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Gibbs Free Energy Calculator: Predicting Reaction Spontaneity

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The Gibbs Free Energy Calculator is an excellent tool for estimating the spontaneity of a reaction. By combining the concepts of enthalpy and entropy, the Gibbs free energy equation, also known as the delta G equation, can be derived.

This article will explain how to calculate Gibbs free energy, discuss the units involved, and provide a practical example of using the delta G calculator. To explore the phase rule, consider using the Gibbs’ phase rule calculator.

Spontaneous and Nonspontaneous Reactions

Gibbs energy, represented by G in equations, is a combination of enthalpy and entropy. The sign of Gibbs free energy indicates the direction of a chemical reaction, provided the following conditions are met:

  1. Constant temperature; and
  2. Constant pressure.

Under these conditions, the outcome of the delta G formula will yield two possibilities:

  • If ΔG > 0, the reaction is nonspontaneous and requires external energy, such as heat or photons.
  • If ΔG < 0, the reaction is spontaneous, occurring without any external energy input.

The Delta G Equation

The formula for calculating Gibbs free energy (the delta G equation) is:

ΔG = ΔH − T × ΔS


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

The Interplay between Enthalpy and Entropy

Enthalpy (H) represents a form of energy, while entropy (S) measures the randomness of molecules. Systems strive to achieve minimum enthalpy and maximum entropy.

Enthalpy is expressed in J·mol-1, and entropy is expressed in J/K. The Gibbs free energy equation yields units of energy, typically in joules (J).

Using the Gibbs Free Energy Calculator

The calculator employs the formula to solve real-life examples. To use it, you need to know three out of the four variables: ΔH, ΔS, T, or ΔG. Input the known data, and the calculator will provide the fourth variable.

Here are the formulas used:

ΔG = ΔH − T × ΔS
ΔH = ΔG + T × ΔS 
ΔS = (ΔH − ΔG) / T

Practical Application of Delta G Calculator

To demonstrate the usefulness of the calculator, consider a reaction with known initial and final enthalpy and entropy values, occurring at 20°C:

Reaction: N2 + 3H2 = 2NH3

Initial enthalpy: H0 = 0; Final enthalpy: H1 = -92.22 kJ·mol-1; Initial entropy: S0 = 583.65 J/K; Final entropy: S1 = 384.9 J/K; and T = 20°C = 20 + 273.15 = 293.15 K.

Calculating the change in enthalpy and entropy:

ΔH = -92.22 kJ·mol-1; and ΔS = -198.75 J/K = -0.19875 kJ/K.

Applying the delta G formula:

ΔG = ΔH − T× ΔS

ΔG = -92.22 - (-0.19875 × 293.15)

ΔG = -33.96 kJ

The result of the delta G equation is negative, indicating that the reaction is spontaneous. Always verify that the units of Gibbs free energy are consistent with the units of enthalpy and entropy.

Frequently Asked Questions

How do I calculate Gibbs free energy?

Follow these steps:
Determine the reaction temperature.
Find the change in entropy by subtracting the initial value from the final value.
Calculate the change in enthalpy similarly.
Multiply the change in entropy by the temperature.
Subtract the product from the change in enthalpy to obtain the Gibbs free energy.

Why is Gibbs free energy 0 at equilibrium?

At equilibrium, there is no net change in any quantities that Gibbs free energy depends on. Since everything is constant, no energy is available to do work unless the process is disturbed.

What information does Gibbs free energy provide about a reaction?

Gibbs free energy indicates the maximum energy available in a system to perform work. It helps determine if a reaction is spontaneous, nonspontaneous, or at equilibrium.

What happens when Gibbs free energy is zero?

When Gibbs free energy equals zero, the forward and backward processes occur at equal rates, meaning the system is at equilibrium. The concentrations of reactants and products remain constant.

How can I determine if a reaction is spontaneous or nonspontaneous?

Calculate the Gibbs free energy for the reaction. A positive value indicates a spontaneous (exergonic) reaction, while a negative value signifies a nonspontaneous (endergonic) process.


In conclusion, the Gibbs Free Energy Calculator is a valuable tool for predicting the spontaneity of a chemical reaction under specific conditions. By understanding and applying the principles of enthalpy, entropy, and the delta G equation, you can determine if a reaction is spontaneous, nonspontaneous, or at equilibrium. With this knowledge, you can make informed decisions in various scientific and industrial applications. Moreover, by considering the factors that influence the spontaneity of a reaction, it is possible to manipulate the conditions to achieve the desired outcome.

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