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11

Chapter 12Chemical Kinetics

Thermodynamics tells us what can happen andThermodynamics tells us what can happen andhow far towards completion a reaction willhow far towards completion a reaction willproceed.proceed.

Kinetics tells us how fast the reaction will go.Kinetics tells us how fast the reaction will go.Kinetics is the study of rates of reactions andKinetics is the study of rates of reactions and

factors that affect these rates.factors that affect these rates.Demo of thermite reactionDemo of thermite reaction

oscillatingoscillatingreactionreaction

thermite reactionthermite reaction22

12.1 Reaction Rates

Rates vary from very slow (geologicalRates vary from very slow (geologicalprocesses) to very fast (combustion of Hprocesses) to very fast (combustion of H22))

Must be able to measure rates to determineMust be able to measure rates to determinehow to control them with concentration orhow to control them with concentration ortemperature.temperature.

Reaction rates may depend on many factors!Reaction rates may depend on many factors!

33

How is Reaction Rate Measured?

The amount of The amount of reactant that is used upreactant that is used up in a in acertain amount of time.certain amount of time.

The amount of The amount of product that is formedproduct that is formed in a in acertain amount of time.certain amount of time.

The reaction rate is the same, regardless ofThe reaction rate is the same, regardless ofwhich method is used.which method is used.

44

Is Reaction Rate Constant?

NO!! Rate NO!! Rate slowsslows over time. over time.As the reaction proceeds, the reactants getAs the reaction proceeds, the reactants get

used up.used up.This decreases the concentration, causing theThis decreases the concentration, causing the

reaction to slow.reaction to slow.

55

[Br[Br--]]

[BrO[BrO33--]]

[Br[Br22]]

0.0850.085

0.0170.017

0.0510.05166

Measuring Reaction Rates

Can define an average rate of reaction:Can define an average rate of reaction:A measurement of the rate of reaction during a timeA measurement of the rate of reaction during a time

intervalintervalRate = Rate = ΔΔconc/conc/ΔΔtime time or or --ΔΔconc/conc/ΔΔtime time so it will alwaysso it will always

have a positive valuehave a positive value

222

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Average Rate

The rate changes as the reaction proceeds.The rate changes as the reaction proceeds.Usually it gets smaller with time.Usually it gets smaller with time.The rates calculated for a particular time intervalThe rates calculated for a particular time interval

are are averageaverage rates. rates.

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Instantaneous Rate

A measurement ofA measurement ofthe rate of reactionthe rate of reactionat an instant inat an instant intime.time.

99

Instantaneous Rate

Found by determiningFound by determiningthe tangent to thethe tangent to theconcentration vs. timeconcentration vs. timecurvecurve

Rate of reaction alwaysRate of reaction alwaysrefers to anrefers to aninstantaneous rateinstantaneous rate

1010

Initial Rates

If there is not much change inIf there is not much change inconcentration, then -concentration, then -ΔΔC/C/ΔΔt is a goodt is a goodapproximation to the rate.approximation to the rate. We know concentration values much better atWe know concentration values much better at

the beginning of the reaction than we do laterthe beginning of the reaction than we do later What is the relationship between initial rate

and concentration?

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Initial Rate

To get around theTo get around theproblem of the rateproblem of the ratechanging with time,changing with time,we sometimes measurewe sometimes measureinitial rates:initial rates:

A measurement of the instantaneous rate ofA measurement of the instantaneous rate ofreaction at the start of the reaction.reaction at the start of the reaction.

Found by determining the slope of the tangent lineFound by determining the slope of the tangent lineat time at time ““zero.zero.””

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Measuring Rates of Reaction

Choosing whether to measure change inChoosing whether to measure change inconcentration of reactants or products and theconcentration of reactants or products and thetechnique with which to measure that change reallytechnique with which to measure that change reallydepends on what will be easiest.depends on what will be easiest.Depends on reaction under study.Depends on reaction under study.Usually concentration isnUsually concentration isn’’t measured directly, but byt measured directly, but by

using some signal related to changing concentration.using some signal related to changing concentration.Data logging devices may be very helpful.Data logging devices may be very helpful.Raw data from experiments will be in a variety of units,Raw data from experiments will be in a variety of units,

not usually concentration in not usually concentration in molarity molarity (M), but isn(M), but isn’’t reallyt reallya problem since we can still use data to determine rate.a problem since we can still use data to determine rate.

