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

  1. Exothermic reactions usually increase the entropy of surroundings → are favored (bcuz heat is not required for the process)
  2. 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°