Chapter 14 Chemical Kinetics Dr. Nick Blake Ventura Community College Ventura, California.

Chapter 14Chemical Kinetics

Dr. Nick BlakeVentura Community College

Ventura, California

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• Kinetics is the study of the factors that affect the speed of a reaction and the mechanism by which a reaction proceeds

• Experimentally it is shown that there are 4 factors that influence the speed of a reaction: nature of the reactants, temperature, catalysts,concentration

Kinetics

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Defining Rate• rate is how much a quantity changes in a given period of

time• the speed you drive your car is a rate – the distance your

car travels (miles) in a given period of time (1 hour) so the rate of your car has units of mi/hr (mph)

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Defining Reaction Rate

• the rate of a chemical reaction is measured as the change of concentration of a reactant (or product) in a given period of time interval

• To make the rate > 0, for reactants, a negative sign is placed in front of the definition

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Reaction Rate: Changes Over Time

• as time goes on, the rate of a reaction generally slows down because the concentration of the reactants decreases

• at some time the reaction stops, either because the reactants

run out or because the system has reached equilibrium.

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at t = 0[A] = 8[B] = 8[C] = 0

at t = 0[X] = 8[Y] = 8[Z] = 0

at t = 16[A] = 4[B] = 4[C] = 4

at t = 16[X] = 7[Y] = 7[Z] = 1

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at t = 16[A] = 4[B] = 4[C] = 4

at t = 16[X] = 7[Y] = 7[Z] = 1

at t = 32[A] = 2[B] = 2[C] = 6

at t = 32[X] = 6[Y] = 6[Z] = 2

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at t = 32[A] = 2[B] = 2[C] = 6

at t = 32[X] = 6[Y] = 6[Z] = 2

at t = 48[A] = 0[B] = 0[C] = 8

at t = 48[X] = 5[Y] = 5[Z] = 3

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Hypothetical Reaction Red Blue

Time (sec)

Number Red

Number Blue

0 100 0

5 84 16

10 71 29

15 59 41

20 50 50

25 42 58

30 35 65

35 30 70

40 25 75

45 21 79

50 18 82

• In this reaction, one molecule of Red turns into one molecule of Blue

• The number of molecules will always total 100

• The rate of the reaction can be measured as the speed of loss of Red molecules over time, or the speed of gain of Blue molecules over time

Tro, Chemistry: A Molecular Approach 10

Hypothetical Reaction: Red Blue


Hypothetical Reaction Red Blue

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Reaction Rate and Stoichiometry• in most reactions, the coefficients of the balanced equation are not all the

same

H2 (g) + I2 (g) 2 HI(g)

• for these reactions, the change in the number of molecules of one substance is a multiple of the change in the number of molecules of another for the above reaction, for every 1 mole of H2 used, 1 mole of I2 will also be

used and 2 moles of HI made therefore the rate of change will be different

• in order to be consistent, the change in the concentration of each substance is multiplied by 1/coefficient

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Average Rate

• the average rate is the change in measured concentrations in any particular time period linear approximation of a curve

• the larger the time interval, the more the average rate deviates from the instantaneous rate

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Hypothetical Reaction Red BlueAvg. Rate Avg. Rate Avg. Rate

Time (sec)

Number Red

Number Blue

(5 sec intervals)

(10 sec intervals)

(25 sec intervals)

