Factors affecting Rates of Reaction

The reaction between the reactants occurs only when they collide in correct orientation in space.
Greater the probability of collisions between the reactants with proper orientation, greater is the rate of reaction.
The orientation of molecules affect the probability factor, p.
The simple molecules have more ways of proper orientations to collide. Hence their probability factor is higher than that of complex molecules.
The orientation factor also affect the interaction between reactants and catalysts.
- For example in case of biological reactions, which are catalyzed by enzymes, the biocatalysts.
The enzymes activate the reactant molecules (or substrates) at a particular site on them. These sites are called as active sites and have definite shape and size.
The size, stereochemistry and orientation of substrates must be such that they can fit into the active site of the enzyme.
Then only the reaction will proceed. This is also known as lock and key mechanism.
The enzymes lose their activity upon heating or changing the pH or adding certain chemical reagents. This is due to deformation of the configuration of active site.

The rate of a reaction increases with increase in the surface area of solid reactant, if any used.
The surface of a solid can be increased by grinding it to a fine powder.

The rate of some photochemical reactions, which occur in presence of light, increases with increase in the intensity of suitable light used.
With increase in the intensity, the number of photons in light also increases.
Hence more number of reactant molecules get energy by absorbing more number of photons and undergo chemical change.
However, some photochemical reactions involving the free radicals, generated in a chain process, are not greatly affected by the intensity of the light.
Just one photon is sufficient to trigger the formation a free radical.
This in turn initiate a chain process in which more free radicals are formed repeatedly in each cycle without the need of extra photons.

The solvent may affect the rate in many ways as explained below:
The solvents are used to dissolve the reactants and while doing so they help in providing more interactive surface between reactant molecules which may be otherwise in different phases or strongly bonded in solid phase.
Usually solvents help in breaking the cohesive forces between ions or molecules in the solid state.
The polar molecules tend to dissolve more in polar solvents with more dielectric constants and react faster in them. Whereas non polar molecules prefer non polar solvents.