REACTION RATE AND RATE LAW Chemical kinetics is the branch of chemistry which deals with the speed of chemical reaction, factors affecting the rate and the mechanism by which the reaction proceeds to completion. Chemical kinetics illustrates the connection between mathematics and chemistry. On the basis of rate of reaction, the chemical reactions are classified as very fast reactions, moderate reactions and very slow reactions. During chemical reaction, breaking of existing bonds and formation of new bonds takes place. Breaking and formation of bonds involve certain amounts of absorption and evolution of energy. Rate of reaction is the change in the concentration of any one of the reactants or products per unit time. Average reaction rate is the rate of change of concentration of a reactant or of a product over a specified measurable period of time. The rate of reaction at a given time is called an instantaneous reaction rate. Concentration, catalyst, temperature, pressure and surface area are the factors that affect the reaction rate. Rate law is the expression which expresses the rate of reaction in terms of molar concentrations of the reactants with each term raised to some power which may or may not be same as the stoichiometric coefficient of that reactant in the balanced chemical equation. Rate constant is the rate of the reaction when the molar concentration of each reactant is taken as unity. ORDER AND MOLECULARITY OF REACTION Order of the reaction is the sum of the exponents to which the concentration terms are raised in the rate law expression. Order of reaction may not be equal to the sum of the stoichiometric coefficients of the reactants in the balanced equation. On the basis of order chemical reactions are classified as zero order reactions, first order reactions, second order reactions and third order reactions. Molecularity of a reaction is the number of atoms or ions or molecules that must collide simultaneously so as to result into a chemical reaction. Chemical reaction in which one or more chemical species react directly to form products in a single reaction step and with a single transition state is called elementary reaction. The reactions that take place in a sequence of steps, and each step is represented by a simple balanced chemical equation, are called complex reactions. Order of a reaction can be determined by initial rate method, integrated rate law method, half life method, graphical method (using differential rate law equation) and Ostwald’s isolation method. CATALYST AND CONCEPT OF ENERGY Substance that alters the rate of a chemical reaction without being used up in the reaction is known as catalyst. Small amount of catalyst is capable of changing the rate of reaction to a large extent. Threshold energy is the minimum energy that a colliding molecule should have so as to make the collision effective. The minimum extra amount of energy absorbed by the reactant molecule so that its energy becomes equal to the threshold value is called “activation energy”. Lower the activation energy, faster will be the rate of reaction and vice-versa. According to intermediate complex theory, the reactants first combine with catalyst (having variable valence) to form a short lived intermediate complex, which eventually decomposes to give product and regenerates the catalyst. The rate of this reaction nearly doubles for 10 degree rise of the temperature. This shows that the temperature has a marked effect on the reaction rate. Quantitatively, the effect of the temperature on the rate of the reaction and hence on the rate constant k, was proposed by Arrhenius in 1889. Arrhenius equation explains the effect of the temperature on the rate constant k. COLLISION THEORY Collision theory was proposed by Max Trautz and William Lewis. According to this theory, the reactant molecules have to collide in a certain way with a certain amount of energy in order to form a new product. The minimum amount of energy that must be possessed by the reacting molecule to make the collision effective is called the threshold energy. Effective collisions are those collisions that lead to the formation of the products. The factors that govern the number of effective collisions are: energy factor and orientation factor Number of collisions per second per unit volume of the reaction mixture is known as collision frequency. There were certain limitations of collision theory such as it considers atoms or molecules to be hard sphere.
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