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### Course: AP®︎/College Chemistry>Unit 9

Lesson 2: Gibbs free energy and thermodynamic favorability

# Introduction to Gibbs free energy

The standard Gibbs free energy change, ΔG°, indicates the thermodynamic favorability of a physical or chemical process. When ΔG° < 0, the process is thermodynamically favored. For a given process, the value of ΔG° can be calculated directly from the values of ΔH° and ΔS° using the following equation: ΔG° = ΔH° - TΔS°. Created by Jay.

## Want to join the conversation?

• How is the Gibbs free energy of a system different from its enthalpy? What different usages or significances would they have?
• Enthalpy is essentially the amount of heat lost or gained by a reaction. Gibbs free energy takes into account enthalpy and entropy of a reaction.

Hope that helps.
• What does the number in ΔG mean? For example, what is the difference between a ΔG value of -50 KJ/(mol rxn) vs -500 KJ/(mol rxn)?
(1 vote)
• G stands for Gibbs free energy, and so ΔG stands for the change in Gibbs free energy. Change in Gibbs free energy is a measure of how spontaneous a process like a chemical reaction is. Spontaneity meaning a process progresses naturally without the need of outside work or energy to move it along.

The change in Gibbs free energy is also related to the change in the universe’s entropy at a specific temperature because of a certain process. Specifically, the formula is: ΔG = -TΔSuniv, where T is temperature and ΔSuniv is the change in the universe’s entropy. And according to the second law of thermodynamics, if a process increases the change in the universe’s entropy, then that process is spontaneous. So for a process to be spontaneous the ΔSuniv value should be positive which would correspond to a negative ΔG value. So we can tell is a process is spontaneous quickly depending on the sign of the ΔG.

The more negative a ΔG value is the more spontaneous the process is, or the further it will go toward the products to reach equilibrium.

Hope that helps.
- ∆Go vs ∆G
- ∆Ho vs ∆H
- ∆So vs ∆S

I don't understand the difference between the two especially when talking about reversible reactions.

It would be great if they also covered the actual meaning of the value of ∆G? What does the actual numerical value indicate? Thermodynamics confuses me so much!