Chemical Kinetics Class 12th Notes PDF

PRANOTI PATIL DK SCIENCE ACADEMY Chemical kinetics The branch of chemistry which deals with the rate of chemical Reaction and the factors which effect the rate is called as chemical kinetics. Rate of Reaction Average Rate...

PRANOTI PATIL DK SCIENCE ACADEMY Chemical kinetics The branch of chemistry which deals with the rate of chemical Reaction and the factors which effect the rate is called as chemical kinetics. Rate of Reaction Average Rate Instantaneous Rate The change in concentration of The change in concentration reactant or product divided by of Reactant or product at the time interval over which the different time interval, is change occurs, is called as called as Instantaneous Rate Average Rate of Reaction of Reaction A→B It is obtained by drawing Average −∆[𝐴] +∆[𝐵] tangent to the curve = = Rate of reaction ∆𝑡 ∆𝑡 A→B Instantaneous −𝑑[𝐴] +𝑑[𝐵] = 𝑑𝑡 = Rate of reaction 𝑑𝑡 PRANOTI PATIL Important points. Average Rate for reactant = -ve Rate of reaction and Instantaneous for product= +ve Rate xA + yB → zC Instantaneous = - 1 𝑑[𝐴] = - 1 𝑑[𝐵] = +1 𝑑[𝐶] 𝑥 𝑑𝑡 𝑦 𝑑𝑡 𝑧 𝑑𝑡 Rate of Reaction Rate law:- The rate of reaction at a given time is proportional to the molar concentration of reactants, raised to the simple powers. aA + bB → cC + dD Rate of reaction α [A]x [B]y Rate of reaction = K [A]x [B]y Imp point Rate = K [A]x [B]y The reaction is x order with respect to A and the reaction is of y order with respect to B PRANOTI PATIL Order of reaction:- aA + bB → cC + dD Rate of reaction = [Reactant] Rate of reaction = [A]x [B]y Overall order of reaction = (x+y) Overall of reaction with respect to A = x Overall of reaction with respect to B = y Overall order The overall order of a reaction is defined as the sum of concentration terms in the rate law expression. Elementary reaction:- The reaction that occur in a single step and cannot be broken down further into simpler reactions are called as elementary reactions. Molecularity of Reaction:- The number of reactant molecules taking part in a chemical reaction, is called molecularity of reaction. Rate determining step(R.D.S) The slowest step in the reaction, is called as Rate determining step (R.D.S). PRANOTI PATIL Reaction intermediate The chemical species that is formed in one step and consumed in the next step is called as reaction intermediate The reaction intermediate does not appear in the rate law. Difference between order and molecularity Points of Order Molecularity difference Experimental/ Order is Molecularity is theoretical experimental theoretical property property Definition The order is the The sum of the molecularity is powers of the the number of concentration reactant terms of molecules reactant in rate taking part in a law. chemical reaction Nature of values It may be integer It is an integer fraction or zero PRANOTI PATIL Intergrated Rate Law for the first order Reaction Consider a first order Reaction 1 A P ∫. 𝑑𝑥 = 𝑙𝑛𝑥 𝑥 Reactant product 1 ∫ ∫. 𝑑[𝐴] = 𝑙𝑛[𝐴] −𝑑[𝐴] = + k [A] 𝐴 Rate = 𝑑𝑡 ∫ dx = x 𝑑[𝐴] So = - k.dt [𝐴] ∫ dt = t Integrating both sides 𝑚 ln (m) – ln (n) = ln( ) 𝑛 𝐴𝑡 −𝑥 ∫𝐴𝑜 1 𝑑[𝐴] = −𝑘. 𝑑𝑡 = +𝑦 [𝐴] 𝑦 𝑥 𝐴𝑡 −[𝐴] [𝐴] In [𝐴]𝐴𝑜 = - k.t + 0 𝑡 𝑜 [𝐴] = [𝐴] 𝑜 𝑡 In [At] –ln [Ao] = -kt Also In [𝐴] 𝑡 [𝐴] 𝑜 = -k.t ln x = 2.303 x log10 x k=- 1 ln [𝐴]𝑡 𝑡 [𝐴]𝑜 𝐴𝑜 k= 1 ln [ ] 𝑡 𝐴𝑡 2.303 [ ]……… …….