Gibbs Free Energy Main Concept: If a chemical or physical process is not driven by both entropy and enthalpy changes, then the Gibbs free energy change can be used to determine whether the process is thermodynamically favored.

Thermodynamic Favoritism Gibbs Free Energy Gibbs Free Energy and Thermodynamic Favoritism Thermodynamic Favoritism Thermodynamic Favored Definition Reverse Reactions and Thermodynamic Favoritism

- Some exothermic reactions involve decreases in entropy - When ΔG°>0, process is not thermodynamically favorable - When ΔG°<0, process is thermodynamically favorable.

- In some reactions, we must consider both enthalpy and entropy to determine if reaction us thermodynamically favorable - freezing of water and dissolution of sodium nitrate in water are examples of need to look at enthalpy and entropy Freezing Water ∆H < 0 ∆S < 0 Dissolution of NaNO3 ∆H > 0 ∆S > 0

Thermodynamic Favoritism Main Concept: Some physical or chemical processes involve both a decrease in the internal energy of the components (∆Hº < 0) under consideration and an increase in the entropy of those components (∆Sº > 0). These processes are necessarily “thermodynamically favored” (∆Gº < 0).

Thermodynamic Favoritism Thermodynamic Favored Definition Reverse Reactions and Thermodynamic Favoritism

- enthalpy and internal energy are the same thing - “thermodynamically favored” means that products are favored at equilibrium (K>1); they will be produced

- before, “spontaneous” was used to describe processes where ΔG°<0 - “thermodynamically favored” is used to avoid confusion b/c “spontaneous” ≠ “immediately”/“without cause”

Spontaneous at Lower Temperatures Spontaneous at Higher Temperatures – + Always Spontaneous Spontaneous at Lower Temperatures Spontaneous at Higher Temperatures Never Spontaneous

- You should be able to determine the signs of ΔH° and ΔS° for a physical or chemical process - where ΔH°<0 and ΔS°>0, no need to calculate ΔG° b/c reaction is thermodynamically favored

- a process that is thermodynamically favored does not mean that it will proceed at a measureable rate - Any process where both ΔH°>0 and ΔS°<0 are not thermodynamically favored, (ΔG°>0) and must favor reactants at equilibrium (K<1) - This should make sense as ΔS° and ΔH° reverse when a chemical or physical process is reversed

Overcoming Unfavored Reactions Main Concept: External sources of energy can be used to drive change in cases where the Gibbs free energy change is positive.

Overcoming Unfavored Reactions Ways to Overcome Unfavored Reactions Examples Electricity Light

- Electricity may be used to cause a process to occur that is not thermodynamically favored Examples: - charging of a battery - electrolysis.

- Light may be a source of energy for driving a process is not thermodynamically favored. - Examples: photoionization of an atom (although separation of negatively charged electrons from remaining positively charged ion is highly endothermic, ionization is observed to occur with the absorption of a photon)

overall conversion of carbon dioxide to glucose through photosynthesis [6CO2 (g) + 6 H2O (l) → C6H12O6 (aq) + 6 O2 (g)] has ΔG°= +2880 kJ/molrxn, yet is observed to occur through a multistep process initiated by absorption of several photons in the range of 400-700 nm

- thermodynamically unfavorable reactions may be made favorable by coupling it to favorable reactions, such as the conversion of ATP to ADP in biological systems - Here, coupling means the process involves a series of reactions with common intermediates, so the reactions add up to produce an overall reaction with a negative ΔG°