Chemical Kinetics Rates of Reactions ©2011 University of Illinois Board of Trustees .

Chemical Kinetics

Rates of Reactions

©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

What is a Reacation Rate?

Deals with how the reactants concentrations change with time

12

12

-

at time ofion concentrat - at time ofion concentrat Rate

tt

tAtA

t

A

Rate


Factors that Affect Reaction Rate

TemperatureCollision Theory: When two chemicals react, their molecules have to

collide with each other with sufficient energy for the reaction to take place.

Kinetic Theory: Increasing temperature means the molecules move faster.

Concentrations of reactants More reactants mean more collisions if enough energy is present

Catalysts Speed up reactions by lowering activation energy

Surface area of a solid reactant Bread and Butter theory: more area for reactants to be in contact

Pressure of gaseous reactants or productsIncreased number of collisions


How do reactions take place?

Collision Theory

Reactants must have…

1. Correct orientation to each other

2. Enough energy for the reaction to occur


Activation Energy Diagrams

Exothermic vs Endothermic


A + B C + D

Exothermic Reaction Endothermic Reaction

The activation energy (Ea) is the minimum amount of energy required to initiate a chemical reaction.

13.4$ What is 13.4?


4PH3(g) P4(g) + 6H2(g)

If 0.0048 mol of PH3 is consumed in a 2.0 L container during each second of the reaction, what are the rates of production of P4 and H2?

t

- consumed being is PH rate 3

$ The picture at the top show a solution reaction, not gaseous. Is thetemperature of the desired reaction high enough that P4(g) is produced Instead of solid? Missing term inside [ ].


Solution…

41-1-

3

43P s L mol 0.0060

PH mol 4

P mol 1

s L

PH mol 0024.0

31-1-3

PH s L mol 0.0024 s x L 2.0

PH mol 0.0048 rate

21-1-

3

23H s L mol 0.0036

PH mol 4

H mol 6

s L

PH mol 0024.0


2NO2(g) 2NO(g) + O2(g) at 300oCTime (s) [NO2] [NO] [O2]

0 0.0100 0 0

50 0.0079 0.0021 0.0011

100 0.0065 0.0035 0.0018

150 0.0055 0.0045 0.0023

200 0.0048 0.0052 0.0026

250 0.0043 0.0057 0.0029

300 0.0038 0.0062 0.0031

350 0.0034 0.0066 0.0033

400 0.0031 0.0069 0.0035©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

Rate Laws

Deal only with the concentration of the reactants

Rate = k[NO2]n

k = proportionality constant (rate constant)

n = order (may be a fraction) & must be determined experimentally

$ Change k to italics.©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

The Rate Law

13.2

The rate law expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers.

aA + bB cC + dD

Rate = k [A]x[B]y

reaction is xth order in A

reaction is yth order in B

reaction is (x +y)th order overall

$ Mention that x and y must be determinedexperimentally. What is the 13.2?


Types of Rate Laws

Differential Rate Law (Rate Law): shows how rate depends on concentration

Integrated Rate Law: shows how concentrations of a species changes depends on time


Finding the Form

• The order of a reactant is not related to the stoichiometric coefficient of the reactant in the balanced chemical equation.

F2 (g) + 2ClO2 (g) 2FClO2 (g)

rate = k [F2][ClO2]1

$ Add and must be determined experimentally.


Method of Initial Rates

Rate just after the reaction has begun but before there is a significant change in concentration


Determine the rate law and calculate the rate constant for the following reaction from the following data:S2O8

2- (aq) + 3I- (aq) 2SO42- (aq) + I3

- (aq)

Experiment [S2O82-] [I-]

Initial Rate (M/s)

1 0.08 0.034 2.2 x 10-4

2 0.08 0.017 1.1 x 10-4

3 0.16 0.017 2.2 x 10-4


Terms

Molecularity – the number of molecules that must collide to produce the reaction indicated by that step

Elementary Step – a reaction for which the rate law can be written from its molecularity

Unimolecular – reactions involving one molecule

Bimolecular – reactions involving two molecules


So what is a mechanism?

A series of elementary steps that must add up to the overall balanced equation AND agree with the rate law

Step 1 Step 2 Step 3


Reaction Mechanisms

Series of steps by which a chemical reaction occurs

Balanced equation does not tell us HOW the reactants become the products

It is a summary of the overall process

Ex – 6CO2 + 6H2O C6H12O6 + 6O2


Elementary Steps

A products unimolecular Rate = k[A]

A + A products bimolecular Rate = k[A]2

A + B products bimolecular Rate = k[A][B]

A + A + B products termolecular Rate = k[A]2[B]

A + B + C products termolecular Rate = k[A][B][C]

Termolecular reactions are rare due to infrequent collisions of 3 molecules simultaneously.


Reactions Mechanisms Con’t

The sum of the intermediate steps must agree with stoichiometry of the reaction

Intermediates – are neither products or reactants and must cancel out


NO2(g) + CO(g) NO(g) + CO2(g)

NO2(g) + NO2(g) NO3(g) + NO(g)

NO3(g) + CO(g) NO2(g) + CO2(g)

NO2(g) + CO(g) NO(g) + CO2(g)

Step 1 is the rate determining step (slow) and the rate law can be written

Rate = k[NO]2

Step 1

Step 2


zero first second

Rate Law

Integrated Rate Law

Straight line Graph

Half-life

Rate = k Rate = k[A] Rate = k[A]2

[A] = -kt + [A]0 ln[A] = -kt + ln[A]0

[A] vs t

Slope = -k

ln[A] vs t

Slope = -k

0A][

1

[A]

1kt

k

t

Slope

vs[A]

1

Summary of Rate Laws

02/1 A][

1

kt

kt

2ln2/1

k

At

2

][ 02/1


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Chemical Kinetics Rates of Reactions ©2011 University of Illinois Board of Trustees .

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