Gibbs Free Energy and Thermodynamic Favorability
- Gibbs Free Energy: available energy of substance that can be used for work
○ Used to predict if a chemical reaction will occur spontaneously
- -ΔG° = thermodynamically favored
○ The reaction favors the products
- +ΔG° = unfavored → not going to occur on its own
○ The reaction favors the reactants
- Method #1:
= kJ/mol
○ Note: when see f or “standard” in a question, use this equation
○ 
of an element in its standard state is zero
■ Units: kJ
- Method #2 for calculating Free Energy Change:
○ ΔG° = 0 → are at equilibrium
■ Often when substance is changing phase or at its MP/BP
■ Find MP/BP = set ΔG° equal to 0 and solve for T
■ Units must be the same!
- Method #3:
○ Use this equation to determine the equilibrium concentration
○ R = gas constant → in thermodynamic calculations use the units with Joules (8.314)
○ Remember that K is the equilibrium constant; must convert ΔG° into Joules
- Method #4: use Hess law
Effect of ΔH and ΔS on Spontaneity
Direction of Heat Flow
- Exothermic reactions usually increase the entropy of surroundings → are favored (bcuz heat is not required for the process)
- Endothermic reactions usually decrease the entropy of surroundings → are not favored (bcuz heat is required
| ΔS | ΔH | Favorability and Value of ΔG |
| + | – | Favored at all temp (ΔG always negative) |
| + | – | Favored at high temp (ΔG negative)
● “Using the equation, ΔG° = ΔH° – TΔS° increasing the tempwith a + ΔS° & ΔH° causes ΔH° < TΔS°, making ΔG° morenegative and the reaction more favored” |
| – | – | Favored at low temp (ΔG negative) |
| – | + | Not favored at any temp (ΔG always positive) → never spontaneous |
- +ΔH = not favorable; -ΔH = favorable
- Reactions can be ‘driven by enthalpy’ (where a very exothermic reaction (- ΔH) overcomes a decrease in entropy) or ‘driven by entropy’ where an endothermic reaction occurs because of a highly positive ΔS
- Note: Hess’s law works the same for ΔG° & ΔS° as it does for ΔH°