11

Post Lab Whiteboards

Include IV and DV (not rate), and reaction Include IV and DV (not rate), and reaction studied in board titlestudied in board title

Summarize, using multiple representations if Summarize, using multiple representations if possible:possible:Did your IV factor affect the rate?Did your IV factor affect the rate?If so, how?If so, how?

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Summary of Lab Results

Reaction Rate Depends on. . . Reaction Rate Depends on. . . Mainly the Mainly the concentration of reactantsconcentration of reactants

Can also depend on product concentrationCan also depend on product concentration

The rate can also be affected by:The rate can also be affected by:Amount of solidAmount of solidPressure (of gases)Pressure (of gases)TemperatureTemperaturePresence of other particlesPresence of other particles

33

Developing a model for Kinetics

What do all of these factors suggest about reaction kinetics? Is there a particle level connection? Discuss and put your main ideas on your boards.

44

Modeling Reaction KineticsModeling Reaction Kinetics

Demo of collision frequency with Phet Gas Demo of collision frequency with Phet Gas simulatorsimulator

55

Modeling Reaction KineticsModeling Reaction Kinetics

Can our experiments be used to determine Can our experiments be used to determine quantitatively how a reactant concentration quantitatively how a reactant concentration affects reaction rate?affects reaction rate?

Sample experiment:Sample experiment:2 g Na2 g Na22COCO33 + 50 mL 1 M, 3 M, 6 M HCl in + 50 mL 1 M, 3 M, 6 M HCl in

balloon; which reaction is fastest?balloon; which reaction is fastest?

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Modeling Reaction Kinetics

Have to be careful in designing experiments:Have to be careful in designing experiments:Rate generally Rate generally changeschanges over time – refer to over time – refer to

data/graphs!!! data/graphs!!!

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Rate and Concentration

As the reaction proceeds, the reactants get used up As the reaction proceeds, the reactants get used up (fewer molecules to collide, so collision frequency (fewer molecules to collide, so collision frequency decreases)decreases)

Concentration of products also changing. Concentration of products also changing. So we usually focus experiments on So we usually focus experiments on initialinitial rates (an rates (an

instantaneous rate)instantaneous rate)

88

Toward a Mathematical Model: Data Analysis

Whiteboard Work.Whiteboard Work.Each station has a couple of sample Each station has a couple of sample

experiments for which concentration data experiments for which concentration data was taken for a given reaction. was taken for a given reaction.

Determine and describe how the rate of Determine and describe how the rate of reaction depends on concentration for each reaction depends on concentration for each data set.data set.

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The Differential Rate LawThe Differential Rate Law

We can describe the dependence of rate on We can describe the dependence of rate on concentration by a simple expression:concentration by a simple expression:Rate = k [A]Rate = k [A]nn

k is the rate constantk is the rate constant n is the reaction order for the individual speciesn is the reaction order for the individual species

This relationship is called the differential This relationship is called the differential rate law (or just rate law)rate law (or just rate law)

1010

The Differential Rate Law

k and n must be determined experimentallyk and n must be determined experimentally (they are not stoichiometric coefficients, must (they are not stoichiometric coefficients, must

be determined experimentally (multi-step be determined experimentally (multi-step reaction possibilities)reaction possibilities)

1111

Meaning of Reaction Orders

Rate = k [A]Rate = k [A]00

Zero orderZero order = rate is = rate is independentindependent of the of the concentration concentration

Changing the concentration of a zero order Changing the concentration of a zero order reactant has NO effect on the rate.reactant has NO effect on the rate. Rate = k = constant!Rate = k = constant!

1212

Meaning of Reaction Orders

Rate = k [A]Rate = k [A]11

First orderFirst order = rate is = rate is directly proportionaldirectly proportional to to the concentrationthe concentrationDoubling the concentration of a first order Doubling the concentration of a first order

reactant, doubles the ratereactant, doubles the rateTripling the concentration triples the rateTripling the concentration triples the rateCutting the concentration in half, cuts the rate in Cutting the concentration in half, cuts the rate in

half.half.

1313

Meaning of Reaction Orders

Rate = k [A]Rate = k [A]22

Second OrderSecond Order = rate is = rate is proportional to the proportional to the squaresquare of the concentration of the concentrationDoubling the concentration increases the rate by Doubling the concentration increases the rate by

four timesfour timesTripling the concentration increases the rate by Tripling the concentration increases the rate by

nine timesnine timesCutting the concentration in half decreases the Cutting the concentration in half decreases the

rate by four timesrate by four times

1414

Meaning of Reaction Orders

Orders are usually positive integersOrders are usually positive integers Can be negative Can be negative Other orders are possible, but are rare.

Each order tells us how the rate depends on that Each order tells us how the rate depends on that concentration; this order must be determined concentration; this order must be determined experimentally; only occasionally does it match the experimentally; only occasionally does it match the coefficients from the balanced reaction equationcoefficients from the balanced reaction equation

Something to think about: What does the reaction Something to think about: What does the reaction order imply about the reactant in terms of a collision order imply about the reactant in terms of a collision model? model?

