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Kinetics VI
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First order reaction

## Shaillee Kaushal is teaching live on Unacademy Plus

Shaillee Kaushal
Faculty in Chemistry with a teaching experience of 14 years. Specialised in teaching for boards and competitions.

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1. XII-04 KINETICs - V

2. Initial rate method to calculate order of reaction:- The graphical methods cannot be applied for the reactions which involve more than one reactants. The rates of such reactions can be determined by initial rate method.This method involves the following steps. I. The rate at the beginning of the reactions is measured. This may be taken as the rate over an initial time interval that is short enough so that concentrations of the reactants do not change appreciably from their initial values. 2. The initial concentration of only one reactant is changed and the rate is determined again. From this the order with respect to that particular reactant is calculated. 3. The ssame procedure is repeated with respect to each reactant untill overall rate law is determined 4. The sum of individual orders with respect to each reactant gives the order of reaction.

3. Example: I he experimental data for the reaction Is as follows Exp. No. [A] mol L [B2lmol L Rate mol L-s- 0.50 0.50 1.00 0.50 1.00 1.00 1.6 10-4 3.2 x 10-4 3.2 x 104 2. From an examination of the above data it is clear that when the concentration of B2 is doubled the rate is doubled. Hence the order of reaction with respect to B2 is one Further when concentration of A is double, the rate remains unchanged. So order of reaction with respect to 'A' is zero . the probable rate law for the reaction will be Rate k[A] [B211

4. Reaction mechanism: The series of steps (known as elementary process) leading to the formation of products is called The mechanism of a reaction predicts the order of the reaction which is formulated by the slow step of The following reaction takes place in two steps: reaction mechanism. the reaction. 2NO Br2NOBr k' (i) NO Br2- NOBr2. fast NOBr2 +NO 2NOBr slow Rate = RINOBT2] [NO] As NOBr2 is an intermediate and its concentration at the beginning of the second step cannot be directly measured Conc. of NOBr2 [NOBT2] = k[NO] [Br2] By substituting this in the above reaction Rate = kk' [NO]2[Br] - k[NO]2Br2l

5. I.Zero order reactions: > A reaction is said to be of zero order if its rate is independent of the concentration of the reactants, i.e the rate is proportional to the zeroth power of the concentration of the reactants. AProducts Rate k[A ] dx = k dt x = kt k Some photochemical reactions and a few heterogenous reactions are zero order reaction

6. Examples: hv () H2s) + Cl2 2HCI(B) (ii) Decomposition of H20 on hot platinum surface: 2(g) v20 2 Rate x [N20] (iii) Decomposition of NH3 in presence of molybednum or tungsten is a zero order reaction [Mo] 2NH3N2 3H2 Rate k[NH3]

7. Characteristics of zero order reaction: (a) The concentration of reactant decreases linearly with time [A]t [A],-kt (b) The time required for the reaction to be complete i.e time at which [A] is zero composition [Ao] (c) The units of k are mol Ltime1

8. (ii) First order reaction: Consider a reaction: A-Products Ratedt Rate k[A] -d[A] dJA k[A] dt [AJ Integrating on both side In[A] kt I (i) At time t 0 let concentration be [Ao] -In[AolI Substituting the value of I in equatio - In[A] kt Ino[Ao] In[Ao] - In[A] kt Converting in to log 2.303 log (Aolkt

9. If 'a' is the concentration of A at the start and after time t the concentration becomes (a - x).The rate of reaction is given by kt 2.303 log This is known as the kinetic equation for a reaction of first order. The following two conclusions are drawn from this equation: A change in concentration unit will not change the numerical value k. The time taken for the completion of same fraction of change is independent of initial concentration a-X

10. For example for half reaction: x = 0.5a and t=t1/2 2.303 log 0.5a 2.303 0.5a t1/2 2.303 !og2 t1/2 0.693 t1/2 0.693 2 t1/2 is the half life period which is independent of initial concentration of a

11. Examples of first order: (i) H202-H20 + 02 (i) CH3COCH3 + H20CH3COOH CH3OH (iii) NH4NO2 N2 + 2H2O In case of gases pressure can be used in place of concentration