Pre-exponential Factor
The constant A in the Arrhenius equation, representing the frequency of appropriately oriented collisions.
Formula / equation
A = pZ
Unit
Various
In depth
Also called the frequency factor, A combines the collision frequency with the steric factor that accounts for orientation. It has the same units as the rate constant and varies only weakly with temperature, so it is usually treated as constant over modest ranges.
Examples in the real world
Simple atomic recombinations have A near 10¹¹ L/(mol·s), while reactions of large molecules are far smaller.