## 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

• Hess's Law:

If a reaction is the sum of two or more other reactions, then ΔH for the overall process must be the sum of the ΔH values of the constituent reactions.
Some practical rules to follow:

• When a reaction is reversed, the magnitude of ΔH remains the same, but its sign changes.

• When the balanced equation for a reaciton is multiplied by an integer, the value of ΔH for that reaction must be multiplied by the same integer.

• The change in enthalpy for a given reaction can be calculated from the enthalpies of formation of the reactants and products, namely ΔH°reaction = ΣΔH°f(products) - ΣΔH°f(reactants)

• Elements in their standard states are not included in the ΔHreaction calculations; that is, ΔH°f for an element in its standard state is zero.

### 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)?

• -74.8 kJ
• +74.8 kJ
2. Is this equation correct? P4(s) + 6 Cl2(g) --> 4 PCl3(l) ΔH°f = -319.7 kJ
• No
• Yes
3. What is ΔH° for this reaction: C6H6(l) + 15/2 O2(g) --> 6 CO2(g) + 3 H2O(l)
• -2126.2 kJ
• -3265.3 kJ
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).
• -986.1 kJ
• +855.7 kJ
5. Calculate ΔH°rxn for the reaction 4 NH3(g) + 7 O2(g) --> 4 NO2(g) + 6 H2O(l).
• +466.7 kJ
• -1394.4 kJ
6. Calculate ΔHrxn for the combustion of methanol, CH3OH(l). Assume all products are gaseous and assume only one mole of methanol is used.
• +637.75 kJ
• -637.75 kJ
7. Calculate ΔHrxn for the combustion of octane, C8H18(l). Assume all products are gaseous and assume only one mole of octane is used.
• +1077.8 kJ
• -5052.4 kJ
8. Using your answer in question #7, how many kJ of energy will be released per gram of octane burned?
• -22.1 kJ/g
• -44.2 kJ/g
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).
• -233 kJ
• -465 kJ
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).
• -296.1 kJ
• +112.7 kJ