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Free Energy (G)

Topic: Energy Changes In Chemical Reactions

Free Energy (Also known as Gibbs Free Energy, G), represents the ability for a system to do work and is represented by ΔG = ΔH -TΔS.

The free energy of a system can be represented by the difference between the change in enthalpy and the temperature times the change in entropy.

The standard Gibbs free energy of the reaction can also be determined according to:

As with standard heats of formation, the standard free energy of a substance represents the free energy change associated with the formation of the substance from the elements in their most stable forms as they exist under the standard conditions of 1 atm pressure and 298K. Standard Gibbs free energies of formation are normally found directly from tables.

When ΔG is positive, a reaction is said to be non-spontaneous, which means energy is absorbed in a reaction. When ΔG is negative, a reaction is said to be spontaneous. The unit for Gibb’s free energy is in Joules.

  • : The reaction will occur spontaneously to the right.
  • : The reaction will occur spontaneously to the left.
  • : The reaction is at equilibrium and will not proceed in either direction.


Practice Questions 

Khan Academy 

Control of glucose levels

The thermodynamics of ATP hydrolysis in living cells


MCAT Official Prep (AAMC)

Chemistry Question Pack Passage 18 Question 101

Official Guide C/P Section Question 25

Practice Exam 1 C/P Section Question 47

Practice Exam 2 C/P Section Passage 7 Question 34

Practice Exam 3 C/P Section Passage 8 Question 41

Practice Exam 4 C/P Section Passage 1 Question 1

Online Flashcards Biochemistry Question 8

Official Guide B/B Section Question 26

Key Points

• Gibbs free energy represents the ability for a system to do work and can be calculated from enthalpy, temperature, and entropy.

Key Terms

Spontaneous reaction: a reaction that occurs without external activity

Non-spontaneous reaction: a reaction that requires the input of additional action to proceed

Enthalpy: in thermodynamics, a measure of the heat content of a chemical or physical system

Entropy: a thermodynamic property that is the measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work

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