Chemical kinetics deals with the rate of reaction, the effect of temperature and pressure on the rate of reaction and the mechanism by which the reaction takes place.
Rate of Reaction
Some reactions are slow while others are fast. For example, iron combines with oxygen in presence of moist air, when it rusts (a slow reaction); while a piece of phosphorus when exposed to air bursts into flame (a spontaneous or fast reaction). Thus chemical reactions are said to proceed at different rates. The rate of fast reactions and hence kinetics cannot be studied, i.e. study of kinetics is limited to slow reactions.
Rate of reaction is the change in concentration (number of moles per litre) of reactant or product per unit time. Mathematically,
When a reaction commences, the concentration of the reactant(s) starts decreasing while the concentration of product(s) starts appearing or increasing. If dc is the change in concentration in a very small time interval dt, then the reaction rate is dc/dt. Conventionally(rule wise), the rate of change in concentration is taken to be negative for reactant (i.e. reaction rate for reactant is written as -dc/dt) and positive for a product (i.e. reaction rate for the product is written as dc/dt).
For example, for a reaction
According to the law of mass action, the rate of reaction is proportional to the active mass of the reactants. Thus for the general reaction,
Rate =k[A]n [B]m
where K is a constant and known as rate constant or velocity constant. This relation is known as rate equation or rate law.
Factors Affecting Rate of Reaction
The rate of the reaction involving collisions of particles is influenced by
(i) nature of the reacting substances.
(ii) the concentration of the reactants.
(iii) temperature of the system.
(iv) presence of a catalyst.
(v) radiation and
(vi) surface area.
Rate constant or specific Reaction rate
It is the rate of reaction when the concentration of each reactant is 1 mol/L. For a given reaction it is constant at a particular temperature and is independent of the concentration of reactants. The units of the rate constant of a reaction depend on the order of the reaction. For an nth order of reaction,
For zero order reaction, units of K is mol L-1
first order reaction, unit of K is time-1.
second order reaction, unit of K is mol L-1 time-1.
In the case of gaseous reactions, concentration is expressed in terms of pressure having units of the atmosphere.
Molecularity in Rate of Reaction
It is the number of reactant species which collide simultaneously in a step leading to a chemical reaction. Alternatively, the total number of molecules present in the reactant(s) of a balanced equation call as molecularity of the reaction. For example,
(i) Molecularity of a reaction is always a whole number.
(ii) Molecularity of a reaction is never zero.
(iii) Reactions having molecularity of more than three are rare. It is because the chances of simultaneous collisions between three or more particles are rare.
(iv) In a case of complex reactions having molecularity more than 3, molecularity of the slowest step is taken as molecularity of reaction.
This is whole about the article Rate of Reaction.