chemical kinetics chapter 13 13.1-13.6. chemical kinetics in learning chemical kinetics, you will...

Chemical KineticsChemical Kinetics

Chapter 13Chapter 13

13.1-13.613.1-13.6

Chemical KineticsChemical Kinetics

In learning chemical kinetics, you will In learning chemical kinetics, you will learn how to:learn how to:– Predict whether or not a reaction will Predict whether or not a reaction will

take place.take place.– Once started, determine how fast a Once started, determine how fast a

reaction will proceed.reaction will proceed.– Learn how far a reaction will go before it Learn how far a reaction will go before it

stops.stops.

Rate of a ReactionRate of a Reaction

Thermodynamics-Thermodynamics- Does a reaction take Does a reaction take place?place?

Kinetics-Kinetics- How fast does a reaction How fast does a reaction proceed?proceed?

Chemical Kinetics-Chemical Kinetics- the area of chemistry the area of chemistry concerned with the speeds or rates at concerned with the speeds or rates at which a chemical reaction occurs.which a chemical reaction occurs.

Reaction Rate-Reaction Rate- the change in the the change in the concentration of a reactant or product with concentration of a reactant or product with time. (M/s)time. (M/s)


Why do we need to know the rate of Why do we need to know the rate of a reaction?a reaction?– Practical knowledge is always usefulPractical knowledge is always useful– Preparation of drugsPreparation of drugs– Food processingFood processing– Home repairHome repair


General equation for a reaction:General equation for a reaction:– A A → B→ B– Reactant → ProductReactant → Product

In order to monitor a reaction’s speed or In order to monitor a reaction’s speed or rate, we can look at one of two things:rate, we can look at one of two things:– Decrease in Decrease in [ reactant ][ reactant ]– Increase in [ product ]Increase in [ product ]– Can be represented as:Can be represented as:

rate = - rate = - ΔΔ [A] / [A] / ΔΔ t t ororrate = rate = ΔΔ [B] / [B] / ΔΔ t t


How do we measure this How do we measure this experimentally?experimentally?– For reactions in solution:For reactions in solution:

Changes in concentration can be measured Changes in concentration can be measured spectroscopicallyspectroscopically

– For reactions involving gases:For reactions involving gases:Changes in pressure can be measuredChanges in pressure can be measured

– For reactions in solution with ions present:For reactions in solution with ions present:Change in concentrations can be measured Change in concentrations can be measured

through electrical conductancethrough electrical conductance

Rate of a Reaction Rate of a Reaction

So if we have an aqueous solution of So if we have an aqueous solution of molecular bromine and formic acid, molecular bromine and formic acid, how do we determine the reaction how do we determine the reaction rate?rate?

BrBr22(aq)(aq)+HCOOH+HCOOH(aq)(aq) →→ 2Br 2Br––(aq)(aq)+2H+2H++(aq)(aq)+CO+CO22 (g)(g)

time


Look for color changesLook for color changes Molecular bromine is usually reddish-Molecular bromine is usually reddish-

brown in color. Formic acid is brown in color. Formic acid is colorless.colorless.

As the reaction progresses, the color As the reaction progresses, the color of the solution changes.of the solution changes.

It fades until it becomes colorless.It fades until it becomes colorless. What does this mean?What does this mean?

Rate CalculationsRate Calculations

How do we calculate the rate of a How do we calculate the rate of a reaction?reaction?– We first need this information:We first need this information:

Time (s) Time (s) [reactant][reactant]


BrBr2 2 (aq)(aq) + HCOOH + HCOOH (aq)(aq) → → 2Br 2Br–– (aq)(aq) + 2H + 2H++ (aq)(aq) + CO + CO22 (g)(g)


Instantaneous Rate–Instantaneous Rate– rate of a rate of a reaction for a specific point in time.reaction for a specific point in time.

Average Rate vs. Instantaneous rateAverage Rate vs. Instantaneous rate– Examples????Examples????


