Introduction

We tend to state that substances are 'reactive' or 'unreactive', 'stable' or 'unstable', but these terms are relative and may depend on several issues. Is significant to specify the circumstances of pressure and temperature and what other substances are exist or could act as potential routes to decomposition. Thermodynamic and kinetic issues can also be significant.

Thermodynamics deals with whole entropy and energy changes and their relation to the direction of reaction and the position of equilibrium. Such types of quantities depend only on the first and final states and not at all on the reaction pathway. It is frequently possible to assess the thermodynamic feasibility of a reaction without any knowledge of the technique. Alternatively, the rate of a reaction does rely on the pathway; this is the subject of chemical kinetics and thermodynamic considerations alone cannot expect how fast a reaction will occur.

Several known substances are thermodynamically stable but others are only kinetically stable. For instance, the hydrides B2H6 and SiH4 are thermodynamically unstable with respect to their elements but in the nonexistence of heat or a catalyst (and of atmospheric moisture and oxygen) the rate of decomposition is very slow. To measure why some

substances are not known, it is significant to consider different possible routes to decomposition. For instance, the unknown CaF(s) is possibly thermodynamically stable with respect to the elements themselves, but surely unstable (thermodynamically and kinetically) with respect to the reaction

2CaF(s) → Ca(s) + CaF₂ (s)

Thermodynamic and kinetic issues rely on temperature and other conditions. For instance, CaF(g) can be formed as a gas-phase molecule at high temperatures and low pressures.