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1. Change in Gas Volume

Convenient if a product is a gas.Convenient if a product is a gas.Collect gas and measure change in volume atCollect gas and measure change in volume at

regular time intervals. Then plot volumeregular time intervals. Then plot volumeagainst time.against time.

Two main methods:Two main methods:Gas syringe collectionGas syringe collectionWater displacement (if gas has low solubility inWater displacement (if gas has low solubility in

water!)water!)

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1. Change in Gas Volume

Sample apparatus for measuring rate of reactionSample apparatus for measuring rate of reactionby following change in volume against time.by following change in volume against time.

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2. Change in Mass

Many reactions involve mass changes thatMany reactions involve mass changes thatcan be conveniently measured.can be conveniently measured.Usually do to evolution of a gas.Usually do to evolution of a gas.DoesnDoesn’’t really work well for hydrogen (too light)t really work well for hydrogen (too light)

Method allows for continuous readings.Method allows for continuous readings.Then a graph can be plotted directly of massThen a graph can be plotted directly of massagainst time.against time.

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2. Change in Mass

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3. Change in light transmission

Technique referred to as Technique referred to as colorimetry colorimetry ororspectrophotometryspectrophotometry..

Useful if one of the reactants or products isUseful if one of the reactants or products iscolored and will give characteristiccolored and will give characteristicabsorption in the visible region (i.e.absorption in the visible region (i.e.wavelengths about 320-800 nm).wavelengths about 320-800 nm).Sometimes indicators may be chosen thatSometimes indicators may be chosen that

generate a colored compound.generate a colored compound.

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3. Change in light transmission

The colorimeter passes light of selected wavelengthThe colorimeter passes light of selected wavelengththrough the solution and measures the lightthrough the solution and measures the lightabsorbed by the reaction components.absorbed by the reaction components.

As concentrationAs concentration of a colored compound increases,of a colored compound increases,it absorbs proportionally more light, so less isit absorbs proportionally more light, so less istransmitted.transmitted.Photocell sensors generate electric current depending onPhotocell sensors generate electric current depending on

the light transmitted. Computer reads the current andthe light transmitted. Computer reads the current andrecords absorbance or transmittance.records absorbance or transmittance.

Method allows for easy plotting of absorbance vs.Method allows for easy plotting of absorbance vs.time data.time data.

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3. Change in light transmission

Main components of a colorimeterMain components of a colorimeter

2020

4. Change in concentrationmeasured using titration

Possible to measure concentration of aPossible to measure concentration of areactant or product by titrating it against areactant or product by titrating it against aknown known ‘‘standardstandard’’

Involves chemically changing the reactionInvolves chemically changing the reactionmixture, so cannot be done continuously asmixture, so cannot be done continuously asreaction proceeds. How useful then?reaction proceeds. How useful then?Draw small samples from the mixture as reactionDraw small samples from the mixture as reaction

proceeds at regular time intervals.proceeds at regular time intervals.Then analyze by titration.Then analyze by titration.

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4. Change in concentrationmeasured using titration

Problem? Titration takes time and samplesProblem? Titration takes time and samplestaken will continue to react.taken will continue to react.

Technique called Technique called ““quenchingquenching”” can be used. can be used.Add a substance which effectively stops theAdd a substance which effectively stops the

reaction in the sample at the moment taken.reaction in the sample at the moment taken.Gives a Gives a ““freeze framefreeze frame”” of the reaction at a of the reaction at a

particular time interval.particular time interval.Process must be repeated at several intervals ofProcess must be repeated at several intervals of

time to generate data necessary for graphing.time to generate data necessary for graphing.

2222

5. Change in concentrationmeasured using conductivity

Total electrical conductivity of a solutionTotal electrical conductivity of a solutiondepends on the total concentration of its ionsdepends on the total concentration of its ionsand on their charges.and on their charges.

If ions or charges change during reaction,If ions or charges change during reaction,this can provide a convenient way to followthis can provide a convenient way to followreaction progress.reaction progress.

