Dr. Mihelcic Honors Chemistry 1

Chemical KineticsRates and Mechanisms of Chemical

Reactions

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• KINETICS — the study of REACTION RATES and the MECHANISM (steps) the reaction follows.

• We can use thermodynamics to tell if a reaction is spontaneous.

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

• KINETICS — the study of REACTION RATES and the MECHANISM (steps) the reaction follows.

• We can use thermodynamics to tell if a reaction is spontaneous.

• 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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Units

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

moles/L.time

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

aA + bB cC +dD

Disappearance of A = -1 ∆[A] a ∆time

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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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• Instantaneous rate – the slope of the tangent line at a given point

• Average rate – the rate over a given time interval

• Initial rate – rate at “time zero”

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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. Concentration 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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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

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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

Use data from experiments 1 and 2

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 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.

1 0.0010 0.0040 0.12

2 0.0020 0.0040 0.48

3 0.0030 0.0040 1.08

4 0.0040 0.0010 0.48

5 0.0040 0.0020 0.96

6 0.0040 0.0030 1.44

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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.

1 0.1 0.1 4.0x10-5

2 0.1 0.2 4.0x10-5

3 0.2 0.1 16.0x10-5

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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.

1 0.1 0.1 12

2 0.1 0.2 24

3 0.2 0.1 48

4 0.3 0.1 108

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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

the minimum amount of energy required for the reaction to occur, Ea.

activated complex – a temporary-transition state formed from the reactants; can form products or reform reactants

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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

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Potential Energy Diagrams and Catalysts

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CATALYSISA catalyst provides an alternate reaction pathway, which has a lower activation energy than an uncatalyzed reaction.

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REACTION MECHANISMSREACTION MECHANISMS

mechanism: the simple steps involved in a chemical reaction.

Most DON’T 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) Slow

H2O2(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)

SlowClO(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?