# Chemical Kinetics

Pages: 74 (24485 words) Published: August 28, 2012
CHEMICAL KINETICS
SYLLABUS
LEARNING OBJECTIVES
Rate of a reaction (average and instantaneous)
Factors affecting rates of reaction; concentration, temperature, catalyst Order and molecularity of a reaction
Rate law and specific rate constant
Integrated rate equations and half life (only for zero and first order reactions) Concept of collision theory (elementary idea, no mathematical treatment).
After studying this Unit, you should be able to:
define the average and instantaneous rate of a reaction ;
express the rate of a reaction in terms of change in concentration of either of the reactants or products with time; distinguish between elementary arid complex reactions ;
differentiate between the molecularity and order of a reaction, define rate constant; discuss the dependence of rate of reactions on concentration, temperature and catalyst; derive integrated rate equations for the zero and first order reactions ; determine the rate constants for zeroth and first order reactions ; describe collision theory.

CONTENTS
4.1. Rate of a chemical reaction
4.2. Average rate and its determination
4.3. Instantaneous rate and its determination
4.4. Factors affecting the rate of a chemical reaction
4.5. Effect of concentration
4.6. Molecularity and mechanism of reaction elementary and complex 4.7. Order of reaction
4.8. Pseudo first order reaction
4.9. Integrated rate equations
4.10. .Different methods for calculating the value of rate constant 4.11. Half life of a reaction
4.12. Effect of temperature on reaction rate
4.13. Concept of activation energy
4.14. Arrhenius equation—temperature dependence of rate constant 4.15. Effect of catalyst on rate of reaction
4.16. Collision theory of chemical reactions NCERT Textbook Questions (Answers I Solutions) For Quick Revision Exercises Competition File Chemical Kinetics
Chemistry, by its very nature, is concerned with change. Substances with well defined properties are converted by chemical reactions into other substances with different properties. For any chemical reaction, chemists try to find out (а) the feasibility of a chemical reaction which can be predicted by thermodynamics (as you know that a reaction with AG < 0, at constant temperature and pressure is feasible); (б) extent to which a reaction will proceed which can be determined from chemical equilibrium; (c) speed of a reaction i.e., time taken by a reaction to reach equilibrium. In addition to the feasibility and extent, it is equally important to know the rate and the factors controlling the rate of a chemical reaction for its complete understanding. For example, which parameters determine as to how rapidly food gets spoiled ? How to design a rapidly setting material for dental filling ? Or what controls the rate at which fuel burns in an auto engine ? All these questions can be answered by the branch of chemistry, which deals with the study of reaction rates or speed of reaction and their mechanisms, called chemical kinetics. The kinetic studies not only help us to determine the reaction rates but also describe the conditions by which the reaction rates can be altered. Besides this the mechanism governing the reactions could also be predicted on the basis of logical kinetic data. During the course of our studies we come across a wide variety of chemical reactions having different rates. In thermodynamics (chapter 6, class XI) we have studied that the decrease in free energy (AG < 0) is quite helpful in predicting the feasibility of chemical reactions. However, knowledge of free energy change of reaction does not give any idea about the speed or rate of the reaction. For example, thermodynamic data indicate that diamond shall convert to graphite but in reality the conversion rate is so slow that the change is not perceptible at all. Hence, the quotation, diamond.is forever. In general, various reactions can be categorised into three types depending upon the reaction rates. 1. Very fast reactions (Type 1)....