By John Brennan
Updated Aug 30, 2022
Chemical reactions arise when two or more substances collide and rearrange to form new compounds. These processes are not only omnipresent in nature but also underpin every living system—NASA even defines life as a self‑sustaining chemical system capable of Darwinian evolution. Understanding the forces that govern whether a reaction will occur—and how quickly—requires a look at three core concepts: collisions, entropy, and equilibrium.
For a chemical transformation to begin, molecules must meet with the correct orientation and enough kinetic energy to break existing bonds. Not every encounter leads to a reaction; the reactants must be able to recombine into more stable products. For example, helium atoms are chemically inert because their outer electron shell is complete, so they rarely form new bonds with other gases. In contrast, when atoms possess unpaired electrons or incomplete shells, they can share or transfer electrons, allowing bonds to form and releasing energy.
Thermodynamics lets us predict whether a reaction will be favorable: if the total energy of the new compound is lower than that of the individual reactants, the resulting molecule is stable and the reaction is energetically downhill.
Entropy measures the degree of randomness or disorder in a system. The Second Law of Thermodynamics states that the entropy of a closed system can never decrease. A reaction that increases the combined entropy of the system and its surroundings is spontaneous. When a reaction is not spontaneous—such as the biosynthesis of proteins—organisms couple it to an energy‑generating process like glucose metabolism, which releases a large amount of entropy and drives the overall process forward.
Because total entropy is difficult to quantify directly, chemists use Gibbs free energy (ΔG) to assess spontaneity. The formula ΔG = ΔH – TΔS compares the enthalpy change (ΔH) to the temperature (T) times the entropy change (ΔS). A negative ΔG indicates that a reaction can occur spontaneously under the given conditions.
Even a spontaneous reaction can be slow; the conversion of carbon atoms in diamond, for example, is chemically favorable yet proceeds over geological timescales. Moreover, many reactions reach a dynamic equilibrium where the forward and reverse rates balance, leaving no net change in concentrations of reactants or products. Whether a reaction proceeds to completion, stalls, or reverses depends on kinetic barriers, thermodynamic favorability, and the specific conditions present.
By examining collisions, entropy, and equilibrium together, scientists can predict not only if a reaction will happen, but also how fast it will occur and under what circumstances it will produce a particular product.
