Predicting Chemical Reactions: A Guide to Gibbs Free Energy

In chemistry, we often want to know if a reaction will happen on its own, without a continuous input of external energy. A reaction that can proceed on its own is called a spontaneous reaction. This doesn't mean the reaction is fast; it only means it's thermodynamically favorable. The key to predicting spontaneity is a thermodynamic quantity called Gibbs Free Energy (G). The change in Gibbs Free Energy (ΔG) for a reaction combines the effects of enthalpy (heat) and entropy (disorder) to determine if a reaction will occur under a given set of conditions.

This calculator is designed to compute the Gibbs Free Energy change using its fundamental equation. By providing the change in enthalpy (ΔH), the change in entropy (ΔS), and the absolute temperature (T), the tool instantly calculates ΔG and tells you whether the reaction is spontaneous, non-spontaneous, or at equilibrium. This is a critical calculation for chemists and chemical engineers designing new reactions, predicting the stability of compounds, and understanding the driving forces behind chemical transformations.

The Gibbs Free Energy Equation

The relationship between free energy, enthalpy, and entropy is given by the Gibbs-Helmholtz equation:

ΔG = ΔH - TΔS

Where:

ΔG (Gibbs Free Energy Change): The maximum amount of non-expansion work that can be extracted from a closed system. It is the ultimate arbiter of spontaneity.
ΔH (Enthalpy Change): The heat absorbed or released by the reaction. A negative ΔH (exothermic reaction) favors spontaneity.
T (Temperature): The absolute temperature of the reaction in Kelvin (K).
ΔS (Entropy Change): The change in the disorder or randomness of the system. A positive ΔS (increase in disorder) favors spontaneity.

It is crucial that the units are consistent. ΔH is typically in kJ/mol, while ΔS is often in J/mol·K. You must convert one of them so they match (e.g., convert ΔS to kJ/mol·K by dividing by 1000).

Interpreting the Sign of ΔG

The sign of the calculated ΔG value tells you about the spontaneity of the reaction:

If ΔG < 0 (negative), the reaction is spontaneous in the forward direction as written.
If ΔG > 0 (positive), the reaction is non-spontaneous in the forward direction. However, the reverse reaction will be spontaneous.
If ΔG = 0, the system is at equilibrium, and the rates of the forward and reverse reactions are equal.

Gibbs Free Energy Calculator