Average Rate =Average Rate =--Δ Δ [Br[Br22] / ] / ΔΔt = - [Brt = - [Br22]]final final – [Br– [Br22]]initial initial / [t]/ [t]finalfinal – –

[t][t]initialinitial

Instantaneous Rate = Instantaneous Rate = rate for specific instance in time

[Br[Br22] / t ] / t


Using this Using this information, information, calculate the calculate the average rate of the average rate of the bromine reaction bromine reaction over the first 50s of over the first 50s of the reaction. the reaction.


Average Rate = Average Rate = - [Br- [Br22]]final final – [Br– [Br22]]initial initial / [t]/ [t]finalfinal – – [t][t]initialinitial

Average Rate = - (0.0101- 0.0120)M / (50s – Average Rate = - (0.0101- 0.0120)M / (50s – 0s)0s)

Average Rate = -0.002M / 50sAverage Rate = -0.002M / 50s

Average Rate = 3.80 x 10Average Rate = 3.80 x 10-5 -5 M/sM/s

Average RateAverage Rate

Reaction Rates and Reaction Rates and StoichiometryStoichiometry

For reactions more complex than A For reactions more complex than A → → B we cannot use the rate expression B we cannot use the rate expression initially described.initially described.

Example:Example:– 2A → B2A → B– Disappearance of A is twice as fast Disappearance of A is twice as fast

formation of Bformation of B– Rate = - ½ Rate = - ½ ΔΔ[A] /[A] /ΔΔtt

Reaction Rates and Reaction Rates and StoichiometryStoichiometry

In general, for the reaction In general, for the reaction – aaA + A + bbB B →→ ccC + C + ddDD

– Rate = Rate = - 1/a - 1/a ΔΔ[A] /[A] /ΔΔt = - 1/b t = - 1/b ΔΔ[B] /[B] /ΔΔt = 1/c t = 1/c ΔΔ[C] /[C] /ΔΔt = 1/d t = 1/d ΔΔ[D] [D] //ΔΔt t

Reaction StoichiometryReaction Stoichiometry

Write the rate expression for the following reaction:

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g)

rate = -[CH4]

t= -

[O2]t

12

=[CO2]

t=

[H2O]t

12

Rate ConstantRate Constant

Look back to molecular bromine chart.Look back to molecular bromine chart. What is k?What is k?

– K-K- the rate constant. A constant of the rate constant. A constant of proportionality between the reaction rate and proportionality between the reaction rate and the concentration of the reactant.the concentration of the reactant.

– K may change slightly over time.K may change slightly over time.– K is represented as: K is represented as:

K = K = rate/ [reactant]rate/ [reactant] K is not affected by the [reactant] or rate alone, since K is not affected by the [reactant] or rate alone, since

it is a ratio of these two. At any given point on a it is a ratio of these two. At any given point on a graph, k should be similar in value to it’s value at graph, k should be similar in value to it’s value at other points in the same graph.other points in the same graph.

Rate ConstantRate Constant

The Rate LawThe Rate Law

Rate Law-Rate Law- expresses the expresses the relationship of the rate of a reaction relationship of the rate of a reaction to the rate constant and the to the rate constant and the concentrations of the reactants concentrations of the reactants raised to some power.raised to some power.

Using the general reaction:Using the general reaction: aaA + A + bbB B →→ ccC + C + ddDD Rate Law is:Rate Law is:

rate = k [A]rate = k [A]xx[B][B]yy

The Rate LawThe Rate Law

aA + bB cC + dD

Rate = k [A]x[B]y

reaction is xth order in A

reaction is yth order in B

reaction is (x + y)th order overall

Reaction OrderReaction Order

Reaction Order-Reaction Order- the sum of the powers the sum of the powers to which all reactant concentrations to which all reactant concentrations appearing in the rate law are raised.appearing in the rate law are raised.

