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Rate coefficient

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Main article: Rate equation

The rate coefficient or rate constant of a chemical reaction is the coefficient that precedes reactant concentrations in a simple rate equation. For a chemical reaction n A + m B → n ' C + m ' D, with rate equation:

$\,r = k(T)[A]^{n}[B]^{m}$

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k(T) is the reaction rate coefficient or rate constant.

Rate coefficient includes all factors that affect reaction rate, except for concentration, which is explicitly accounted for. Rate coefficient is therefore not constant; because of that reason the name rate coefficient is preferred over rate constant. Rate coefficient is mainly affected by temperature as described by Arrhenius equation but also, ionic strength, surface area of the adsorbent (for heterogeneous reactions), light irradiation...

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