Reaction mechanism and reaction orderChemical 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 orderIs a first order reaction is always an unimolecular reaction?Consecutive reaction processesRX + 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 , B C -dA/dt = k1 A; -d(a-x)/dt = k1 (a-x)dB/dt = k1(a-x) - k2B = k1 a e-k1t - k2B dB/dt + k2B = k1 a e-k1t Plot of concentrations of A, B, Cin consecutive processMaximum BFormation 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 lowFormation of an intermediate complex A + B X k1; X A + B k-1 X + C P + Q k2Two limiting cases: k-1 k2C, k-1 a B- = aKa/(Ka + H+) A = aH+/(Ka + H+)We can formulate rate equation by A BX*t=0 = b;AX*t=0 = BXt=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 AXt+ AX*t = a, BXt+ BX*t = b = (a+b)/ab(xe-x) Integration with x=0 at t=0 lnxe/(xe-x) = (a+b)/abt no information on reaction order for Determined by initial rate with different a and b = kn apbqlnxe/(xe-x) = kn ap-1bq-1 (a+b)t