1515

12.3 How can we determine reaction order and rate laws?12.3 How can we determine reaction order and rate laws?

Get the data! Measure initial rates for Get the data! Measure initial rates for various concentration of species. Observe various concentration of species. Observe how the initial rate depends on the initial how the initial rate depends on the initial concentrations. Simple Example:concentrations. Simple Example:

2O2O33(g) (g) 3O3O22(g)(g)

Initial [OInitial [O33]] Initial Rate (M/s)Initial Rate (M/s)

0.00150 M0.00150 M 3.08 x 103.08 x 10-8-8

0.00300 M0.00300 M 1.28 x 101.28 x 10-7-7

0.00600 M0.00600 M 5.03 x 105.03 x 10-7-7

1919

Method of Initial Rates

Compare two concentrations, determine the Compare two concentrations, determine the effect on the initial rates! Mathematically:effect on the initial rates! Mathematically:

Whiteboard #27Whiteboard #27

In General: In General: RateRate22 = = ConcConc2 2 nn

RateRate11 ConcConc11

(why is there no k in the generic expression?)(why is there no k in the generic expression?)

2020

Method of Initial RatesMethod of Initial Rates

What if there are multiple reactants?What if there are multiple reactants? If the rate depends on the concentration of If the rate depends on the concentration of

more than one substance, design the more than one substance, design the experiment so only one concentration experiment so only one concentration changes appreciably. changes appreciably.

Vary the concentration of one reactant at a Vary the concentration of one reactant at a time and measure the rates.time and measure the rates.

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Group Practice

Whiteboards:Whiteboards:#25#25#23 – units of the rate constant#23 – units of the rate constant

If time…If time…#28#28#29#29

2222

Units of rate constant, kUnits of rate constant, k

Units of k = MUnits of k = M--(n(n--1)1)ss--11 for an nth order reaction for an nth order reaction in solutionin solution

2323

Rate constant = CONSTANT!

While the rate of reaction does change with While the rate of reaction does change with the concentration of the reactants, the rate the concentration of the reactants, the rate constant does NOT!constant does NOT!

2424

Other mathematical models

We can also describe the dependence of We can also describe the dependence of concentration on time by different analysisconcentration on time by different analysis

This relationship is always related to the This relationship is always related to the differential rate law.differential rate law.

More details later!!!More details later!!!

2525

OLD GROUP WORK SLIDES

2626

Group WorkGroup Work

Derive the rate law and value of k for the Derive the rate law and value of k for the reaction, Freaction, F22 + 2ClO + 2ClO22 2FClO2FClO22

[F[F22]]oo [ClO[ClO22]]oo Rate (M/s)Rate (M/s)0.1000.100 0.01000.0100 0.00120.00120.1000.100 0.04000.0400 0.00480.00480.2000.200 0.01000.0100 0.00240.0024

RR22/R/R11 = 4 and C = 4 and C22/C/C11 = 4, so 1st order in ClO = 4, so 1st order in ClO22

RR33/R/R11 = 2 and C = 2 and C33/C/C11 = 2, so 1st order in F = 2, so 1st order in F22

Rate = k[FRate = k[F22][ClO][ClO22] and k = 1.2 M] and k = 1.2 M-1-1ss-1-1

2727

Rate LawRate Law

4V4V33++ + O + O22 + 2H + 2H22O O 4VO4VO22++ + 4H + 4H++

Rate = k[VRate = k[V33++][O][O22]/[H]/[H++]]Rate = k[VRate = k[V33++][O][O22][H][H++]]--11

| | || | | 1 1 -1 }sum = 1, so 1st order1 1 -1 }sum = 1, so 1st order

What are the individual and overall orders?What are the individual and overall orders?5Br5Br-- + BrO + BrO33

-- + 6H + 6H++ 3Br3Br22(aq) + 2H(aq) + 2H22O(l)O(l)

Rate = k[BrRate = k[Br--][BrO][BrO33--][H][H++]]22

1,1,2 } overall = 41,1,2 } overall = 4

2828

Group Quiz

Turn in one sheet of paper with names of each Turn in one sheet of paper with names of each participatingparticipating member on the top. member on the top.

Determine the rate law and the value of the rate Determine the rate law and the value of the rate constant for the following reaction:constant for the following reaction:

2I2I--(aq) + 2VO(aq) + 2VO22++(aq) + 4H(aq) + 4H++(aq) (aq) I I22(aq) + 2VO(aq) + 2VO22++(aq) (aq)

+ 2H+ 2H22O(l)O(l)

Rate (M minRate (M min-1-1)) [I[I--]] [VO[VO22++]] [H[H++]]

26.026.0 0.002000.00200 0.01000.0100 0.1000.10052.152.1 0.004000.00400 0.01000.0100 0.1000.10051.951.9 0.002000.00200 0.02000.0200 0.1000.1006.506.50 0.002000.00200 0.01000.0100 0.05000.0500