the reaction 4 NH3(g) + 7 O2(g) --> 4 NO2(g) + 6 H2O(l).
6. Calculate ΔHrxn for the combustion of methanol, CH3OH(l). Assume all products are gaseous and assume only one mole of methanol is used.
7. Calculate ΔHrxn for the combustion of octane, C8H18(l). Assume all products are gaseous and assume only one mole of octane is used.
8. Using your answer in question #7, how many kJ of energy will be released per gram of octane burned?
9. Given the following data:
2 O3(g) --> 3 O2(g) ΔH° = -427 kJ
O2(g) --> 2 O(g) ΔH° = +495 kJ
NO(g) + O3(g) --> NO2(g) + O2(g) ΔH° = -199 kJ
Calculate ΔH° for the reaction NO(g) + O(g) --> NO2(g).
10. Given the following data:
S(s) + 3/2O2(g) --> SO3(g) ΔH° = -395.2 kJ
2 SO2(g) + O2(g) --> 2 SO3(g) ΔH° = -198.2 kJ
Calculate ΔH° for the reaction S(s) + O2(g) --> SO2(g).