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Topic 16 Kinetics. Rate expressions Reaction mechanism Activation energy. A + B → C + D. 16.1 Rate expression. Change in concentration usually affects the rate of reaction The change in rate isn’t the same for all reactants (A and B)
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Topic 16 Kinetics • Rate expressions • Reaction mechanism • Activation energy A + B → C + D
16.1 Rate expression • Change in concentration usually affects the rate of reaction • The change in rate isn’t the same for all reactants (A and B) • Must be determined by experiment. (Change the concentrations of one reactant and hold the others constant) A + B → C + D rate =k*[A]a*[B]b
If a reaction involves the reactants A, B etc => The Rate expression Rate of reaction = - d[A]/dt = k[A]a[B]b k = rate constant • Order of reaction:“a” in substance A and “b” in substance B • Overall order of reaction:a+b
Reactants A, B and C in four experiments with altering concentrations:A + B + C => ……
Compare Experiment 1 and 2: [A]: 2x [B] and [C] : constant • 2X rate => [A]1 The reaction is first order in [A]
Compare Experiment 1 and 3: [A]: constant [B]: ½ [C] : constant • same rate => [B]0 The reaction is zero order in [B]
Compare Experiment 2 and 4: [A]: constant [B]: constant [C] : 3X • 9X=> [C]2 The reaction is second order in [C]
Conclusion • Rate = k*[A]1’*[B]0*[C]2= k*[A]1*[C]2 • Overall order 1+2 = 3 • k can be calculated using the data from one of the experiments above
Exercises • 1-2 on page 120
The order can also be found in a graph where initial concentration is set against initial rate.The gradient of the graph => rate of the reaction.
First order reactionsThey show an exponential decrease:the time to half the concentration is equal to go from ½ to 1/4Half life, t½ = 0.693/k
16.2 Reaction mechanism Types of reactions: Molecularity • A Products Unimolecular • A + B Products Bimolecular • In a Bimolecular reaction the reactants collide and form an activated complex
2 molecules Nucleophilic Substitution bimolecular, SN2- topic 10
If we study the reaction: CH3COCH3+ I2CH3COCH2I + HI H+ • It could be a bimolecular process with a rate expression rate = k*[CH3COCH3] *[I2] • The rate is independent of [I2], but first-order in propanone and acid => rate = k*[CH3COCH3]*[H+] • The reaction must proceed through a series of steps, a mechanism must be found:
Intermediate CH3C(OH+)CH3 is known as a intermediate, not an activated complex, though it occur at an energy minimum. In the mechanism there will be several activated complexes
Exercises • 1 and 2 on page 122
16.3 Activation energy Recall: Maxwell-Boltzmann energy distribution curve. Temperature Average speed Higher temperature =>More particles with higher speed => Greater proportion of particles with energy enough to react
The Arrhenius equation • The rate constant, k, can be given if collision rate and orientation is given • Ea = activation energy • T = temperature, K • R = Gas constant