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Unformatted text preview: 1 Chemical kinetics Prepared by V. Aditya vardhan adichemadi @ gmail.com P r e p a r e d b y V . A D I T Y A V A R D H A N V A A G D E V I J U N I O R C O L L E G E W A R A N G A L CHEMICAL KINETICS Chemical kinetics deals with 1) Study of rates of reactions 2) Factors affecting rate of reaction 3) Study of mechanisms of reactions Rate of reaction : The rate of reaction is defined as the change in molar concentration of either reactants or products in unit time. change in concentration rate r = time interval Let ‘dc’ is the decrease in concentration of reactants in a small interval of time ‘dt’ then dc r=- dt ( negative sign indicates decrease in concentration ) If ‘dx’ is the increase in concentration of products in a small interval of time ‘dt’ then dx r = dt ( positive sign indicates increase in concentration ) E.g., For a reaction, A B d A d B rate r dt dt In general for a reaction, pP + qQ rR + sS the relations between rates of reaction with respect to P,Q,R and S can be written as d P d Q d R d S 1 1 1 1- =- =+ =+ p dt q dt r dt s dt Eg., for the reaction, 2 g 2 g 3 g N +3H 2NH , the relations can be expressed as 2 2 3 d N d H d NH 1 1-- dt 3 dt 2 dt Factors affecting the rate of reaction 1) Nature of reactants: Rate of a reaction depends on the nature of bonding in the reactants. Ionic compounds react faster than covalent compounds. The reactions between ionic compounds in water occur very fast as they involve only exchange of ions. Eg., AgCl is precipitated out immediately when AgNO 3 solution is added to NaCl solution. 3 3 AgNO +NaCl AgCl NaNO This reaction involves only exchange of ions as shown below and hence occurs very fast. +- +- +- 3 3 Ag +NO +Na +Cl AgCl Na NO Reactions between covalent compounds take place slowly because they require energy for the cleavage of existing bonds. Eg., Following esterification of acetic acid occurs slowly as the breaking of bonds require energy. C H 3 C OH O O C 2 H 5 H C H 3 C OC 2 H 5 O + O H 2 + H 2 SO 4 2 Chemical kinetics Prepared by V. Aditya vardhan adichemadi @ gmail.com P r e p a r e d b y V . A D I T Y A V A R D H A N V A A G D E V I J U N I O R C O L L E G E W A R A N G A L 2) Concentration: Rate of a reaction is directly proportional to the concentration of reactants. rate (r) c n where c = concentration n = order of the reaction or r = kc n where k = specific rate The number of collisions increases with increase in concentration and hence the rate of the reaction also increases. The rate of a reaction decreases with time as the concentration of reactants is decreasing. This can be shown graphically as follows....
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