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Chemical KineticsChapter 11

H2O2 decomposition in an insect

H2O2 decomposition catalyzed by MnO2

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• KINETICS — the study of REACTION RATES and their relation to the way the reaction proceeds, i.e., its MECHANISM(steps in the process).

• We can use thermodynamics to tell if a reaction is product- or reactant-favored.

• Only KINETICS will tell us HOW FAST the reaction happens!

• KINETICS — the study of REACTION RATES and their relation to the way the reaction proceeds, i.e., its MECHANISM(steps in the process).

• We can use thermodynamics to tell if a reaction is product- or reactant-favored.

• Only KINETICS will tell us HOW FAST the reaction happens!

Chemical Kinetics

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Meaning of Rate of Reaction

rate – the change per interval of time

Ex—speed (distance/ time) is a RATE

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• Types of rates -– initial rate, average rate, and

instantaneous rate

Reaction Rates Reaction Rates

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Determining a Reaction Rate

Blue dye is oxidized with bleach.

Its concentration decreases with time.

The rate — the change in dye conc with time — can be determined from the plot.

Blue dye is oxidized with bleach.

Its concentration decreases with time.

The rate — the change in dye conc with time — can be determined from the plot.

NOTE THE UNITS OF RATE IN CHEMICAL REACTIONS!!!

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Units

• [ ] = Molarity = moles/L• Time= hr, min, s• Overall= M/time or

moles/L.time

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C. MeasurementGeneral reaction rate: calculated by dividing rate expressions by stoichiometric coefficients

EOS

t

D

dt

C

ct

B

bt

A

aRate

][1][1][1][1

For aA + bB cC + dD,

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• Example: Calculate the rate of N2O5 decomposition at 1.0 minute using this graph. How are the rates of formation of NO2 and O2 related to the rate of decomposition of N2O5?

Decomposition of N2O5

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0 20 40 60 80 100

Time (sec)

Mo

lar

Co

nce

ntr

atio

n o

f N

2O5

(mo

l/L

)

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Example 2: Sample Calculation of Average RateRate of decomposition of N2O5(g) at 67oC:Time(min) 0 1 2 3 4

[N2O5]0.160 0.113 0.080 0.056 0.040

What is the average rate of decomposition of dinitrogen pentoxide during the first three minutes?

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II. Reaction Rate and ConcentrationA. Reactant concentration and

collision theory*Reactions occur when

molecules collide to exchange or rearrange atoms

*Effective collisions occur when molecules have correct energy and orientation

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1. Concentrations and physical state of reactants and products

2. Temperature 3. CatalystsRate Law: relates the rate of the

reaction to the concentration of the reactants

Factors Affecting Factors Affecting Rate: Rate:

Factors Affecting Factors Affecting Rate: Rate:

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Concentrations & RatesConcentrations & Rates

0.3 M HCl 6 M HCl

Mg(s) + 2 HCl(aq) ---> MgClMg(s) + 2 HCl(aq) ---> MgCl22(aq) + H(aq) + H22(g)(g)

As concentration increases, what happens As concentration increases, what happens to rate?to rate?

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•Physical state of reactants

Factors Affecting Factors Affecting RatesRates

Greater surface area means greater rate of reaction!

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•Temperature

Factors Affecting Factors Affecting RatesRates

Bleach at 54 ˚C Bleach at 22 ˚C

As Temperature increases, what happens to rate? Why?

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Catalysts: catalyzed decomp of H2O2

Catalysts are substances that speed up a reaction but are unchanged by the reaction

2 H2O2 --> 2 H2O + O2

Factors Affecting Factors Affecting RatesRates

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Effect of Concentration on Reaction Rate

To propose a reaction mechanism, we study

• reaction rate and• its concentration

dependence

To propose a reaction mechanism, we study

• reaction rate and• its concentration

dependence

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B. Rate Expression and Rate Constant

The rate law is: R = k[A]m[B]n[C]p

EOS

The exponents in a rate law must be determined by experiment. They are not the stoichiometric coefficients and k is the rate constant.

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B. continued

m, n, and p • are the reaction order• can be 0, 1, 2 or fractions• must be determined by experiment!