Conductivity meters involve an electrodeConductivity meters involve an electrodeimmersed in solution which directly measuresimmersed in solution which directly measuresconductivity.conductivity.

2323

Use of conductivity probe:

Apparatus can beApparatus can becalibrated withcalibrated withsolutions of knownsolutions of knownconcentration toconcentration toconvert conductivityconvert conductivityreadings toreadings toconcentrations of ionsconcentrations of ionspresent.present.

2424

6. Non-continuous methods ofdetecting change during a reaction

Sometimes it is difficult to record the continuousSometimes it is difficult to record the continuouschange in the rate of a reaction.change in the rate of a reaction.

May be more convenient to measure the time itMay be more convenient to measure the time ittakes for the a reaction to reach a certain fixedtakes for the a reaction to reach a certain fixedpoint point –– something observable which can be used as something observable which can be used asan arbitrary an arbitrary ‘‘end pointend point’’ by which to stop the clock. by which to stop the clock.

The time taken to reach this point for the sameThe time taken to reach this point for the samereaction under different conditions can then bereaction under different conditions can then becompared and used as a means of judging thecompared and used as a means of judging thedifferent rates of reaction.different rates of reaction.

555

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6. Non-continuous methods ofdetecting change during a reaction

Recording increaseRecording increasein opaquenessin opaquenessduring a reaction.during a reaction.

A cross drawn ontoA cross drawn ontopaper is placedpaper is placedunder the reactionunder the reactionbeaker to showbeaker to showwhen the end pointwhen the end pointis reached.is reached.

The experiment isThe experiment istimed.timed.

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Group Practice:

Consider the following reaction:Consider the following reaction: 2 MnO 2 MnO44

–– ((aqaq) + 5 C) + 5 C22OO442- 2- ((aqaq) + 16 H) + 16 H+ + ((aqaq) )

2 Mn2 Mn2+2+((aqaq) + 10 CO) + 10 CO22(g) + 8 H(g) + 8 H22O(l)O(l)

Describe 3 ways in which you could measure the rateDescribe 3 ways in which you could measure the rateof this reaction.of this reaction.

2727

2 MnO2 MnO44–– ((aqaq) + 5 C) + 5 C22OO44

2- 2- ((aqaq) + 16 H) + 16 H+ + ((aqaq) ) 2 Mn2 Mn2+2+((aqaq) + 10 CO) + 10 CO22(g) + 8 H(g) + 8 H22O(l)O(l)

Possibilities:Possibilities:Change in volumeChange in volumeChange in massChange in massColorimetryColorimetryConductivityConductivity

All techniques enable continuous measurements to beAll techniques enable continuous measurements to bemade from which graphs could be plotted of themade from which graphs could be plotted of themeasured variable against time.measured variable against time.

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Rates and Stoichiometry

Use reaction stoichiometry to specify reference pointsUse reaction stoichiometry to specify reference pointsfor rates. When reacting in ratios not equal to 1:1, onefor rates. When reacting in ratios not equal to 1:1, onesubstance disappears or appears faster than the other.substance disappears or appears faster than the other.

To find an equivalent rate for all substances involved,To find an equivalent rate for all substances involved,insert insert reciprocal of coefficient from balanced equation.reciprocal of coefficient from balanced equation.

5Br5Br- - ((aqaq) + BrO) + BrO33- - ((aqaq) + 6H) + 6H+ + ((aqaq) ) →→ 3Br3Br22(aq) + 2H(aq) + 2H22O(l)O(l)

BrBr-- disappears 5 times as fast as BrO disappears 5 times as fast as BrO33--

--ΔΔ [BrO [BrO33--] -1 ] -1 ΔΔ [Br [Br--] -1 ] -1 ΔΔ [H [H++] +1 ] +1 ΔΔ [Br [Br2 2 ]] ———————— = = —— —————— = = —— —————— = = —— ——————

ΔΔt 5 t 5 ΔΔt 6 t 6 ΔΔt 3 t 3 ΔΔttCanCan’’t measure change in Ht measure change in H22O because itsO because its

concentration is so high, it stays constant.concentration is so high, it stays constant.

