2.12 : 反応結果の予測

Chemical kinetics describes the rate and path by which reactants move from one state to another in the reaction pathway, whereas thermodynamics considers the relative stabilities of the states themselves.

Compounds A and B can react with each other in two possible pathways, one leading to products C and D, and the other leading to products E and F.

Products C and D are kinetically favored over E and F, as their formation requires smaller activation energy. Moreover, they are thermodynamically more favorable on account of their lower energies.

In most cases, a reaction pathway is both thermodynamically and kinetically favored, although there are instances where thermodynamics and kinetics oppose each other.

In this case, products C and D are favored by thermodynamics because they have lower energy. However, products E and F are preferred by kinetics because their formation involves lower energy of activation.

Temperature plays an important role in determining the major product. At low temperatures, products E and F form rapidly, whereas at high temperatures, equilibrium concentrations are quickly achieved, producing C and D.

If the activation energy barrier is sufficiently high, the reactants are considered 'kinetically stable'. However, adding energy can overcome this barrier.

For instance, petroleum fuels and atmospheric oxygen do not spontaneously react at room temperature. However, in a car engine, the fuel is ignited by a spark from an external energy source, and the reaction between the fuel and atmospheric oxygen reaches equilibrium.

The size of the reacting particles impacts their collision frequency, as several smaller particles have a larger overall surface area than one large particle. The greater the surface area on which collisions can occur, the faster the reaction.

For example, breaking larger logs into smaller pieces of kindling increases their surface area. With more accessible fuel, the fire more rapidly provides the needed energy to sustain the combustion reaction.