Chemical Stability

You have a mixture of hydrogen and oxygen gases contained in a closed system at 298 K. Does this mixture ever reach chemical equilibrium?

2H2(g) + O2(g)      2H2O(l)

The answer is no. It will keep indefinitely; no change in composition has ever been detected (provided flames, sparks, and catalysts are kept away to avoid an explosive reaction!).

Kinetic Stability and Thermodynamic Stability

The mixture is said to be kinetically stable: this means that it reacts extremely slowly, or not at all. The time it takes for a reaction to occur is a measure of kinetic stability. The slower a reaction, the greater the kinetic stability.

If this mixture could reach equilibrium at 298K, then the reaction has an equilibrium constant, Kc, equal to 3 x 1037. This very large value of Kc shows that at equilibrium it would nearly all have reacted to form water (except for reactant present in excess). Though a mixture of H2(g) and O2(g) is kinetically stable, it is certainly not thermodynamically stable (energetically stable). A substance (or mixture) that would be mostly converted into something else at equilibrium is said to be thermodynamically unstable.

So, chemical stability, or 'resistance to change', has two meanings. It must be considered in terms of both kinetic stability and thermodynamic stability. Just because a system is thermodynamically unstable (like H2(g) and O2(g)) does not mean it will react. It is not kinetically unstable.

On the other hand, the decomposition of pure hydrogen peroxide may occur explosively.

H2O2(l) ® H2O(l) + ŻO2(g)

H2O2(l) is both thermodynamically unstable and kinetically unstable.

Thermodynamic measurements are concerned with initial and final states of reactants and products in a chemical reaction at certain conditions; they give no information about what happens in going from reactants to products.

If you want to read more¼

If you want to read more about this, start by looking up Gibbs free energy. Thermodynamic stability is measured by the change in Gibbs free energy, DG, for the reaction. The more negative is DG, the smaller is the thermodynamic stability, and the greater is the tendency for the reaction to go as written. However, the reaction may be kinetically stable and immeasurably slow, effectively not taking place. Chemical kinetics is the study of rates of reaction, that is, how fast reactions take place and what happens in going from reactants to products. Understand about Chemical Equilibrium and Entropy too. Take a look at Using Standard Electrode Potentials.