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 DH for the overall process must be the sum
of the DH values of the constituent reactions.
Some practical rules to follow:
- When a reaction is reversed, the magnitude of DH remains the same, but its sign changes.
- When the balanced equation for a reaciton is multiplied by an integer, the value of DH 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 DH°reaction = SnproductsDH°f(products) - SnreactantsDH°f(reactants)
- Elements in their standard states are not included in the DHreaction calculations; that is, DH°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 (DH°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 DH°f for the reaction C(s) + 2H2(g) --> CH4(g)?
2. Is this equation correct? P4(s) + 6Cl2(g) --> 4PCl3(l) DH°f = -319.7 kJ
3. What is DH° for this reaction: C6H6(l) + 15/2O2(g) --> 6CO2(g) + 3H2O(l)
4. Find DH°f for Ca(OH)2(s) if DH°rxn = -65.2 kJ for the reaction CaO(s) + H2O(l) --> Ca(OH)2(s).
5. Calculate DH°rxn for the reaction 4NH3(g) + 7O2(g) --> 4NO2(g) + 6H2O(l).
6. Calculate DHrxn for the combustion of methanol, CH3OH(l). Assume all products are gaseous and assume only one mole of methanol is used.
7. Calculate DHrxn 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:
|
2O3(g) --> 3O2(g) |
DH° = -427 kJ |
|
O2(g) --> 2O(g) |
DH° = +495 kJ |
|
NO(g) + O3(g) --> NO2(g) + O2(g) |
DH° = -199 kJ |
Calculate DH° for the reaction NO(g) + O(g) --> NO2(g).
10. Given the following data:
|
S(s) + 3/2O2(g) --> SO3(g) |
DH° = -395.2 kJ |
|
2SO2(g) + O2(g) --> 2SO3(g) |
DH° = -198.2 kJ |
Calculate DH° for the reaction S(s) + O2(g) --> 2SO2(g).