5.3 Hess's Law and Enthalpies of Formation
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Hess’s Law
3NO(g) ➔ N2O(g) + NO2(g) ΔH°rxn = ?
| THERMODYNAMIC DATA | |
|---|---|
| 2NO2(g) ➔ 2NO(g) + O2(g) | ΔH° = +113kJ |
| 2N2(g) + O2(g) ➔ 2N2O(g) | ΔH° = +163kJ |
| N2(g) + O2(g) ➔ 2NO(g) | ΔH° = +181kJ |
C2H4(g) + 6F2(g) ➔ 2CF4(g) + 4HF(g) ΔH°rxn = ?
| THERMODYNAMIC DATA | |
|---|---|
| H2(g) + F2(g) ➔ 2HF(g) | ΔH° = -537kJ |
| C(s) + 2F2(g) ➔ CF4(g) | ΔH° = -680kJ |
| 2C(s) + 2H2(g) ➔ C2H4(g) | ΔH° = +52.3kJ |
C3H8(g) + 5O2(g) ➔ 3CO2(g) + 4H2O(l) ΔH°rxn = ?
| THERMODYNAMIC DATA | |
|---|---|
| 2C3H8(g) + 7O2(g) ➔ 6CO(g) + 8H2O(g) | ΔH° = -2390kJ |
| CO(g) + ½O2(g) ➔ CO2(g) | ΔH° = -283kJ |
| H2O(l) ➔ H2O(g) | ΔH° = +40.7kJ |
Enthalpies of Formation
ΔH°rxn = ∑nΔH°products - ∑mΔH°reactants
Given the following standard enthalpy of formation values, calculate the ΔH°rxn of the following reaction:
3NO(g) ➔ N2O(g) + NO2(g) ΔH°rxn = ?
| ΔHf° (kJ/mol) | |
|---|---|
| NO(g) | 90.4 |
| N2O(g) | 81.6 |
| NO2(g) | 33.8 |
What is a Formation Reaction?
1) Form 1 mole of a single product
2) Reactants are elements in their standard states.
| STANDARD STATES | |
|---|---|
| Gases | He, Ne, Ar, Kr, Xe, Rn, H2, N2, O2, F2, Cl2 |
| Liquids | Br2, Hg |
| Solids | Everything Else |
| Special | C(graphite), S8 |