(for first order reaction) Where, [Ao] = Initial concentration = (a) [At] = concentration after time t = (a-x) t = time k = rate constant PRANOTI PATIL 𝑎 So k = 2.303 log10( ) 𝑡 𝑎−𝑥 [𝐴] k= 1 ln 𝑜 𝑡 [𝐴] 𝑡 So [𝐴] kt = ln [𝐴] 𝑡 [𝐴] [𝐴] [𝐴] [𝐴] Half life The time required for the reactant concentration to fall one half of its initial value is called as half life of a chemical reaction. Expression for half life of a first order reaction. The integrated rate law for the first order reaction is given as [𝐴] 2.303 K= log10 𝑜 [𝐴] ………………………………….(1) 𝑡 𝑡 PRANOTI PATIL So now substitute t= t1/2 [𝐴] and [A]t = 𝑜 2 so eqn (1) becomes [𝐴] 2.303 k= log10 𝑜 [𝐴]/2 t𝟏/𝟐 𝑜 2.303 k= log10 (2) t𝟏/𝟐 2.303 𝑥 0.3010 k= t𝟏/𝟐 or t𝟏/𝟐 = …………………….(for first order reaction) t𝟏/𝟐 0.693 Graphical representation of the first order reactions. i) Graph of rate versus initial concentration y Rate x initial concentration −𝑑[𝐴] rate = = k[A]t + o y m x c (slope) PRANOTI PATIL ii) Graph of concentration versus time (t) :- [𝐴] iii) Graph of log [𝐴]0 versus time(t) :- 𝑡 [𝐴] 2.303 k = 𝑡 = log10 [𝐴]0 𝑡 [𝐴] So log [𝐴]0 =( 𝑘 )𝑡 + 0 2.303 𝑡 X y-interept © y m(slope) PRANOTI PATIL [𝐴] 0 Log [𝐴] 𝑘 𝑡 Slope = 2.303 Time (t)→ iv) Graph of log [A]t versus time (t):- [𝐴] 2.303 0 k= 𝑡 log10 [𝐴] 𝑡 𝑘𝑡 So = log10[A]0 – log10 [A]t 2.303 −𝑘 log10[A]t = ( ) 𝑡 + log10[A] 0 2.303 y m x c y −𝑘 Slope = 2.303 log[A]t log[A]o x t (time) PRANOTI PATIL Examples of first order reaction i) 2H2O2(l) → 2H2O(l) + O2(g) ii) 2N2O5(g) →4NO2(g) + O2(g) Integrated rate law for the gaseous phase(f) reaction 2.303 𝑃𝑖 k= log10 𝑡 2𝑃𝑖−𝑃 Where, k= rate constant t = time in sec Pi = Initial pressure P = final pressure (pressure after time t) Zero order Reaction The reaction which is independent of the concentration of reactant is called as zero order reaction. Integrated rate law for zero order Reaction A → P −𝑑[𝐴] Rate = = k[A]0 = k 𝑑𝑡 𝑑[𝐴] = -k…………………………………….[A] 0 = 1 𝑑𝑡 Integrating both sides,…………………..(anything)0 = 1 We get [𝐴]𝑡 ∫[𝐴] 𝑑[𝐴] = - ∫ 𝑡𝑑𝑡 0 𝑜 PRANOTI PATIL [A]t – [A]o = -k.t kt = [A]o – [A]t……………………………………….(for zero order reaction) Half life of zero order Reaction So, substitute kt = [A]0 – [A]t [𝐴] [𝐴]𝑜 = [𝐴] − [𝐴] 𝑡 2 𝑜 t= 𝑡 𝑘 t–t 1/2 [𝐴] −[𝐴] 𝑜 𝑡 2 [𝐴]− [𝐴] [𝐴] t1/2= 2 = 𝑜 𝑜 = 𝑜 𝑘 2 𝑘 2𝑘 [𝐴] 𝑜 so t = 1/2 ………….(for zero order reaction) 2𝑘 Examples of zero order Reaction a) Decomposition of ammonia (NH3) on Platinum metal surface 2NH3(g) → N2(g) + 3H2(g) b) Decomposition of Nitrous oxide in the presence of Platinum(Pt) catalyst 2N2O(g) → 2N2(g) + O2(g) PRANOTI PATIL Graphical representation of zero order reaction:- i) Graph of [A]t versus time(t) [A]t Pseudo-first order reaction The reaction which are expected to be of higher order, but follow first order kinetics, are called as pseudo first order reaction. Consider a reaction. CH3COOCH3 (aq) + H2O(l) → CH3COOH(aq) + CH3OH(aq) methyl acetate Rate Law Rate α [Reactant] = k [Reactant] Rate = k [CH3COOCH3][H2O] As here solvent water is present in large excess, so there is negligible change (almost no change) in the concentration of water(H2O). Rate = k[CH3COOCH3] [H2O] = constant Hence, pseudo first order reaction is of first order. PRANOTI PATIL Units General formula of calculation unit Unit = ( 𝑚𝑜𝑙 )1-n sec-1 𝑙𝑖𝑡𝑟𝑒 Where n = order of reaction