Reaction order is Reaction order is alwaysalways defined in terms defined in terms of reactant concentration.of reactant concentration.

Overall reaction order-Overall reaction order- x + y x + y Example:Example:

Rate = k [FRate = k [F22] [ClO] [ClO22]] Reaction order = firstReaction order = first Overall reaction order = secondOverall reaction order = second


What is the rate expression for What is the rate expression for aaA + A + bbB B →→ ccC + C + ddD where x=1 and D where x=1 and y=2?y=2?– Rate = k[A][B]Rate = k[A][B]22

What is the reaction order?What is the reaction order?– First in A, second in BFirst in A, second in B

Overall reaction order?Overall reaction order?– 2 +1 = 32 +1 = 3


F2 (g) + 2ClO2 (g) 2FClO2 (g)

rate = k [F2]x[ClO2]y


If initially [FIf initially [F22] = 1.0M and [ClO] = 1.0M and [ClO22]=1.0M, what ]=1.0M, what will happen to the reaction rate if Fwill happen to the reaction rate if F22 is is doubled?doubled?

RateRate11 = k(1.0M)(1.0M) = k(1.0M)(1.0M)22

RateRate11 = k(1.0M = k(1.0M33)) [[FF22 ] = 1.0M ] = 1.0M

RateRate22 = k(2.0M)(1.0M) = k(2.0M)(1.0M)22

RateRate22 = k(2.0M = k(2.0M33)) [[FF22 ] = 2.0M ] = 2.0M

RateRate2 2 = 2 x Rate= 2 x Rate11


What will happen in the same reaction if What will happen in the same reaction if the [ClOthe [ClO22] is doubled?] is doubled?

RateRate11 = k(1.0M)(1.0M) = k(1.0M)(1.0M)22

RateRate11 = k(1.0M = k(1.0M33)) [[ClOClO22 ] = 1.0M ] = 1.0M

RateRate22 = k(1.0M)(2.0M) = k(1.0M)(2.0M)22

RateRate22 = k(4.0M = k(4.0M33)) [[ClOClO22 ] = 2.0M ] = 2.0M

RateRate22 = 4 x Rate = 4 x Rate11

Determination of Rate LawDetermination of Rate Law

ExperimentExperiment [F[F22]] [ClO[ClO22]] Rate Rate (M/s)(M/s)

11 0.040.04 0.030.03 1.0x101.0x10-2-2

22 0.040.04 0.040.04 2.0x102.0x10-2-2

33 0.020.02 0.040.04 1.0x101.0x10-2-2

44 0.040.04 0.060.06 2.0x102.0x10-2-2

F2 (g) + 2ClO2 (g) 2FClO2 (g)

Determination of Rate LawDetermination of Rate Law

Experiments 1 & 4Experiments 1 & 4

As [FAs [F22] doubles, so does the rate] doubles, so does the rate Experiments 2 & 3Experiments 2 & 3

As [ClOAs [ClO22] doubles, so does the rate] doubles, so does the rate 2:2 ratio…..1:1 ratio2:2 ratio…..1:1 ratio

x = 1 and y = 1x = 1 and y = 1 Rate = k [FRate = k [F22] [ClO] [ClO22] ]

Rate law/Expression Rate law/Expression CalculationsCalculations

ExperimenExperimentt

[S[S22OO882-2-]] [I[I--]]

Initial Initial Rate Rate ((MM/s)/s)

11 0.080.08 0.0340.034 2.2 x 102.2 x 10--

44

22 0.080.08 0.0170.017 1.1 x 101.1 x 10--

44

33 0.160.16 0.0170.017 2.2 x 102.2 x 10--

44

rate = k [S2O82-]x[I-]y

Double [I-], rate doubles (experiment 1 & 2)

y = 1

Double [S2O82-], rate doubles (experiment 2 & 3)

x = 1

k = rate

[S2O82-][I-]

