This branch of chemistry which deals with the study of rates of chemical reactions and the mechanism by which they occur. While studying reaction, one deals with:
(a) how fast (or slow) the reactants get converted into products
(b) the steps or paths through which the products are formed (reaction mechanism)
Chemical reaction kinetics deals with the rates of chemical processes. Any chemical process may be broken down into a sequence of one or more single-step processes known either as elementary processes, elementary reactions, or elementary steps.
Elementary reactions usually involve either a single reactive collision between two molecules, which we refer to as a a bimolecular step, or dissociation/isomerization of a single reactant molecule, which we refer to as a unimolecular step.
Very rarely, under conditions of extremely high pressure, a termolecular step may occur, which involves simultaneous collision of three reactant molecules. An important point to recognize is that many reactions that are written as a single reaction equation in actual fact consist of a series of elementary steps. This will become extremely important as we learn more about the theory of chemical reaction rates.
As a general rule, elementary processes involve a transition between two atomic or molecular states separated by a potential barrier. The potential barrier constitutes the activation energy of the process, and determines the rate at which it occurs. When the barrier is low, the thermal energy of the reactants will generally be high enough to surmount the barrier and move over to products, and the reaction will be fast. However, when the barrier is high, only a few reactants will have sufficient energy, and the reaction will be much slower.
Fig: Chemical kinetics
The presence of a potential barrier to reaction is also the source of the temperature dependence of reaction rates. The huge variety of chemical species, types of reaction, and the accompanying potential energy surfaces involved means that the timescale over which chemical reactions occur covers many orders of magnitude, from very slow reactions, such as iron rusting, to extremely fast reactions, such as the electron transfer processes involved in many biological systems or the combustion reactions occurring in flames.
A study into the kinetics of a chemical reaction is usually carried out with one or both of two main goals in mind:
1. Analysis of the sequence of elementary steps giving rise to the overall reaction. i.e. the reaction mechanism.
2. Determination of the absolute rate of the reaction and/or its individual elementary steps.
Rate of a Reaction
In general, for a reaction : R → P, the behaviour of the concentration of the reactant and product, as the reaction proceeds is shown graphically:
From the graph, it is clear that the concentration of the reactant decreases and that of the product increases as the reaction proceeds and the rate of the change of the concentration of the reactant as well as that of the product is also changing.
Rate of a reaction can, now, be defined in two ways:
Average Rate of reaction (rav) given by for the reaction R → P:
rav = =
where Δ[R] and Δ[P] represents the change in the concentrations of 'R' and 'P' respectively over a time interval Δt
The average rate of the reaction between a time interval (tf - ti = Δt) can be determined from the above graph by locating the concentration of 'R' of 'P' on this graph at the time instants tf and ti as shown.
If [R]f and [R]i are the concentrations of the reactant 'A' at the time instants tf and ti then :
Similarly from the plot of 'P' as a function of 't', we have :
Instantaneous Rate of reaction (rinst.) can be calculated from rav
in the limit Δt → 0 and is represented as :
Units of rate of a reaction:
Units of rate are concentration time-1. For example, if concentration is in mol L-1 and time is in seconds then the units will be mol L-1s-1. However, in gaseous reactions, when the concentration of gas is expressed in terms of their partial pressures, then the units of the rate equation will be atm s-1.
Relation between various rates:
In general for a reaction : aA + bB → cC + dD
The rate of reaction can be expressed as follows :
Rate = = = = = kr[A]m[B]n