CHEMICAL KINETICS

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1 CHEMICAL KINETICS Long Answer Questions: 1. Explin the following terms with suitble exmples ) Averge rte of Rection b) Slow nd Fst Rections c) Order of Rection d) Moleculrity of Rection e) Activtion Energy of Rection Ans. ) Averge Rte of Rection: Decrese in concentrtion of rectnts (or) increse in concentrtion of products per unit time is clled verge rte of rection. 1) HI( g ) H ( g) + I( g) Δ[ HI] Δ[ H] Δ[ I] Rte = =+ =+ Δt Δt Δt ) Hg() l + Cl( g) HgCl() s Δ[ Hg] Δ[ Cl] Δ[ HgCl] rte = = =+ Δt Δt Δt b) Slow nd fst rections: Bsed on rte of chemicl rections, rections re clssified in to i. Very fst Rections: occurs instntneously E.g.: All precipittion nd neutrliztion rections ii. Very slow Rections: E.g.: Rusting of Iron iii. Moderte Rections: E.g.: Inversion of cne sugr c) Order of Rection: The sum of powers of concentrtion terms of the rectnts in the rte lw expression is clled the order of tht chemicl rection. Order of rection cn be 0, 1,, 3 nd even frction. x y In the rte eqution rte = K[ A] [ B], order of rection is x + y Generlly the following methods re used to determine the order of rection experimentlly 1) Integrted eqution method or tril nd error method ) Hlf-Life method 3) Vnt s-hoff differentil method 4) Ostwld isoltion method Exmple: H + I HI ( g) ( g) ( g) Rte= K[ H][ I ], order =

2 d) Moleculrity of Rection: The number of recting species (toms, ions or molecules) tking prt in n elementry rection, which must collide simultneously in order to form products is clled moleculrity of the rection. Moleculrity cnnot be zero or non integer nd does not exceed three. E.g.; unimoleculr rection NH4NO N + HO Bimoleculr Rection: HI( g) H( g) + I( g) Termoleculr Rection: NO( g ) + O( g) NO( g) e) Activtion Energy of Rection: The difference between threshold energy nd energy of norml recting molecules is clled ctivtion energy. Activtion energy ( E ) = Threshold energy ( ET )-Energy of norml molecule E p =Energy of products, E R = Energy of rectnts, E =ctivtion energy E T =threshold energy E = ET ER *. Give detiled ccount of the collision theory of rection rtes in bimoleculr gseous rections? Ans. According to this theory 1. Rectnt molecules re ssumed to be hrd spheres nd recting molecules must collide with ech other for ny rection to occur.. The number of collisions per second per unit volume of the rection mixture is known s collision frequency (Z) 3. For bimoleculr elementry rection A + B products, Rte of rection cn be expressed s Rte Z e E/ = AB Where AB Z collision frequency of rectnts A nd B

3 E/ e = Frction of molecules with energies equl to or greter thn E 4. The minimum energy required by the colliding molecules to give products is clled threshold energy 5. The molecules possessing the threshold energy re clled ctivted molecules 6. Collisions occurring between ctivted molecules re clled ctivted collisions (or) effective collisions 7. To ccount for effective collisions, nother fctor P clled the probbility (or) steric fctor is introduced then rte = PZ e AB E/ In collision theory, ctivtion energy nd proper orienttion of the molecules together determine the criteri for effective collisions nd hence the rte of rection for exmple, formtion of CH3OH from CH3Br depends upon orienttion of rectnt molecules. The proper orienttion of rectnt molecules led to bond formtion where s improper orienttion mkes them simply bounce bck nd no products re formed H HO C H + Br H *3. Define order of rection. Give n exmple to first nd second order rection. How is it obtined experimentlly? Ans. The sum of powers of the concentrtion terms of the rectnts in the rte lw expression is clled the order of tht chemicl rection. Order of rection my be whole number, negtive or frction. E.g.: For the rection xa + yb products x y Rte = K[ A] [ B] order of rection = x + y