0 100 0

5 84 16 3.2

10 71 29 2.6 2.9

15 59 41 2.4

20 50 50 1.8 2.1

25 42 58 1.6 2.3

30 35 65 1.4 1.5

35 30 70 1

40 25 75 1 1

45 21 79 0.8

50 18 82 0.6 0.7 1

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H2 I2 HI

Avg. Rate, M/s Avg. Rate, M/s

t (s) [H2]t, M [HI]t, M -Δ[H2]/Δt ½ Δ[HI]/Δt

0.000 1.000

10.000 0.819

20.000 0.670

30.000 0.549

40.000 0.449

50.000 0.368

60.000 0.301

70.000 0.247

80.000 0.202

90.000 0.165

100.000 0.135



0.000 1.000 0.000

10.000 0.819 0.362

20.000 0.670 0.660

30.000 0.549 0.902

40.000 0.449 1.102

50.000 0.368 1.264

60.000 0.301 1.398

70.000 0.247 1.506

80.000 0.202 1.596

90.000 0.165 1.670

100.000 0.135 1.730

Avg. Rate, M/s

t (s) [H2]t, M [HI]t, M -Δ[H2]/Δt

0.000 1.000 0.000

10.000 0.819 0.362 0.0181

20.000 0.670 0.660 0.0149

30.000 0.549 0.902 0.0121

40.000 0.449 1.102 0.0100

50.000 0.368 1.264 0.0081

60.000 0.301 1.398 0.0067

70.000 0.247 1.506 0.0054

80.000 0.202 1.596 0.0045

90.000 0.165 1.670 0.0037

100.000 0.135 1.730 0.0030



0.000 1.000 0.000

10.000 0.819 0.362 0.0181 0.0181

20.000 0.670 0.660 0.0149 0.0149

30.000 0.549 0.902 0.0121 0.0121

40.000 0.449 1.102 0.0100 0.0100

50.000 0.368 1.264 0.0081 0.0081

60.000 0.301 1.398 0.0067 0.0067

70.000 0.247 1.506 0.0054 0.0054

80.000 0.202 1.596 0.0045 0.0045

90.000 0.165 1.670 0.0037 0.0037

100.000 0.135 1.730 0.0030 0.0030

HI


0.000 10.000 20.000 30.000 40.000 50.000 60.000 70.000 80.000 90.000100.0000.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

1.600

1.800

2.000Concentration vs. Time for H2 + I2 --> 2HI

[H2], M[HI], M

t (s)

con

cen

trat

ion

, (M

)

average rate = - slope of the line connecting the [H2] points; = ½ slope of the line for [HI]

the average rate for the first 10 s is 0.0181 M/sthe average rate for the first 40 s is 0.0150 M/sthe average rate for the first 80 s is 0.0108 M/s

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Instantaneous Rate

• the instantaneous rate is the change in concentration at any one particular time slope at one point of a curve

• determined by taking the slope of a line tangent to the curve at that particular point first derivative of the function

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H2 (g) + I2 (g) 2 HI (g) Using [H2], the instantaneous rate at 50 s is:

Using [HI], the instantaneous rate at 50 s is:

For the reaction given, the [I-] changes from 1.000 M to 0.868 M in the first 10 s. Calculate the average rate in the first 10 s

and the Δ[H+].H2O2 (aq) + 3 I-

(aq) + 2 H+(aq) I3

- (aq) + 2 H2O(l)

Solve the equation for the Rate (in terms of the change in concentration of the Given quantity)

Solve the equation of the Rate (in terms of the change in the concentration for the quantity to Find) for the unknown value

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Measuring Reaction Rate• In order to measure the reaction rate you need to be able to measure

the concentration of at least one component in the mixture at many points in time

• There are two ways of approaching this problem (1) for reactions that are complete in less than 1 hour, it is best to use continuous monitoring of the concentration, or (2) for reactions that happen over a very long time, sampling of the mixture at various times can be used when sampling is used, often the reaction in the sample is stopped

by a quenching technique

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Continuous Monitoring• polarimetry – measuring the change in the degree of rotation

of plane-polarized light caused by one of the components over time

• spectrophotometry – measuring the amount of light of a particular wavelength absorbed by one component over time

• total pressure – the total pressure of a gas mixture is stoichiometrically related to partial pressures of the gases in the reaction


Sampling• gas chromatography can measure the

concentrations of various components in a mixture for samples that have volatile components separates mixture by adherence to a surface

• drawing off periodic aliquots from the mixture and doing quantitative analysis titration for one of the componentsgravimetric analysis

