Chemical Kinetics Chapter 16 Kinetics Reaction Rates Factors

Chemical Kinetics Chapter 16

Kinetics Reaction Rates Factors affecting rate Quantitative rate expressions Determination Factors Models for Rates Reaction Mechanisms Effects of catalysts

Rates Change in concentration of a reactant or product per unit time

Factors affecting rates Nature of the reactants State of subdivision/surface area Concentration Temperature Catalysts

Reactants Complexity Bond strengths Etc.

Concentrations as functions of time

Graph: Concentration vs. time

Average Rate Change of concentration in a time interval

Average Rate Slope of line between two points on the graph

Instantaneous rate Slope of tangent line at a point on the graph

Instantaneous Rate 0. 009 M 375 s

Initial Rate (t = 0)

Initial rate Slope of tangent line at time 0 (y intercept)

Rate Laws k = m, n = rate constant order rate = k[NO 2]n

Introduction to Rate Laws Reversible chemical reactions Forward: Backward: Equilibrium:

Introduction Dominant Reaction: Rate Law: k, k’: n: specific rate constant order of reactant can be zero, fractional, or negative

Method of Initial Rates Unknown: k, m, n Initial rate: instantaneous rate just after reaction is initiated

Initial Rates, NO 2 decomposition

Order of Reaction General: Substituting: Solution:

Rate constant Rate 1 7. 1 x 10 -5 M s-1 k Rate 2 2. 8 x 10 -4 M s-1 k = = -k[0. 01 M]2 0. 71 M-1 s-1 = = -k[0. 02 M]2 0. 70 M-1 s-1

You try

O 2 + 2 NO 2 NO 2

Overall Order Sum: 1 = + 6 2 Overall order of reaction: + 6 3

Types Differential: Rate dependence on concentration Integrated: Concentration dependence on time

First Order Reactions For a. A products Differential: Integrated:

Half-life, first order reactions Integrated law: Half-life: Half of initial reacted [A]t = ½[A]0 Independent of [A]0

Second Order Reactions For a. A products Differential: Integrated:

Half-life, second order reactions Integrated law: Half-life: Half of initial reacted [A]t = ½[A]0 Inversely proportional to [A]0

Zero Order Reactions For a. A products Differential: Integrated:

Graphical Method First order Second order Zero order Straight line

First order ln[A]0 slope = -k ln[A] Plot: ln[A] vs. time

Second order slope = k 1 [A] Plot: 1 vs. time [A] 1 [A]o time

Zero order [A]0 slope = -k [A] Plot: [A] vs. time

Summary Conditions set so dominant forward reaction Differential Rate Laws rate as a function of concentration method of initial rates Integrated Rate Laws concentration as a function of time graphical method Experimental data collection Rate law types can be interconverted

Reaction Mechanism Chemical equation: Summary Mechanism: Series of elementary steps Elementary Steps: Reactions with rate laws from molecularity Molecularity: Number of species that must collide to produce reaction

Reaction Mechanism Proposed elementary steps must satisfy conditions: — reasonable reactions — sum of steps = overall balanced reaction — mechanism rate law = experimental rate law

Intermediates appear in steps — produced in one step — used in subsequent — not in overall equation —

Rate-determining step In a multi-step process: SLOWEST step Determines overall reaction rate “Bottleneck”

Model for Kinetics Collision Theory rate determined by particle collisions collision frequency and energy Transition State Theory how reactants convert to products

Collision Theory (Bimolecular Collsions) Z: fa : P: no. of bimolecular collisions per second fraction with Ea fraction with correct orientation Ea: activation energy

Arrhenius Equation k: E a: T: R: A: rate constant activation energy (minimum required) absolute temperature universal gas constant orientation factor Energy & orientation requirements for reaction

Hydrolysis of an ester

Transition State Theory Ea and internal energy: Bonds breaking and forming Atoms rearranging “Transition State” Unstable intermediate At point of highest energy

forward reaction reverse reaction

exothermic reaction

I- + CH 3 Cl Cl- + CH 3 I

Catalysts Speed reaction Are not consumed Alternative pathway for reaction with lower Ea Types Homogeneous Heterogeneous Enzymes are biological catalysts

Adsorption, activation, reaction, desorption