Reaction Kinetics Review Unit 7 Chapter 13

Kinetic Molecular Theory Molecules are in constant, random, straight line motion Molecules are in constant, random, straight line motion Collisions between molecules are elastic Collisions between molecules are elastic KE ave = 3/2 kT = 3/2 RT = 1/2 mv 2 KE ave = 3/2 kT = 3/2 RT = 1/2 mv 2 Molecular volume of an ideal gas is negligible Molecular volume of an ideal gas is negligible IMF’s in an ideal gas are negligible IMF’s in an ideal gas are negligible Last two assumptions are true if molecules are widely spaced which occurs at high T and low P Last two assumptions are true if molecules are widely spaced which occurs at high T and low P

Collision Theory Reaction rate depends on success rate of collisions Reaction rate depends on success rate of collisions In order to react, molecules must collide with In order to react, molecules must collide with Sufficient KE Sufficient KE Proper geometry (correct pieces, head-on) Proper geometry (correct pieces, head-on) KE of collision breaks the old bonds KE of collision breaks the old bonds Anything that effects the energy or number of collisions will affect rate Anything that effects the energy or number of collisions will affect rate

Factors Affecting Reaction Rate Nature of reactants Nature of reactants Bond strength determines E act Bond strength determines E act Temp Temp Affects both energy and number of collisions Affects both energy and number of collisions [ ]/P/V of reactants [ ]/P/V of reactants Affects number of collisions Affects number of collisions Surface area Surface area Affects number of collisions Affects number of collisions Catalyst Catalyst Creates alternate activated complex with lower E act Creates alternate activated complex with lower E act

Transition Theory Concerned with details of the collision Concerned with details of the collision Potential energy diagram simulates this (next slide) Potential energy diagram simulates this (next slide) KE of collision overcomes PE of repulsion KE of collision overcomes PE of repulsion Each collision is a step in the reaction mechanism Each collision is a step in the reaction mechanism Intermediates are produced in one step and consumed in a later step of the mechanism Intermediates are produced in one step and consumed in a later step of the mechanism Catalysts are consumed in one step and reproduced in a later step of the mechanism Catalysts are consumed in one step and reproduced in a later step of the mechanism

Potential-Energy Curve Copyright © Houghton Mifflin Company. All rights reserved 13-13

Potential Energy Curve w/Catalyst

Rate Definition Rate = -∆[reactant]/∆t or +∆[product]/∆t Rate = -∆[reactant]/∆t or +∆[product]/∆t Can be written in terms of any reactant/product Can be written in terms of any reactant/product To express equally, swap coefficients To express equally, swap coefficients Units are always M/time Units are always M/time Since rate is not constant, we calculate ave. rate over an interval Since rate is not constant, we calculate ave. rate over an interval

Rate Laws Rate = k [reactant 1] x [reactant 2] y … Rate = k [reactant 1] x [reactant 2] y … k = rate constant = M z /time k = rate constant = M z /time Depends on T, catalyst, surface area, nature Depends on T, catalyst, surface area, nature T effect calculated with Arrhenius’ Eq’n T effect calculated with Arrhenius’ Eq’n x,y are the orders x,y are the orders NOT the coefficients in the net equation NOT the coefficients in the net equation DO equal the coefficients in the RDS of the mechanism DO equal the coefficients in the RDS of the mechanism Rate decreases as [reactants] decreases Rate decreases as [reactants] decreases Details of the rate law are determined Details of the rate law are determined With the method of initial rates With the method of initial rates Graphical analysis with the integrated rate laws Graphical analysis with the integrated rate laws

Method of Initial Rates Run reaction several times changing [initial]’s Run reaction several times changing [initial]’s Measure initial rate that results from new [ ] Measure initial rate that results from new [ ] Determine order by inspection or Determine order by inspection or Mathematically Mathematically Write rate law twice, using data from two trials Write rate law twice, using data from two trials Divide one rate law by the other, cancel and solve Divide one rate law by the other, cancel and solve Once orders are determined, find k by substituting data from any trial Once orders are determined, find k by substituting data from any trial

Integrated Rate Laws Relate [ ] to time Relate [ ] to time 0 th A t - A 0 = - kt 0 th A t - A 0 = - kt 1 st ln A t - ln A 0 = - ktt 1/2 =.693/k 1 st ln A t - ln A 0 = - ktt 1/2 =.693/k 2 nd 1/A t - 1/A 0 = kt 2 nd 1/A t - 1/A 0 = kt Graphical analysis Graphical analysis Collect [ ] vs. time data Collect [ ] vs. time data Graph 3 ways; one that gives straight line is order Graph 3 ways; one that gives straight line is order Slope of straight line = |k| Slope of straight line = |k| Used to determine [ ] at future time, vice-versa Used to determine [ ] at future time, vice-versa

Mechanisms Mechanism shows the sequence of individual steps/collisions that occur in the reaction Mechanism shows the sequence of individual steps/collisions that occur in the reaction Steps in mechanism must add to give net eq’n Steps in mechanism must add to give net eq’n Mechanism must yield the correct rate law Mechanism must yield the correct rate law Use RDS to determine reactants and orders Use RDS to determine reactants and orders Use K eq to find substitute for intermeds/catalysts Use K eq to find substitute for intermeds/catalysts

Mechanism of the Decomposition of N 2 O 5, Copyright © Houghton Mifflin Company. All rights reserved 13-16AB

Miscellaneous Hump diagrams Hump diagrams PE curves PE curves Illustrate effect of catalyst; E act and ∆H’s Illustrate effect of catalyst; E act and ∆H’s Maxwell-Boltzman distribution of molecular energies Maxwell-Boltzman distribution of molecular energies Illustrate effect of ∆T and of catalyst Illustrate effect of ∆T and of catalyst Arrhenius Equation (k as function of t) Arrhenius Equation (k as function of t) ln (k 1 /k 2 ) = - E act /(RT) (1/T 1 - 1/T 2 ) ln (k 1 /k 2 ) = - E act /(RT) (1/T 1 - 1/T 2 ) On formula sheet? On formula sheet?

Maxwell-Boltzmann Distribution of Molecular Energies