A-Factor
Definition and meaning of A-Factor in chemistry.
The A-factor, also called the pre-exponential factor or frequency factor, is the constant A in the Arrhenius equation, k = A·e^(−Ea/RT). It represents how often reactant particles collide with the correct orientation for a reaction to occur.
In more detail
The A-factor combines the frequency of collisions between particles with a steric factor that accounts for whether those collisions are correctly aligned. It has the same units as the rate constant, k, which depend on the overall order of the reaction. Although it varies slightly with temperature, the A-factor is often treated as a constant over a limited temperature range.
Key facts
| Symbol | A |
|---|---|
| Also called | Pre-exponential / frequency factor |
| Appears in | The Arrhenius equation |
| Field | Physical chemistry (kinetics) |
For a first-order reaction, a large A-factor means the reactant molecules collide and align favorably very frequently, allowing a fast reaction once the activation energy is available.
Frequently asked questions
What does the A-factor represent?
It represents the frequency of collisions between reactant particles that are oriented correctly for reaction. A larger A-factor means more frequent, effective collisions.
Is the A-factor the same as the rate constant?
No. The rate constant k depends on both the A-factor and the activation energy through the Arrhenius equation; the A-factor is only the collision-frequency part.