The steric factor, usually denoted ''ρ'', is a quantity used in

collision theory Collision theory is a principle of chemistry used to predict the rates of chemical reactions. It states that when suitable particles of the reactant hit each other with correct orientation, only a certain amount of collisions result in a percept ...

. Also called the ''probability factor'', the steric factor is defined as the ratio between the experimental value of the

rate constant In chemical kinetics a reaction rate constant or reaction rate coefficient, ''k'', quantifies the rate and direction of a chemical reaction. For a reaction between reactants A and B to form product C the reaction rate is often found to have the ...

and the one predicted by collision theory. It can also be defined as the ratio between the

pre-exponential factor In chemical kinetics, the pre-exponential factor or A factor is the pre-exponential constant in the Arrhenius equation (equation shown below), an empirical relationship between temperature and rate coefficient. It is usually designated by A when ...

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 specie ...

, and it is most often less than unity. Physically, the steric factor can be interpreted as the ratio of the

cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Abs ...

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 bioche ...

s, the lower the steric factors. Nevertheless, some reactions exhibit steric factors greater than unity: the

harpoon reaction A harpoon reaction is a type of chemical reaction, first proposed by Michael Polanyi in 1920, whose mechanism (also called the harpooning mechanism) involves two neutral reactants undergoing an electron transfer over a relatively long distance to ...

s, 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 acc ...

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 In chemistry, the cage effect (also known as geminate recombination) describes how the properties of a molecule are affected by its surroundings. First introduced by Franck and Rabinowitch in 1934, the cage effect suggests that instead of acting ...

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 {{Physical-chemistry-stub