Chemical Kinetics 1

(a) the feasibility of a chemical reaction which can be predicted by thermodynamics ( as you know that a reaction with DG < 0, at constant temperature and pressure is feasible); (b) extent to which a reaction will proceed can be determined from chemical equilibrium; (c) speed of a reaction i.e. time taken by a reaction to reach equilibrium.

the branch of chemistry, which deals with the study of reaction rates and their mechanisms, called chemical kinetics. Kinetic studies not only help us to determine the speed or rate of a chemical reaction but also describe the conditions by which the reaction rates can be altered.

the speed of a reaction or the rate of a reaction can be defined as the change in concentration of a reactant or product in unit time. it can be expressed in terms of: (i) the rate of decrease in concentration of any one of the reactants, or (ii) the rate of increase in concentration of any one of the products.

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 gases is expressed in terms of their partial pressures, then the units of the rate equation will be atm s–1

It can be determined graphically by drawing a tangent at time t on either of the curves for concentration of R and P vs time t and calculating its slope

Rate of reaction depends upon the experimental conditions such as concentration of reactants (pressure in case of gases), temperature( van’t hoff eqn) and catalyst ( may be +ve,-ve or neutral). it depends on nature of reacting species, pH, pressure.

The representation of rate of reaction in terms of concentration of the reactants is known as rate law. It is also called as rate equation or rate expression. rate law is the expression in which reaction rate is given in terms of molar concentration of reactants with each term raised to some power, which may or may not be same as the stoichiometric coefficient of the reacting species in a balanced chemical equation.

The rate of a chemical reaction at a given temperature may depend on the concentration of one or more reactants and products. rate of a reaction decreases with the passage of time as the concentration of reactants decrease. Conversely, rates generally increase when reactant concentrations increase. So, rate of a reaction depends upon the concentration of reactants.

the powers, are usually equal to stoichiometric coefficients, but not always. Rate law for any reaction cannot be predicted by merely looking at the balanced chemical equation, i.e., theoretically but must be determined experimentally.

Rate = k [CHCl3] [Cl2]1/2 Rate = k [CH3COOC2H5]1 [H2O]0

Rate = k [A]x [B]y x and y indicate how sensitive the rate is to the change in concentration of A and B. Sum of these exponents, i.e., x + y in gives the overall order of a reaction whereas x and y represent the order with respect to the reactants A and B respectively.

the sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction. Order of a reaction can be 0, 1, 2, 3 and even a fraction. A zero order reaction means that the rate of reaction is independent of the concentration of reactants.

The reactions taking place in one step are called elementary reactions.

When a sequence of elementary reactions (called mechanism) gives us the products, the reactions are called complex reactions. These may be consecutive reactions (e.g., oxidation of ethane to CO2 and H2O passes through a series of intermediate steps in which alcohol, aldehyde and acid are formed), reverse reactions and side reactions (e.g., nitration of phenol yields o-nitrophenol and p-nitrophenol).

rate= (concentration)^1-n x (time)^-1 n- order for zero order, mol/Ls for 1st order, /s for 2nd order L/mol s