4 First order: Rection in which rte of the rection is directly proportionl to the single concentrtion term of the recting substnce is clled first order rection. E.g.: Therml decomposition of nitrogen pentoxide N O 5 (g) N O 4 (g) + O (g), Rte = k [N O 5 ] Decomposition of queous hydrogen peroxide H O (q) H O (l) + O (g), Rte = k [H O ] Second order: Rection in which the rte depends on two concentrtion terms is clled second order rection. E.g.: Decomposition of chlorine monoxide i.e. Cl O (g) Cl (g) + O (g), Rte = k [Cl O] Therml decomposition of nitrous oxide, N O (g) N (g) + O (g), Rte = k [N O] Following methods re used to determine the order of rection experimentlly. 1) Integrted eqution method or tril nd error method ) Hlf -life method 3) Vn t Hoff differentil method 4) Ostwld isoltion method Hlf-life method The time required for the initil concentrtion () of the rectnts to become hlf of its vlue (/) during the progress of the rection is clled hlf life ( t 1/) of the rection The hlf-life ( t 1/) is inversely proportionl to ( n 1) where is initil concentrtion of the rectnt nd n is the order of the rection t 1 1/ n 1 Sy given rection is strted seprtely with two initil concentrtions 1, nd. The hlf - time vlues (t1/)1 nd (t1/) re determined experimentlly nd the order is estblished from the eqution. ( t ) 1/ 1 ( t ) = 1/ 1 n 1

5 Short Answer Questions *1. Wht is moleculrity of rection? How is different from the order of rection? Nme one bimoleculr nd one trimoleculr gseous rection? Ans. The number of recting species (toms, molecules or ions) tking prt in n elementry rection, which must collide simultneously in order to bring bout chemicl rection is clled moleculrity. Order 1. The sum of the powers of the concentrtion terms of the rectnts in rte eqution is clled order of rection Moleculrity 1. The number of toms or ions or molecules tking prt in n elementry rection is clled moleculrity.. Order of rection is n experimentl quntity 3. Order my be whole number/frction/negtive or zero. It is theoreticl quntity nd it cn be clculted by using rection Mechnism 3. It cnnot be zero/non- integer nd does not exceed three 4. Order is pplicble to elementry s well As complex rections 4. It is pplicble only for elementry rections Dissocition of hydrogen iodide in to H nd I is bimoleculr rection. HI H + I Formtion of NO from NO nd O is trimoleculr rection. NO + O NO

6 . Wht is hlf-life ( t1/) of rection? Derive the eqution for the hlf-life vlue of zero nd first order rections? Ans. The hlf life of rection is the time in which the concentrtion of rectnt is reduced to one hlf of its initil concentrtion For zero order rection [ R0 ] [ R] K = t [ R0 ] At t = t1/ [ R] = [ R0 ] [ R0 ] K [ R] 0 = = t K 1/ For the first order rection.303 [ R] K =.log 0 t [ R ] [ R] 0 At t = t1/,[ R] = K = [ R].303.log[ 0 t1/ R ] 0 /.303log = = = t t t 1/ 1/ 1/ k = t 1/ *3. Mention the fctors tht ffect the rte of chemicl rection. Discus the effect of ctlyst on the kinetics of chemicl rection with suitble digrm? Ans. Following fctors ffect the rte of chemicl rection. () Nture of rectnts: Ionic substnces rect more rpidly thn covlent compounds becuse ions produced fter dissocition is immeditely vilble for rection. (b) Concentrtion of rectnts: Rte of rection increses when concentrtion of rectnts is incresed. (c) Temperture: Generlly rte of rection increses on incresing the temperture.

