Gibbs Free Energy of Formation

You can use standard enthalpy change (ΔH°) and standard absolute entropy (S°) data to calculate the change in Gibbs free energy (ΔG°) for a chemical reaction or physical change using the equation:

 ΔG° = ΔH° - TΔS°

... OR ....
you can use tables of values for the standard Gibbs free energy of formation of compounds (ΔG_ƒ°).
For a chemical reaction or physical change:

reactants → products 

ΔG° = ΣΔG_ƒ°(products) - ΣΔG_ƒ°(reactants)

If you would like to understand where this comes from and how it can be applied solve problems in chemistry, visit our new tutorial Standard Gibbs Free Energy of Formation Calculations

AUS-e-TUTE Members should log-in to use the new tutorial, game, test and exam resources.

Effect of Temperature on GIbbs Free Energy

Imagine a reaction in which you decompose liquid water to produce oxygen gas and hydrogen gas:

H₂O_(l) → H_2(g) + ½O_2(g)

The reaction is endothermic, ΔH > 0, and the change in entropy is also positive, ΔS > 0
At room temperature and pressure this reaction is not spontaneous ( ΔG > 0).
But could I heat, or cool, the reaction sufficiently to make the reaction spontaneous?
Can I turn a nonspontaneous reaction into a spontaneous reaction by changing the temperature?
Want to know?
Go to https://www.ausetute.com.au/gibbstemp.html

AUS-e-TUTE Members can access the tutorial, game, test and exam by logging-in to the Members Only area of the website.

Gibbs Free Energy Calculations

For a chemical system, either a chemical reaction or a physical change, at a constant temperature and pressure we define a function called the Gibbs Free Energy (G) so that we can determine whether the system will be spontaneous or non-spontaneous:

• spontaneous if ΔG < 0 (ΔG is negative)
• non-spontaneous if ΔG > 0 (ΔG is positive)

In this new tutorial we will calculate the change in Gibbs free energy of a reaction at constant temperature and pressure (ΔG) using:

• standard absolute entropy of each reactant and product (S°)
• standard enthalpy of formation of each reactant and product (ΔH_ƒ°)

For a chemical system under standard conditions, we can calculate the change in standard Gibbs free energy using the equation shown below:

ΔG° = ΔH° - TΔS°

AUS-e-TUTE members can access the new Gibbs free energy calculations tutorial, game, test and exam when they log-in (Go to Physical Chemistry Heading, then "Thermodynamics").

If you are not an AUS-e-TUTE member, there is a "free-to-view" Gibbs free energy calculations tutorial currently available for evaluation purposes at https://www.ausetute.com.au/gibbscalc.html

Gibbs Free Energy and Spontaneity of Reactions

Whether or not a chemical reaction proceeds in a particular direction depends on a balance between the enthalpy of the system and its entropy.
Gibbs Free Energy allows us to quantify this relationship, and determine whether a particular reaction will be spontaneous.
AUS-e-TUTE members can now access a new tutorial, game, test and exam on this topic.
Not a member?
There is a "free-to-view" tutorial currently available at http://www.ausetute.com.au/freeenergy.html