Xuan Cheng Xiamen University Physical Chemistry Reaction Kinetics (2) Xuan Cheng Xiamen University

Simple Order Reactions Physical Chemistry Reaction Kinetics Simple Order Reactions Order Reaction Rate Law t1/2 A  P 1 2 n2

Typical Complex Reactions Physical Chemistry Reaction Kinetics Typical Complex Reactions Reversible reaction Opposing reaction kb (1) Consecutive reaction (2) Competing reaction Parallel reaction (3)

Reversible First-Order Reactions Physical Chemistry Reaction Kinetics Reversible First-Order Reactions (17.1) Consider the reverse reaction (opposing reaction) kb the first order in both forward and back directions

Reversible First-Order Reactions Physical Chemistry Reaction Kinetics Reversible First-Order Reactions (17.31) (17.32)

Reversible First-Order Reactions Physical Chemistry Reaction Kinetics Reversible First-Order Reactions (17.32) At equilibrium, and (17.33) (17.34)

Reversible First-Order Reactions Physical Chemistry Reaction Kinetics Reversible First-Order Reactions (17.34) (17.14)* [A]/[A]o [C]/[A]o kf / kb=2 A  C jt Fig. 17.2 When [A]eq=0, kb=0 [A]/[A]o t Fig. 17.1(a)

Consecutive First-Order Reactions Physical Chemistry Reaction Kinetics Consecutive First-Order Reactions Consider two consecutive irreversible first-order reactions (17.35) (17.36)

Consecutive First-Order Reactions Physical Chemistry Reaction Kinetics Consecutive First-Order Reactions Let only A be present in the system at t = 0 (17.37) (17.36) (17.11) (17.38) (17.39) Integration (17.40) ?

Consecutive First-Order Reactions Physical Chemistry Reaction Kinetics Consecutive First-Order Reactions At all times (conservation of matter) (17.38) (17.40) (17.41)

Consecutive First-Order Reactions Physical Chemistry Reaction Kinetics Consecutive First-Order Reactions The intermediate’s concentration rises to a maximum, and then falls to zero (the position of maximum: k1/k2) ? The concentration of the product C rises from zero and reaches [A]o

Consecutive First-Order Reactions Physical Chemistry Reaction Kinetics Consecutive First-Order Reactions Suppose that in an industrial batch process a substance A produces the desired product B which goes to decay to a worthless product C, each stage of the reaction being first-order. At what time will product B be present in greatest concentration? The time dependence of [B] (17.40) A maximum should occur at

Consecutive First-Order Reactions Physical Chemistry Reaction Kinetics Consecutive First-Order Reactions Since [A]o  0, k1  0, so The maximum concentration of B ? For a given value of k1 > k2, as k2 increases, both the time at which [B] is a maximum and the yield of B increase.

Competing First-Order Reactions Physical Chemistry Reaction Kinetics Competing First-Order Reactions Consider two competing irreversible first-order reactions (17.42) A C D k1 k2 Parallel reactions (17.43)

Competing First-Order Reactions Physical Chemistry Reaction Kinetics Competing First-Order Reactions (17.43) Compare with (17.11) (17.14)* For C (17.44) ?

Competing First-Order Reactions Physical Chemistry Reaction Kinetics Competing First-Order Reactions (17.44) similarly (17.45) Division of (17.44) and (17.45) At any time during the reaction (17.46)

Competing First-Order Reactions Physical Chemistry Reaction Kinetics Competing First-Order Reactions (17.42) Consider the reverse reactions (17.47) Moreover At equilibrium Thermodynamic control When any reverse reactions or interconversion of C and D can be neglected (17.46) Kinetic control

Homework Supplemental Material Page 591 Page 145, Prob. 16 Prob. 17.13 Physical Chemistry Reaction Kinetics Homework Supplemental Material Page 591 Page 145, Prob. 16 Prob. 17.13 Page 149, Prob. 30 Page 158, Prob. 68