Gibbs Free Energy Calculator

This Gibbs free energy calculator is just perfect if you're looking for a tool that estimates whether or not a reaction can happen spontaneously. The Gibbs free energy equation, A.K.A. the delta G equation, combines the enthalpy vs. entropy relation.

The upcoming paragraphs tell you how to calculate Gibbs free energy, provide the Gibbs free energy units, and give the uses of this delta G calculator. To check the phase rule, use our Gibbs' phase rule calculator.

What is Gibbs energy? In equations, it is known as G, and it's a combination of enthalpy and entropy. The sign in front of Gibbs free energy indicates the direction of the chemical reaction as long as two conditions are met:

• Constant temperature; and
• Constant pressure.

In such a case, there are two possible options depending on what you get from the delta G formula:

1. If ΔG > 0, the reaction is nonspontaneous - external energy is crucial for the reaction (you can estimate it with our activation energy calculator to begin. This external energy can be heat, a photon, or any other source of energy.

2. If ΔG < 0, the reaction is spontaneous and happens without any external energy. You don't need to add anything; the atoms within the reaction will initialize it by themselves.

The delta G formula for how to calculate Gibbs free energy (the Gibbs free energy equation) is:

ΔG = ΔH − T × ΔS

where:

• ΔG – Change in Gibbs free energy;
• ΔH – Change in enthalpy;
• ΔS – Change in entropy; and
• T – Temperature in Kelvin.

Again, the answer to "What is Gibbs energy?" is that it combines enthalpy vs. entropy and their relationship.

Enthalpy, here noted as H, is a kind of energy – the sum of the molecules' internal energies and the flow of energy. Entropy, on the other hand, noted as S, is a measurement of the randomness of molecules.

The system tries to reach minimum enthalpy and maximum entropy at all times.

Enthalpy's unit is J, while entropy's is J/K.

Therefore, we can derive the Gibbs free energy units from the Gibbs free energy equation. These are simply units of energy, typically J.

You can check the volume of gas and the number of moles with our standard temperature and pressure calculator.

This tool applies the formula to real-life examples. All you need to know is three out of four variables: change in enthalpy (ΔH), change in entropy (ΔS), temperature (T), or change in Gibbs free energy (ΔG). As the formula can be read backward or in any direction, just put in all the data you have and see the fourth number appear.

For the sake of completeness, here are all the formulas we use:

• ΔG = ΔH − T × ΔS;

• ΔH = ΔG + T × ΔS; and

• ΔS = (ΔH − ΔG) / T.

Knowing the theory behind what Gibbs energy is without knowing how to use it in practice is no use to anyone. That's why we prepared a simple example of how to calculate Gibbs free energy with this tool. Imagine you have a reaction and know its initial entropy, enthalpy and that it happens at 20°C. Moreover, there's also a note on the final entropy and enthalpy.

Now, all you need to figure out is whether the reaction is spontaneous or if it needs external energy. In that case, let's calculate the Gibbs free energy!

Let's work it out with the following reaction:

• Initial enthalpy: H₀ = 0;
• Final enthalpy: H₁ = -92.22 kJ;
• Initial entropy: S₀ = 583.65 J/K;
• Final entropy: S₁ = 384.9 J/K; and
• T = 20 °C = 20 + 273.15 = 293.15 K.

Therefore:

• ΔH = -92.22 kJ; and
• ΔS = -198.75 J/K = -0.19875 kJ/K.

From the delta G formula:

ΔG = ΔH − T × ΔS

ΔG = -92.22 - (-0.19875 × 293.15)

ΔG = -33.96 kJ

The delta G equation result is below zero, which means that the reaction is spontaneous.