Reaction rate constant

Reaction rate constant
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Reaction_rate_constant".

In chemical kinetics a reaction rate constant k or $λ$ quantifies the speed of a chemical reaction.

For a chemical reaction where substance A and B are reacting to produce C, the reaction rate has the form:

$\frac{d[C]}{dt} = k(T)[A]^{m}[B]^{n}$

k(T) is the reaction rate constant that depends on temperature.

[X] is the concentration of substance X in moles per volume of solution assuming the reaction is taking place throughout the volume of the solution. (for a reaction taking place at a boundary it would denote something like moles of X per area.)

The exponents m and n are called orders and depend on the reaction mechanism. They can be determined experimentally.

In a single-step reaction can also be written as

$\frac{d[C]}{dt} = Ae^\frac{-E_a}{RT}[A]^m[B]^n$

E_a is the activation energy and R is the Gas constant. Since at temperature T the molecules have energies according to a Boltzmann distribution, one can expect the proportion of collisions with energy greater than E_a to vary with e^-Ea/RT. A is the pre-exponential factor or frequency factor.

The Arrhenius equation gives the quantitative basis of the relationship between the activation energy and the reaction rate at which a reaction proceeds.

The units of the rate coefficient depend on the global order of reaction:

For order zero, the rate coefficient has units of mol L^-1 s^-1 or mol dm^-3 s^-1
For order one, the rate coefficient has units of s^-1
For order two, the rate coefficient has units of L mol^-1 s^-1 or mol^-1 dm³ s^-1
For order n, the rate coefficient has units of mol^1-n L^n-1 s^-1 or mol^1-n dm^3n-3 s^-1

See also