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[Br[Br--]]

[BrO[BrO33--]]

[Br[Br22]]

0.0850.085

0.0170.017

0.0510.0513030

Average Rate of Reaction

For Example:For Example: N N22 + 3H + 3H22 2NH 2NH33

1 mole of N1 mole of N22 and 3 moles of H and 3 moles of H22 must be used to must be used toproduce 2 moles of NHproduce 2 moles of NH33

This means that the rate of disappearance of HThis means that the rate of disappearance of H22 is isthree times the rate of the disappearance of Nthree times the rate of the disappearance of N22

We can also say that the rate of formation of NHWe can also say that the rate of formation of NH33

is twice the rate of disappearance of Nis twice the rate of disappearance of N22..

666

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Average Rate of Reaction

NN22 + 3H + 3H22 2NH 2NH33

Therefore, we can express the rate ofTherefore, we can express the rate ofthis reaction by saying:this reaction by saying:

Rate = - Rate = - ΔΔ[N[N22]] = - = - 1 1 ΔΔ[H[H22]] = = 1 1 ΔΔ[NH[NH33]] ΔΔtt 3 3 ΔΔtt 2 2 ΔΔtt

Rate ofconsumption of N2

1/3 Rate ofconsumption of H2

½ Rate ofproduction of NH3

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Calculation of Rates

2N2N22OO55 →→ 4NO4NO22 + O + O22(g) in CCl(g) in CCl44 solution solutionStart with 2.330 M solution at 45Start with 2.330 M solution at 45ooC, measureC, measure

the concentration periodically. the concentration periodically. See dataSee data

-1 -1 ΔΔ[N[N22OO55] -1 ] -1 (([N[N22OO55]]22 - [N - [N22OO5 5 ]]11))Rate = Rate = —— ———————— = = —— ———————————————— 2 2 ΔΔt 2 t 2 ((t t 22 - t - t 11))

Rate during time period 52,400 to 72,000 sec,Rate during time period 52,400 to 72,000 sec,where the concentrations are measured to bewhere the concentrations are measured to be1.350 M and 1.110 M.1.350 M and 1.110 M.

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2N2O5 → 4NO2 + O2(g)

52400 1.3572000 1.11112700 0.72138900 0.55188600 0.34

Time (s) [NTime (s) [N22OO55] (M)] (M)

ReturnReturn3434

Calculation of Rates

−Δ−Δ [N [N22OO55] = -(1.110 M - 1.350 M) = 0.240 M] = -(1.110 M - 1.350 M) = 0.240 MΔΔt = 72,000 - 52,400 = 19,600 sect = 72,000 - 52,400 = 19,600 secRate = 1/2 x 0.240 M/19,600 s = 6.12 x 10Rate = 1/2 x 0.240 M/19,600 s = 6.12 x 10--6 6 M/sM/s

3535

Group Work

2N2N22OO55 →→ 4NO4NO22 + O + O22(g)(g)Calculate the rate in terms of NOCalculate the rate in terms of NO22 and of O and of O22

ΔΔt = 72,000 - 52,400 = 19,600 sect = 72,000 - 52,400 = 19,600 sec −Δ−Δ [N [N22OO55] = -(1.110 M - 1.350 M) = 0.240 M] = -(1.110 M - 1.350 M) = 0.240 M

ΔΔ[NO[NO22] = 2 x (] = 2 x (−Δ−Δ [N [N22OO55]) = 2 x 0.240 = 0.480]) = 2 x 0.240 = 0.480 ΔΔ[O[O22] = 1/2 x (] = 1/2 x (−Δ−Δ [N [N22OO55]) = 1/2 x 0.240 = 0.120]) = 1/2 x 0.240 = 0.120Rate = 1/4 Rate = 1/4 ΔΔ[NO[NO22]/]/ΔΔt = 1/4 (0.480)/19,600 = 6.12 xt = 1/4 (0.480)/19,600 = 6.12 x

1010--6 6 M/sM/sRate = Rate = ΔΔ[O[O22]/]/ΔΔt = (0.120)/19,600 = 6.12 x 10t = (0.120)/19,600 = 6.12 x 10--6 6 M/sM/s