Overall Order = Sum of m, n and p

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Rate Constant, k

• Relates rate and concentration at a given temperature

General Formula M(1- overall order) time-

1

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-For one reactant

aA products

Rate = k [A]m

m= order of reaction with respect to A

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• If m = 0 (zero order) Rate = k [A]0 rate is independent of [A]

• If m = 1 (1st order)Rate = k [A]1

If [A] doubles, then rate doubles• If m = 2 (2nd order)

Rate = k [A]2

If [A] doubles, then rate quadruples

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C. Determining Rates

Pick any two points from the given data!

Use a ratio of rate expressions for the 2 chosen points.

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The initial rate of decomposition of acetaldehyde, CH3CHO, was measured at a series of different concentrations and at a constant temperature.

Using the data below, determine the order of the reaction (m) in the equation:

Rate = k[CH3CHO]m

CH3CHO(g) CH4(g) + CO(g)CH3CHO (mol/L) 0.162 0.195 0.273 0.410 0.518

Rate (mol/L*min) 3.15 4.56 8.94 20.2 35.2

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Consider the rate data for the decomposition of CH3CHO given in the above example. Knowing that the reaction is 2nd order, determine the

• a. value of the rate constant (k)

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• b. the rate of the reaction when [CH3CHO] = 0.452mol/L

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Order of a Reaction for more than one reactant

• aA +bB products• Rate = k[A]m[B]n

• Overall order of reaction = m + n

• Use one equation to determine each reactant’s order, where the other reactant(s) concentration is held constant.

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The data below are for the reaction of nitrogen (II) oxide with hydrogen at 800oC.

2NO(g) + 2H2(g) N2(g) + 2H2O(g)Determine the order of the reaction with respect to both

reactants, calculate the value of the rate constant, and determine the rate of formation of product when [NO]=0.0024M and [H2]=0.0042M.

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Ex The initial rate of a reaction A + B C was measured with the results below. State the rate law, the value of the rate constant, and the rate of reaction when [A] = 0.050M and [B] = 0.100M.

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Ex The following data were collected for this reaction at constant temperature:2NO(g) + Br2(g) 2NOBr(g)

State the rate law, and determine the rate of this reaction when [NO]=0.15M and [Br2]=0.25M.

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Properties of ReactionsOrder Rate Expression0 (zero) rate = k1 (first) rate = k[A]2 (second) rate = k[A]2

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Activation Energy

A. definition: the minimum amount of energy required to react, Ea.

activated complex - a short-lived molecule formed when reactants collide; it can return to reactants or form products.

Reaction coordinate diagram

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Effective Collisions

How can the number of effective collisions be increased?

• Increase concentration of reactants,

• Increase temperature of reaction,

• Increase surface area of reactants.

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Potential Energy Diagrams and Catalysts

Energy of Activated Complex

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CATALYSISCatalysis LOWERS activation energy!

Uncatalyzed reactionUncatalyzed reaction

Catalyzed reactionCatalyzed reaction

MnOMnO22 catalyzes catalyzes

decomposition of Hdecomposition of H22OO22

2 H2 H22OO22 ---> 2 H ---> 2 H22O + OO + O22

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REACTION MECHANISMSREACTION MECHANISMS

mechanism: the elementary steps involved in a chemical reaction.

Most chemical reactions DO NOT occur in a single step!

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Reaction Mechanisms

B. Rate Determining Step:the slowest step in the mechanism; thus determining the overall rate

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C. Intermediates:A substance that is not an

original reactant or a final product; it is produced in one step and consumed in another step of the mechanism

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D. Catalysts:A substance that speeds up the

reaction but is not permanently changed by the reaction; both an original reactant and a final product

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Sample ProblemH2O2(aq) + I1-(aq) H2O(l) + IO1-(aq)

SlowH2O2(aq) + IO1-(aq) H2O(l) + O2(g) + I1-

(aq) Fast

Identify the following:

•Rate determining step?

•Are there any intermediates or catalysts in the reaction? If yes who?

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Sample Problem

O3(g) + Cl(g) O2(g) + ClO(g)Slow

ClO(g) + O(g) Cl(g) + O2(g)Fast

Identify the following:

Rate determining step?

Are there any intermediates

or catalysts in the reaction? If

yes who?