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Chapter 4 Chemical Kinetics Chemical Kinetics Introduction Chemical Kinetics (Kinesis: Movement) The branch of chemistry which deals with the study of the rates of chemical reactions, the factors affecting the rates of the reactions and the mechanism by which the reactions proceed is called Chemical Kinetics. Classification of Reactions On the Basis of Rates: Very fast reactions e.g. precipitation of AgCl Very slow reactions e.g. rusting of iron Reactions taking place at moderate speeds e.g. hydrolysis of starch 1. Rate of a Chemical Reaction The rate of a reaction can be defined as the change in concentration of a reactant or a product in unit time. 1.1 Average Rate The rate of reaction measured over a definite time interval is called average rate of a reaction. Consider a hypothetical reaction, R→PR→P ∗20l∗20l Average rate of reaction = (Decrease in concentration of R) (Time taken) = [ΔR]ΔtAverage rate of reaction = (Decrease in concentration of R) (Time taken) = [ΔR]Δt Or
Chapter 4 Chemical Kinetics Average rate of reaction = (Increase in concentration of P) (Time taken) = [ΔP]Δt Average rate of reaction = (Increase in concentration of P) (Time taken) = [ΔP]Δt 1.2 Instantaneous Rate Rate of change of concentration of any one of the reactants or products at that particular instant of time is called instantaneous rate. As Δt→0 Δt→0 or rinst = −d[R]dt = d[P]dt rinst = −d[R]dt = d[P]dt Important: Rate of a reaction is always positive. Since, RR is a negative quantity (as concentration of reactants is decreasing), it is multiplied with – 1 to make the rate of the reaction a positive quantity. 1.3 Units of Rate of a Reaction Units of rate are concentration time–1 e.g. mol L-1s -1; atm s-1mol L-1s -1; atm s-1 (for gaseous reactions) 1.4 Overall Rate of a Reaction When there are several reactants and products the individual rates of the various components may differ as they would depend on the stoichiometric coefficients. For a reaction, A + 2B →3C + 4DA + 2B →3C + 4D Rate of disappearance of B = 2 × Rate of disappearance of A (2:1) Rate of formation of C = 3 × Rate of disappearance of A (3:1) Rate of formation of D = 4 × Rate of disappearance of A
Chapter 4 Chemical Kinetics (4:1) To define a unique value for the overall rate of the reaction we divide the individual rates by the respective coefficients and equate their signs. Overall Rate = - A / t = (-1/2) B/ t = ( + 1/3) C/ t Overall Rate = - A / t = (-1/2) B/ t = ( + 1/3) C/ t = ( + 1/4) D / t = ( + 1/4) D / t 2. Dependence of Rate nn Concentration Factors Influencing Rate of a Reaction concentration of reactants (pressure in case of gases), temperature and Catalyst. Dependence on Concentration: 2.1 Rate Law Consider a general reaction aA + bB →cC + dDaA + bB →cC + dD The rate expression for this reaction is Rate ∝ [A]x [B]yRate ∝ [A]x [B]y where exponents x and y may or may not be equal to the stoichiometric coefficients (a and b) of the reactants. Rate=k⋅[A]x⋅[B]y Rate=k⋅[A]x⋅[B]y −d[R]/dt=k[A]x [B]y −d[R]/dt=k[A]x [B]y Above equation is known as differential rate equation, k is a proportionality constant called rate constant. 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.

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