2.8 : 反応速度に対する温度変化の影響

A reaction's rate law defines the relationship between a reactant concentration and the reaction rate in terms of a rate constant, ‘k’.

The rate constant describes the relationship between temperature and kinetic parameters relating to the collision, orientation, and activation energy of reacting molecules via the Arrhenius equation. ‘A’ is a constant called the Arrhenius factor or frequency factor. ‘e’ is an exponential factor integrating activation energy measured in joules-per-mole, the gas constant, and the temperature in kelvin.

The parameters’ temperature dependence can be explained with the collision model, which states that reacting molecules should collide with sufficient energy in the correct orientation to initiate a chemical reaction.

The frequency factor constitutes two components—the collision frequency and the orientation factor. The collision frequency is the number of molecular collisions per unit time, whereas the orientation factor describes the probability of collisions with a favorable orientation.

Still, only a small fraction of collisions leads to a reaction. This is because the reacting molecules have to overcome an energy barrier, called the activation energy, to transform into products.

Only those molecules colliding with sufficient kinetic energy will have enough potential energy to bend, stretch, or break bonds to transform into a high-energy intermediate called the transition state, or the activated complex. The short-lived, unstable activated complex loses energy to form stable products, whose total energy is lower than that of the reactants.

The exponential factor in the Arrhenius equation represents the fraction of successful collisions resulting in products. An increase in temperature influences both the frequency factor and the exponential factor.

At elevated temperatures, molecules move faster, more forcefully, and with higher thermal energies, leading to more favorable collisions.

For most reactions, a temperature increase results in higher frequency and exponential factors, leading to a rise in the rate constant, consequently translating to an accelerated reaction rate.