The capacity of a system to do work, or use force to move an object, is its **free energy**. This is often expressed as Gibbs free energy, named after the American scientist Josiah Willard Gibbs, known for his work in thermodynamics in the late 1800s. **Gibbs free energy ( G)** is the amount of work that can be done by a system, and it is expressed as $G=H-TS$, where

*H*is enthalpy in kilojoules (which is the total internal energy of the system plus the product of the pressure and volume),

*T*is temperature, and

*S*is entropy.

For any chemical reaction, the change in Gibbs free energy ($\Delta G$) can be calculated. Assuming that temperature is standard (25°C, which is 298.15 K), the change in Gibbs free energy is $\Delta G=\Delta H-T\Delta S$, with *T* = 298.15 K. Importantly, if $\Delta G$ is less than 0, then the reaction is spontaneous; it does not need energy input to proceed. If $\Delta G$ is greater than 0, then the reaction is nonspontaneous and requires energy input to proceed.

*S*°) for each molecule. The standard molar entropy for N

_{2}is 191.61 J/K·mol, for H

_{2}is 130.68 J/K·mol, and for NH

_{3}is 192.77 J/K·mol.

*K*

_{eq}) for that reaction as a function of temperature. It can also be used to determine whether or not a reaction is at equilibrium, in which the forward and reverse reactions are equal.

*K*

_{eq}is the ratio of the concentration of products and reactants of a chemical reaction at equilibrium and indicates whether the reaction favors products or reactants. The relationship between $\Delta{G}$ and

*K*

_{eq}is given as

*R*is the ideal gas constant, 8.314 J/mol⋅K, which relates the kinetic energy of molecules to temperature per mole, and

*T*is temperature.

*K*

_{eq}) to determine the change in standard molar free energy for the reaction ${\rm {N}_{2}}(g)+3{\rm {H}_{2}}( g)\rightarrow2{\rm{NH}_{3}}(g)$ at 25°C (298.15 K) and 2.0 atm.

*K*

_{eq}considering any reaction ${{a\rm{A}}+{b\rm{B}}}\rightarrow{{c\rm{C}}+{d\rm{D}}}$, the equation is as follows:

*K*

_{eq}for the reaction of N

_{2}and H

_{2}using the coefficients of the balanced equation.