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Factors Affecting Reaction RateNature of the Reactants

• nature of the reactants means what kind of reactant molecules and what physical condition they are in. small molecules tend to react faster than large molecules; gases tend to react faster than liquids which react faster than solids; powdered solids are more reactive than “blocks”

more surface area for contact with other reactants certain types of chemicals are more reactive than others

e.g., the activity series of metals ions react faster than molecules

no bonds need to be broken

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• Increasing temperature increases reaction rate chemist’s rule of thumb - for each 10°C rise in temperature, the

speed of the reaction doubles for many reactions

• there is a mathematical relationship between the absolute temperature and the speed of a reaction discovered by Svante Arrhenius which will be examined later

Factors Affecting Reaction RateTemperature

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• catalysts are substances which affect the speed of a reaction without being consumed

• most catalysts are used to speed up a reaction, these are

called positive catalysts catalysts used to slow a reaction are called negative catalysts

• homogeneous = present in same phase

• heterogeneous = present in different phase

• how catalysts work will be examined later

Factors Affecting Reaction RateCatalysts


• generally, the larger the concentration of reactant molecules, the faster the reaction increases the frequency of reactant

molecule contactconcentration of gases depends on the partial

pressure of the gas higher pressure = higher concentration

• concentration of solutions depends on the solute to solution ratio (molarity)

Factors Affecting Reaction RateReactant Concentration

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The Rate Law• the Rate Law of a reaction is the mathematical relationship

between the rate of the reaction and the concentrations of the reactantsand homogeneous catalysts as well

• the rate of a reaction is directly proportional to the concentration of each reactant raised to a power

• for the reaction aA + bB products the rate law would have the form given belown and m are called the orders for each reactantk is called the rate constant

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Reaction Order• the exponent on each reactant in the rate law is called the

order with respect to that reactant• the sum of the exponents on the reactants is called the

order of the reaction • The rate law for the reaction:

2 NO(g) + O2(g) 2 NO2(g)

Rate = k[NO]2[O2]

The reaction is second order with respect to [NO], first order with respect to [O2], and third order overall


Sample Rate Laws

The reaction is autocatalytic, because a product affects the rate.Hg2+ is a negative catalyst, increasing its concentration slows the reaction.

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Reactant Concentration vs. TimeA Products


Half-Life

• the half-life, t1/2, of a reaction is the length of time it takes for the concentration of the reactants to fall to ½ its initial value

• the half-life of the reaction depends on the order of the reaction

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Zero Order Reactions

• Rate = k[A]0 = k constant rate reactions

• [A] = -kt + [A]0

• graph of [A] vs. time is straight line with • slope = -k and y-intercept = [A]0

• t ½ = [A0]/2k• when Rate = M/sec, k = M/sec

[A]0

[A]

time

slope = - k

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First Order Reactions• Rate = k[A]

• ln[A] = -kt + ln[A]0

• graph ln[A] vs. time gives straight line with slope = -k and y-intercept = ln[A]0

used to determine the rate constant

• t½ = 0.693/k

• the half-life of a first order reaction is constant

• the when Rate = M/sec, k = sec-1

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ln[A]0

ln[A]

time

slope = −k

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Half-Life of a First-Order ReactionIs Constant

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Rate Data for C4H9Cl + H2O C4H9OH + HCl

Time (sec) [C4H9Cl], M

0.0 0.1000

50.0 0.0905

100.0 0.0820

150.0 0.0741

200.0 0.0671

300.0 0.0549

400.0 0.0448

500.0 0.0368

800.0 0.0200

10000.0 0.0000

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C4H9Cl + H2O C4H9OH + 2 HCl

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slope = -2.01 x 10-3

k =2.01 x 10-3 s-1

Chapter 14 Chemical Kinetics Dr. Nick Blake Ventura Community College Ventura, California.

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Transcript of Chapter 14 Chemical Kinetics Dr. Nick Blake Ventura Community College Ventura, California.