a) Zero order Reaction (n=o) Units of zero order reaction = ( 𝑚𝑜𝑙 )1-0sec-1 𝑙𝑖𝑡𝑟𝑒 = ( 𝑚𝑜𝑙 )1sec-1 𝑙𝑖𝑡𝑟𝑒 b) First order Reaction (n=1) Unit of first order reaction = ( 𝑚𝑜𝑙 )1-1sec-1 𝑙𝑖𝑡𝑟𝑒 = ( 𝑚𝑜𝑙 )0sec-1 𝑙𝑖𝑡𝑟𝑒 = sec-1 c) Second order Reaction (n=2) Unit of second order reaction = ( 𝑚𝑜𝑙 )1-2sec-1 𝑙𝑖𝑡𝑟𝑒 = ( 𝑚𝑜𝑙 )-1sec-1 𝑙𝑖𝑡𝑟𝑒 = mol-1 litre1 sec-1 PRANOTI PATIL d) Third order reaction (n=3) Unit of third order reaction = ( 𝑚𝑜𝑙 )1-3sec-1 𝑙𝑖𝑡𝑟𝑒 = ( 𝑚𝑜𝑙 )-2sec-1 𝑙𝑖𝑡𝑟𝑒 = mol-2 litre2 sec-1 Collision theory of bimolecular reactions i) Collision between reactant molecules 1) Chemical reaction occurs as a result of collision between the reactant species. 2) So, Rate of reaction = Rate of collision ii) Activation Energy:- (Ea):- The minimum kinetic energy, that the colliding reactant molecules must possess (have) for the reaction to occur, is called as Activation Energy It is represented by(Ea). iii) Orientation of reactant molecules The colliding reactant molecules must have proper orientations, for the reaction to be successful or to occur. PRANOTI PATIL Consider a reaction A + BC → A – B + C 1) 1st way O + O-O → O + O-O A C B A C B (no reaction, as the reactant molecules are not properly oriented) 2) 2nd way O + O-O → O-O + O A BC AB C (Reaction occurs, as the reactant molecules are properly oriented) Activated Complex The configuration in which, all the three atoms are weakly connected together, is called as Activated Complex. A + B + C → A-----B-----C → A – B + C Reactant Activated product Complex PRANOTI PATIL Graph representing potential energy barrier Arrhenius Equation:- Where, k = Rate constant A = frequency factor or pre-exponential factor Ea = Activation energy R = Gas constant T = Temperature PRANOTI PATIL Graphical determination of Activation Energy ……..(1) Taking “In” on both the sides of eqn(1) We get In k = ln A - 𝐸𝑎 𝑅𝑇 also, In k = - 𝐸𝑎 1 + In A 𝑅 𝑇 2.303 x log10k = -𝐸𝑎 1 + 2.303 x log10A 𝑅 𝑇 So log10 k = −𝐸𝑎 1 + log10A 2.303𝑅 𝑇 y m x + c y −𝐸𝑎 Slope = 2.303𝑅 Log10K log10 A x 1/T PRANOTI PATIL Determination of activation Energy(Ea) 𝑘2 𝐸𝑎 𝑇2−𝑇1 log10( )= 2.303𝑅 [ ] 𝑘1 𝑇1 𝑇2 Graphical description of effect of temperature 1) The average kinetic energy is directly proportional to the temperature. At a given temperature, the fraction of molecules having the kinetic Energy equal to or greater than Ea (activation energy), collides and leads to the formation of product. ▪ The total area under the curve at T1& T2 is same. ▪ With the increase in temperature, the fraction of molecules processing energy larger than Ea increases with increase in temperature, so rate of reaction increases with increase in temperature PRANOTI PATIL Effect of catalyst on the rate of reaction catalyst The substance which increases the rate of reaction, without itself being consumed in the reaction, is called as Catalyst Some important points a) The barrier for uncatalysed reaction(Ea) is larger than of the catalysed reaction (Ea”) b) A catalyst lowers the threshold energy. c) The rate of catalysed reaction is larger than that of the uncatalysed reaction Graphical representation of potential energy barriers for catalyst and uncatalyst reaction PRANOTI PATIL i) Graphical representation showing the comparison of fraction of molecules for the catalyst and uncatalyst reaction `THE END