The steric factor, usually denoted ''ρ'',
is a quantity used in
collision theory.
Also called the ''probability factor'', the steric factor is defined as the ratio between the experimental value of the
rate constant and the one predicted by collision theory. It can also be defined as the ratio between the
pre-exponential factor and the
collision frequency
Collision frequency describes the rate of collisions between two atomic or molecular species in a given volume, per unit time. In an ideal gas, assuming that the species behave like hard spheres, the collision frequency between entities of speci ...
, and it is most often less than unity. Physically, the steric factor can be interpreted as the ratio of the
cross section for reactive collisions to the total collision cross section.
Usually, the more complex the reactant
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
s, the lower the steric factors. Nevertheless, some reactions exhibit steric factors greater than unity: the
harpoon reactions, which involve atoms that exchange electrons, producing ions. The deviation from unity can have different causes: the molecules are not spherical, so different geometries are possible; not all the
kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its a ...
is delivered into the right spot; the presence of a
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
(when applied to solutions); and so on.
When collision theory is applied to reactions in solution, the
solvent cage has an effect on the reactant molecules, as several collisions can take place in a single encounter, which leads to predicted preexponential factors being too large. ''ρ'' values greater than unity can be attributed to favorable entropic contributions.
Usually there is no simple way to accurately estimate steric factors without performing trajectory or scattering calculations. It is also more commonly known as the frequency factor.
Notes
Chemical kinetics
Physical chemistry
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