Rate of reaction usually doubles when the temperature is raised by ten degrees near the normal temperature because the rate constant is dependent on temperature. The temperature dependency of rate constant is mathematically expressed by the Arrhenius equation. Arrhenius equation is an empirical equation. The equation indicates exponential dependency on temperature of the rate constant. The effect of temperature on the rate constant can be explained by Collision Theory, especially for gas phase reactions.
The essence of Collision Theory is that reaction can take place only when the colliding molecules collide with energy greater than the threshold energy and they collide with proper orientation. The fraction of collision that have energy greater than the threshold increases rapidly with temperature and this explains the exponential temperature dependence of rate constant. Transition State explains the rate of reaction in term of formation of a transition state of reacting molecules. The formation of transition state requires energy of activation. Transition state theory connects kinetic rates with thermodynamic properties of reactants and the transition state.The chemical reaction may take place in one step or a sequence of steps. Each of the steps are called elementary reactions. Complex reaction consists of a sequence of elementary reactions. The sequence of elementary reactions can occur in series, or there can be set of all parallel reactions that describe the overall reaction. Molecularity characterizes an elementary reaction. It is the number of molecules, which are in simultaneous collision in a reaction. Molecularity can have integral values only. It cannot be zero. Elucidating the mechanism of a reaction involves identification of products, intermediates and then formulating a set of elementary reactions. Then the rate law is predicted and tested with experimental rate law. The rate of a complex reaction depends on the slowest elementary reaction. This is the rate determining step of the complex reaction.
A catalyst is substance that accelerates a reaction but does not take part in the reaction. Catalyst enhances the rate of reaction by lowering the activation energy of the reaction.Photochemical reactions are induced by the absorption of light in the ultraviolet and visible spectrum of the electromagnetic radiation. The light energy may be absorbed directly by the reacting species or it may be absorbed by a photosensitizer. Photosensitizer acts as a carrier of energy, it does not take part in the chemical reaction. The efficiency of photochemical reaction is given by the quantum yield, which is the ratio of number of molecules reacted and the number of photons absorbed.
