Thermodynamically Favorable Reactions
Chemistry: IB Diploma Higher Level, topic 4.3.4
Thermodynamically Favorable Reactions
Free energy and the thermodynamic favorability of a reaction is determined by enthalpy, entropy, and, sometimes, temperature.
Gibbs free energy
- Gibbs free energy change is described by the equation ΔG = ΔH - TΔS.
- This gives four possible outcomes, which will be detailed below.
Both ΔH and ΔS are positive
- This condition describes an endothermic process that involves an increase in system entropy.
- In this case, ΔG will be negative if the magnitude of the TΔS term is greater than ΔH.
- If the TΔS term is less than ΔH, the free energy change will be positive.
- Such a process is thermodynamically favorable at high temperatures and not thermodynamically favorable at low temperatures.
Both ΔH and ΔS are negative
- This condition describes an exothermic process that involves a decrease in system entropy.
- In this case, ΔG will be negative if the magnitude of the TΔS term is less than ΔH.
- If the TΔS term’s magnitude is greater than ΔH, the free energy change will be positive.
- Such a process is thermodynamically favorable at low temperatures and not thermodynamically favorable at high temperatures.
ΔH is positive and ΔS is negative
- This condition describes an endothermic process that involves a decrease in system entropy.
- In this case, ΔG will be positive regardless of the temperature.
- Such a process is not thermodynamically favorable at all temperatures.
ΔH is negative and ΔS is positive
- This condition describes an exothermic process that involves an increase in system entropy.
- In this case, ΔG will be negative regardless of the temperature.
- Such a process is thermodynamically favorable at all temperatures.
Test yourself on Thermodynamically Favorable Reactions
When is the free energy change always favorable?
Popular chemical hand warmers generate heat by the air-oxidation of iron:
4Fe(s) + 3O2(g) ⟶ 2Fe2O3(s)
When is this reaction thermodynamically favorable?