VIEWS: 3 PAGES: 19 POSTED ON: 12/16/2011
Reaction mechanism and reaction order • Chemical reaction: single or sum of many steps =>reaction mechanism • A step: (elementary reaction) – one, two or three reacting molecules => molecularity – unimolecular: isomerization, decomposition – bimolecular: energy transfer, combination – termolecular: energy transfer + combination • Reaction order vs. molecularity – Reaction takes place in an elementary reaction • unimolecular: first order • bimolecular: second order • termolecular: third order – Is a first order reaction is always an unimolecular reaction? Consecutive reaction processes • RX + H2O = ROH + H+ + X- – Unaffected by the PH of solution, SN1 reaction – RX R+ + X- – R+ + H2O ROH + H+ • RX + OH- = ROH + X- – Alkaline solution – RX R+ + X- – R+ + OH- ROH • What happen for 2nd order reaction? Consecutive reaction processes • A B <k1>, B C <k2> – -d[A]/dt = k1 [A]; -d(a-x)/dt = k1 (a-x) – d[B]/dt = k1(a-x) - k2[B] = k1 a e-k1t - k2[B] – d[B]/dt + k2[B] = k1 a e-k1t [ B] ak1 k 2 k1 e k1t e k2t , k1 k 2 , [ B] 0..t 0 Plot of concentrations of A, B, C in consecutive process Consecuti 1 0.8 0.6 [A] 1/a [B] 0.4 [C] 0.2 0 0 20 40 60 time Maximum [B] d [ B] dt ak1 k 2 k1 k 2 e k2t k1e k1t 0 1 k1 tm ln k1 k 2 k 2 Formation of an intermediate complex • A + B X (1) X A + B(2) X C + D(3) • Principles of Stationary state – 1913 Chapman: the net rate of formation of a reaction intermediate may be put equal to zero. – X so reactive, [X] is low d[ X ] k1[ A][ B] k 1[ X ] k 2 [ X ] 0 dt k k [ A][ B] d [C ] / dt k 2 [ X ] 1 2 k 1 k 2 Formation of an intermediate complex • A + B X .. k1; X A + B … k-1 • X + C P + Q … k2 d[ X ] k1[ A][ B ] k 1[ X ] k 2 [ X ][C ] dt d [ P] k1k 2 [ A][ B][C ] k 2 [ X ][C ] dt k 1 k 2 [C ] Two limiting cases: k-1 > k2[C], k-1 < k2[C], Third body effect in atomic recombination • Two atoms molecule + lot of vibrational energy – short life time production: needs a collision with other molecular species to shed some excessive energy • X + X + M X2 + M – before first vibration (10-13 s) effective collision – effective molecule=highly negative activation energy • higher temperature: lower reaction rate – efficiency is different depend on molecule • large molecule Porter’s interpretation(1961) • I + M = I.M (K1); I.M + I I2 + M(k2) – I.M: spectroscopically identified for benzene – [I.M]=K1[I][M] – d[I2]/dt = k2[I][I.M] = K1k2[I]2[M] • RT ln K = -G = -(H-TS) • k = A exp[-Ea/RT] d ln K1k 2 d ln K1 d ln k 2 Ea R R R d (1 / T ) d (1 / T ) d (1 / T ) H1 E2 Parallel reaction • AB (1) AC (2) – -d[A]/dt = (k1 + k2)[A] – (a-x)=a exp[-(k1+k2)t] – k1/k2 = Yield of B/Yield of C B • AC (1), A+BD (2) – -d[A]/dt = (k1 + k2[B])[A] A • AB (1), A+AC (2) C Reactant participating in equilibria • H2 + I2 = 2HI – Bodenstein 1899 – first order with respect to each reactants – Bimolecular process, elementary reaction, four center transition state – Sullivan 1967: 0-200C ; non elementary reaction – I2 + h I2*, I2* 2 I 2 I + H2 2 HI A probable mechanism for H2 + I2 • I2 = I + I … (1) • I + I+ H2 HI + HI … (2) • K1 = [I]2/[I2] [I]=K11/2[I2]1/2 • d[HI]/dt = 2k2[H2][I]2 = 2k2K1[H2][I2] – Ea(k2) = 21 kJ/mole – Ea(k2) + Bond dissociation energy of I2 == Overall activation energy of Bodenstein thermal experiment. A probable mechanism for H2 + I2 • I2 = I + I … (1) • I + H2 = IH2 … (2) • I + IH2 HI + HI … (3) • K1 = [I]2/[I2] [I]=K11/2[I2]1/2 • K2 = [IH2]/[I][H2] [IH2]=K2 [I][H2] • d[HI]/dt = 2k3[IH2][I] = 2k3K1K2[H2][I2] Hydrogen-halogen kinetics • H2-F2, H2-Br2, H2-Cl2 – complicated experimental rate law – interpreted in terms of free radical chain reaction • H2-I2 – simple kinetic behavior – the least understood system Reactant participating in equilibria • nA=An – [A]=Kn-1/n [An]1/n = Kn-1/n [c/n]1/n – [A]: a part of aggregate, – [aggregate] [normal concentration of A]/n – Butyl lithium initiator of anionic polymerization, rate initiator 1/6 – butyl lithium associate to hexamer Reactant participating in equilibria • A = B- + H+ (Ka) – [B-]=Ka[A]/[H+]; [A] = [H+] [B-]/ Ka.; – [A] + [B-] = [A]{([H+] + Ka)/[H+]} – = [B]{(Ka + [H+])/Ka} => a – [B-] = a·Ka/(Ka + [H+]) – [A] = a·[H+]/(Ka + [H+]) – We can formulate rate equation by [A] & [B-] – PH dependent Opposing reaction • AB (k1) B A (k-1) – dx/dt = k1(a-x) -k-1 x = k1a - k1x - k-1 x – final equilibrium. – k1(a-xe) = k-1 xe k1a = (k1 + k-1)xe – dx/dt = (k1 + k-1)(xe-x) – ln {(xe)/(xe-x)} = (k1+k2)t Isotope exchange reaction • AX + BX* = AX* + BX – Assume K=1 G = -RT ln K = 0 – [AX]t=0 = a; [BX*]t=0 = b; [AX*]t=0 = [BX]t=0 = 0 – At equilibrium – x2/(a-x)(b-x) = 1 x = ab/(a+b) – [AX]= a-x = a2/{a+b}, [BX*]= b-x = b2/{a+b} – [AX*]= ab/{a+b}, [BX]= ab/{a+b} – [AX]t+ [AX*]t = a, [BX]t+ [BX*]t = b dx a x b x x 2 dt a b ab dx a x b x x 2 dt a b ab – = {(a+b)/ab}(xe-x) – Integration with x=0 at t=0 – ln{xe/(xe-x)} = {(a+b)/ab}t – no information on reaction order for • Determined by initial rate with different a and b • = kn apbq – ln{xe/(xe-x)} = kn ap-1bq-1 (a+b)t