RATE AND ORDER OF REACTIONS ‘Kinetics is the study of the rate of reactions, or how fast they occur under different conditions. It usually includes a study of the mechanisms of reactions, ie. how the reacting molecules break apart and then form the new molecules. RATE OF REACTION Rate of reaction is the velocity with which the reactant undergo chemical change. During a reaction, the concentuation of reactants changes with time.“Decrease in concentration of reactant or increase in concentration of the products of the reaction at a given moment of time is called rate of reaction”. It is determined by measuring the decrease in the concentration of reacting substances per unit time. de/dt-K ‘Where, de-change in concentration, ét-time taken for change and K~ reaction rate constant ORDER OF REACTION The order of reaction can be defined as the power dependence of rate om the concentration of all seactants. Sum of the powers of the concentration in the rate equation is called overall order of the reaction. Rate lavw or rate equation for a gencral reaction, A ————-> product can be written as: de rae= Sax [a] q tll ‘where m= order of the reaction ‘For a general reaction of two reactants, 2A=0B+ > products the rate law can be written as: r=KlAFIBE where “Prefers to the rate of reaction, ‘k’ is the rate constant of the reaction, [A] and [B] are the concentrations of the reactants. The exponents of the reactant concentrations x and y are referred, to as partial orders of the reaction. The overall order of a reaction is the sum of the individual seaction orders for each reactant. Hence the order of above reaction = x + y. The reactions may ‘be zero order, first order, pseudo first order, second order, and may be of higher order. Let us consider a chemical reaction (CHsCOOGH: + NaOH ——+ CHCOONa+ CoHsOH ‘The rate expression will be Rate = -[CH:COOC:Hs]_ - d]NaOE] =k [CHsCOOC:E5] '. [NaOH] a at ‘So in the above reaction, the overall order of reaction will be 1+1=2. So this will be a second order reaction. ‘Onder ofa reaction is an experimental value. It means it isan experimentally determined parameter. ‘Some characteristics ofthe reaction order for a chemical reaction are listed below. ‘+ Reaction order represents the mumiber of species whose concentration directly affects the rate of reaction, ‘+ The reaction order of a chemical reaction is always defined with the help of reactant concentrations and not with product concentrations, ‘+ The value of the order of reaction can be in the form of an integer or a fraction. It can even have a valve of ze. (Consider the reaction in Which concentration of the reactant changes and inital rate of reaction also changes2NO+ Ci2 — 2NOCI itroryl Chloride) Rate=K [NO} [CIP ital data NO] M- [Cl] M_ Tnitial Rate Mis OL 02 1 [NO] concentration increase ? times and it’ rate increase 4 times, therefore partial order of reaction is ‘422. Samar [C1] concentration increases ? times while it'orateinerenses 2 times, therefore partial order of reactions 1. So, order ofthe sbove rcactin is ?= 1=3 RatefNOF [CIP | , 18 Mis/ (0.1M)* (0.1M) = 180M“S* Zero Order Reaction In these reactions the rate of reaction doesn’t depend upon the concentration of reactants. It means change in concentration of reactants doesnt affect the rate of reaction. Forthereaction A> products tobe of zero onder, dt or = HA) =k Examples of these types of reactions include the enzyme-catalysed oxidation of (CHCH:OH (ethanol) to CHsCHO (acetaldehyde). Fe or Wascatalyst 2NH,(9) N,() + 3H.(9) : 20 ener First Order Reaction In these reactions the rate of reaction depends on the concentration of one reactant only. There can be many reactants in the reaction but concentration of only one reactant will affect the rate of reaction. Concentration of other reactants will have no effect on order of reaction. Tn general terms: A= products -dsidt= KA}Example: NOs —> N20: +0: Rate =K(N,O5] Tn 1" oxder reaction the rate of reaction increases with concentration, as shown in the figure. Second Order Reaction In these reactions the rate of reaction depends on the concentration of two different reactants or square of concentration of one reactant The kinetics of second order reactions are given as follows: A+B products -dv/dt=K{A].[B] Example: 2NO; + 2NO+0, Rate =k[NO;]* CH,CO0CH, + OH» CH;COO'+ C,H,0H Ethyl acetate) Rate =k{CHsCOOC>Hs] [OH]fate = HIN Rate HA tomas) Stoondorder fate> Wa Pseudo First Order Reaction Originally of higher order but made to behave like lower order reaction. Here a second order or bimolecular reaction is made to behave like first order. This is found in the case in which one reacting material is present in great excess or is maintained at constant concentration as compared with other substance. Here reaction rate is determined by one reactant even though two are present. Decomposition of ascorbic acid tablet and Aspirin hydrolysis. Example ~ Hydration of alkyl halide CH,I+H,0 — CH,OH + H-+T Rate of reaction = k [CH:I] [ H:0] As methyl iodide is also used in aqueous solution form so the concentration of water is for higher than methyl iodide [CHI] =<<[ 1,0] So, concentration of water doesn’t change much and can be approximated as no change or constant. ‘Now we can write — Rate of reaction = k [CHI] Where k* = k [H20] Thus, the reaction appears to be first order, but itis actually of second order that’s why it is known as pseudo first order reaction. Pseudo Zero Order Reaction Ic frequently occur in drugs formulated as pharmaceutical suspensions. Suspension is the case of 2210 order kinetics in which the concentration in solution depends on the drug's solubility. As the drug decomposes in solution, more drug is released from the suspended particles so that the concentration remains constant. Once all the suspended putticles have been couverted into deug in solution, the system changes to a first order reaction. Eg. in aspirin suspeusion the reaction appears to be zero till, aspiria is present in solid form and only when all of aspiria gets dissolved the reaction becomes first order. In solid state, many drug decomposes by pseudo zero order ie. reaction between dug and moisture in solid dosage form. The system behaves like suspensions and because of the presence of excess solid rug; the first order rate actually becomes pseudo zero order. Equation for it is similar to zero order except K is replaced by K’Third order reactions : Total order of the reaction is three Examples: 1) Reaction between NO and 0; to give NO» £=k(NO}{O3] 2) Reaction between NO and Cl: to give NOCI 2NO + Cl, 2NOoCcL F=K{NOP [Ch] MOLECULARITY OF THE REACTION ‘The molecularty of 2 chemical reaction is the number of molecules or atoms of the reactants that give the product. Tfone type of molecule undergoes change to give a product, such a reaction is termed 2s unimolecular. Bo —> Br ‘Two molecules reacting to give @ product follow bimolecular reaction Eek a ‘Trimolecular reactions are those involving three molecules reacting simmltaneousty to give product 2NO+0> —> 2NO: Difference Between Molecularity and Order of Reaction ‘Moleculatity and order of reaction both give information zbout the chemical reaction but are very ifferent fiom each other 2s molecularity tells about the aumber of molecules taking part in reaction while order of reaction tells about the relationship between rate of reaction and concentration of reactants, ‘Molecularity Order of reaction doesn’t depent on pressure and temperature. | ¥ SePeeds on pressure. temperature zad Ys Trean be zero, integer or vena Inisalways awhole number are ‘We can determine molecularity by looking at | Order of reaction can be determined by reaction mechanisms. experiments, ‘Molecularity cannot be a negative munber. ‘Order of reaction can be ancgative