7 (d) Surfce re of the rectnts: Rte of rection increses with increse in surfce re of the rectnts. Tht is why powdered form of rectnts is preferred thn their grnulr form. (e) Ctlyst: Presence of ctlyst lso effects the rte of rection. It decreses the ctivtion energy by forming rection intermedite nd hence lowers the potentil energy brrier. So rte of rection increses. 4. Wht is Arrhenius eqution? Derive n eqution which describes the effect of rise of temperture (T) on the rte constnt (k) of rection? Ans. Arrhenius rte eqution K = Ae E/ Where A is the Arrhenius fctor (or) frequency fctor (or) pre-exponentil fctor R is the gs constnt ' E ' is ctivtion energy Tking nturl logrithm on both sides of eqution E ln k = + ln A E At temperture T 1,lnk 1 = + ln A (1) 1 E And t temperture T,lnk = + ln A () E E Eqution () (1) ln k ln k1 = 1 K E 1 1 K E 1 1 ln = log = K1 R T1 T K1.303R T1 T Where K 1 nd K re the vlues of rte constnts t temperture T 1 nd T respectively

8 5. Derive the integrted rte eqution for first order rection? Ans. For first order rection, rte of the rection is proportionl to the concentrtion of the rectnt R For exmple R P dr [ ] Rte = = KR [ ] dt dr [ ] = = Kdt [ R] Integrting this eqution, we get ln[ R] = Kt+ I (1) Where, I is the constnt of integrtion When t = 0, [ R] = [ R] 0 From (1) ln[ R] 0 = K(0) + I I = ln[ R] 0 Substitute I in eqution (1) Then ln[ R] = Kt + ln[ R0 ] 1 ln[ R] K = 0 (ln =.303log) t [ R ].303 [ R] K = log 0 t [ R Or ] K.303 = log t x Where = initil concentrtion of rectnt X = concentrtion of rectnt consumed This is integrted from of first order rte eqution

9 Very Short Answer Questions 1. Wht is the effect of temperture on the rte constnt of rection? How cn this temperture effect on rte constnt be represented quntittively? Ans. The rte constnt of rection increses with increse in temperture nd becomes nerly double for every 10 o rise in temperture. The effect cn be represented quntittively by Arrhenius eqution. k = Ae E /. Wht is pseudo first order rection or pseudo unimoleculr rection? Ans. A First order rection with moleculrity two is clled pseudo first order rections or pseudo unimoleculr rection. Ex; Acid ctlysed hydrolysis of ethyl cette is pseudo first order rection, becuse wter is tken lrge in excess nd hence ethyl cette is sid to be isolted. CH3COOCH5 + HO H + CH3COOH + CH5OH Rte of the rection, rte = k [CH3COOCH5] Its moleculrity is but the order of rection is Wht is zero order rection? Give n exmple? Ans. The Rection in which rte of the rection is independent of the concentrtion of the recting substnces is clled zero order rection. E.g. Decomposition of mmoni gs on hot pltinum surfce NH 3 (g) N (g) + 3H (g), Rte = k [NH 3 ] o 4. A rection hs hlf life of 10 minutes. Clculte the rte constnt for the first order rection? Ans. K = = = min t 10 1/ 1 5. In first order rections, the concentrtion of the rectnt is reduced from 0.6 mol/l to 0. mol/l in 5min. Clculte the rte constnt?

10 Ans K = log = log = log 3 = = min t x The hlf life for first order rection is Ans sec will rect in hours? K = log = log 6 t x x On solving x = % wht percentge of the initil rectnt 1 7. Show tht in the cse of first order rection, the time required for 99.9% completion of the rection is 10 times tht required for 50% completion? (log = ).303 Ans. We know, K = log 10...( i ) t x k =, = 100, x = 99.9 t 1/ From Eqution (i) we get = log t t / = log10 t t 1/ = t t 1/ t = 10 t 1/ 3 8. For rection, A + B product, the rte lw is given by of the rection? r ka B 1/ = [ ] [ ]. Wht is the order Ans. Rte lw for the rection is r ka B 1/ = [ ] [ ] Order of the rection 1 5 = + = 9. A rection is 50% completed in hours nd 75% completed in 4hours. Wht is the order of the rection?

11 Ans. First order rection becuse t 1/ is independent of initil concentrtion.

4. CHEMICAL KINETICS

4. CHEMICAL KINETICS Synopsis: The study of rtes of chemicl rections mechnisms nd fctors ffecting rtes of rections is clled chemicl kinetics. Spontneous chemicl rection mens, the rection which occurs on