In this lecture, I will introduce Gibbs free energy. What is Gibbs free energy? Gibbs free energy, like enthalpy, is a human-made concept which means it cannot be measured.
So you cannot use an instrument to measure the Gibbs free energy of some object. Gibbs’s free energy can only be measured experimentally. Now that’s because a formula defines Gibbs free energy and this formula only holds under certain conditions. If these conditions aren’t met, Gibbs’s free energy breaks down.
What is Gibbs free energy?
Josiah Gibbs was an American mathematician who came up with working out. We call free energy, and free energy is calculated by taking the enthalpy change of the reaction and taking it away from the temperature multiplied by the entropy. Delta G (∆G) is the symbol that symbolizes the Gibbs free energy.
The Gibbs function is defined as the enthalpy minus the temperature times the entropy. If there’s a change in Gibbs free energy, if there’s a negative change, if there’s a downhill direction for Gibbs free energy, that’s the favored direction for a chemical process or a physical strategy.
This equation is,
ΔG = ΔH − TΔS
Delta H (ΔH) is the enthalpy change in kilojoules per mole (KJ/mole), the temperature is measured in Kelvin, and the entropy change is measured in joules per kelvin per mole.
Factors affecting Gibbs free energy
If you put all the products and reactants at one molar if it’s a concentration, and one atmosphere, if it’s a gas or pure liquids or pure solids, this standard free energy difference gives you the relative ordering of those standard states reactants and standard state products.
- If the standard state products are higher in energy than the standard state reactants, then that’s a positive ΔG, which says that the reactants are favored. The reactants have stronger bonds than our products, and therefore, our reactants are more stable. If it becomes more positive, that means we have more freedom in our products than our reactants.
- If the products are lower in free energy in their standard states than the reactants, a negative free energy difference, and the products are favored. Also, If we have an exothermic reaction, our products have stronger bonds with more stable bonds than our reactants. The reaction will be spontaneous.
So, you can talk about the various changes in enthalpy, entropy, and free energy. For a process to be spontaneous, to be favored by the universe. It could have a decrease in enthalpy. It could release energy, or it could absorb energy. Or the system could go towards a higher entropy state, more microstates to disperse the energy. Or it could go to a lower entropy state, where the number of microstates is smaller.
Now, since free energy is a state function, I can calculate the free energy change for a reaction by taking the standard free energies of the formation of the products minus the standard free energies of the reactants’ formation. So this gives me a powerful tool to determine whether a reaction is favored or unfavored based on the products’ free energies and reactants’ formation.
Let’s look at these conditions. These conditions are constant temperature and pressure. The reaction must be reversible, and there is no mechanical work done. Only PV work is allowed to be done.
- In an isolated system, the number of moles stays the same. That’s because there is no change in mass. So the number of moles stays the same, the temperatures are constant, and pressure is constant. According to the ideal gas law, volume remains constant. So there is no volume change.
According to the formula for change in enthalpy, the PV work done is zero if there is no volume change. We can approximate the change in enthalpy to equal the internal energy or change in energy or heat.
Unit of Gibbs free energy equation
ΔG = Gibbs free energy, Unit: Joules per mole or J/Mol.
ΔH = Enthalpy change, Unit: Joules per mol or J/Mol.
T = Temperature, Unit: Kelvin (K). Temperature is always positive.
ΔS = Entropy change, Unit: Joules per kelvin per mole (J K⁻¹ mol⁻¹).
Examples & Problems
Example: CH₄[gas] + 2O₂[gas] → CO₂[gas] + 2H₂O[steam] + Energy
Here, Gibbs free energy problems:
4KClO3(s) ⟶ 3KClO3(s) + KCl(s)
ΔG= ΔH−TΔS = −144KJ−298K (−0.036KJK) = −133KJ
Cu2O(s) + C(s)⟶2Cu(s) + CO(g) ; Where, ΔH= 50, ΔS= 0.165
ΔG= 50 − 0.165T
Conditions for spontaneity
Gibbs free energy is an excellent indicator of if we have a spontaneous reaction or a non-spontaneous reaction.
- If Gibbs Free Energy is ever negative or less than 0, then it’s a spontaneous reaction. If enthalpy increases entropy, that’s going to be a spontaneous reaction. Here, ΔG < 0; Spontaneous & Exergonic reaction.
- If Gibbs Free Energy is ever positive or greater than 0, then it’s not a spontaneous reaction. It’s not going to occur spontaneously. In other words, we’re going to have to put a little bit of energy in for it to work.
Here, ΔG > 0; Spontaneous Backwards & Endergonic reaction.
- If Gibbs Free Energy is equal to zero, then it will be in equilibrium.
Here, ΔG = 0; Equilibrium.
If Delta G is 0 or -1, the reaction is feasible at the state and temperature. But it might not be feasible depending on the temperature.
|–||+||Always at any temperature.|
|+||–||Never at any temperature.|
|+||+||Depends on the temperature.|
|–||–||Depends on the temperature.|
I hope you understand the Gibbs free energy properly. If you have any questions, then please comment down below.
Learn More: How Do Enzymes Lower Activation Energy
Perrot, Pierre. A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6.
Gibbs, Josiah Willard. “A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces.”
Peter Atkins; Loretta Jones. Chemical Principles: The Quest for Insight. W. H. Freeman.