=2.2 x 10-4 M/s

(0.08 M)(0.034 M)= 0.08/M•s

rate = k [S2O82-][I-]

Determine the rate law and calculate the rate constant for the following reaction from the following data:S2O8

2- (aq) + 3I- (aq) 2SO42- (aq) + I3

- (aq)

Rate Law/Reaction OrderRate Law/Reaction Order

Rate laws are always determined Rate laws are always determined experimentallyexperimentally

Reaction order is always defined in terms Reaction order is always defined in terms of reactantof reactant

Reactant order is not related to the Reactant order is not related to the stoichiomteric coefficient in the overall stoichiomteric coefficient in the overall reaction.reaction.

F2 (g) + 2ClO2 (g) 2FClO2 (g)

rate = k [F2][ClO2]

Relation between Reactant Relation between Reactant Concentration and TimeConcentration and Time

First Order Reaction-First Order Reaction- a reaction whose a reaction whose rate depends on the reactant rate depends on the reactant concentration raised to the first power.concentration raised to the first power.

Reaction Type: AReaction Type: A→B→B

Rate of: Rate of: --Δ Δ [A]/[A]/ΔΔt or k[A]t or k[A]

Combining and simplifying these Combining and simplifying these equations brings us to the following rate equations brings us to the following rate equation:equation:

ln[A]ln[A]tt = -kt + ln[A = -kt + ln[A00]]

Relation between Reactant Relation between Reactant Concentration and TimeConcentration and Time

Reaction TimeReaction Time

The reaction 2A B is first order in A with a rate constant of 2.8 x 10-2 s-1 at 800C. How long will it take for A to decrease from 0.88 M to 0.14 M ?

ln[A] = ln[A]0 - kt

kt = ln[A]0 – ln[A]

t =ln[A]0 – ln[A]

k= 66 s

[A]0 = 0.88 M

[A] = 0.14 M

ln[A]0

[A]

k=

ln0.88 M

0.14 M

2.8 x 10-2 s-1=

Decomposition of Nitrogen Decomposition of Nitrogen PentoxidePentoxide

Data on page 560Data on page 560 Plot of ln[N2O5] (M) vs. t (s) will allow Plot of ln[N2O5] (M) vs. t (s) will allow

us to see and calculate more us to see and calculate more information about the reaction taking information about the reaction taking placeplace

Decomposition of Nitrogen Decomposition of Nitrogen PentoxidePentoxide

Gas Phase ReactionsGas Phase Reactions

First order gas phase reactions have First order gas phase reactions have a linear relationship between partial a linear relationship between partial pressure of gas and time.pressure of gas and time.

lnPlnPt t = -kt + lnP= -kt + lnP00

Gas Phase ReactionsGas Phase Reactions

Reaction Half-lifeReaction Half-life

As a reaction proceeds, the As a reaction proceeds, the concentrations of the reactants concentrations of the reactants decreases. decreases.

Another way to measure [reactant] Another way to measure [reactant] over time is to use the over time is to use the half-life.half-life.

Half-life, tHalf-life, t1/21/2 – – the time required for the time required for the concentration of a reactant to the concentration of a reactant to decrease to half of its initial decrease to half of its initial concentration.concentration.


Expression for half-life of a first order Expression for half-life of a first order reaction is:reaction is:

tt1/2 1/2 = ln2/k= ln2/k

oror

tt1/2 1/2 = 0.693/k= 0.693/k


What is the half-life of N2O5 if it decomposes with a rate constant of 5.7 x 10-4 s-1?

t½ln2k

=

0.6935.7 x 10-4 s-1

=

= 1200 s

= 20 minutes

t½

t½

t½

Second-Order ReactionsSecond-Order Reactions

Second-order reaction-Second-order reaction- a reaction whose rate a reaction whose rate depends on the concentration of one reactant depends on the concentration of one reactant raised to the second power OR on the raised to the second power OR on the concentrations of two different reactants, each concentrations of two different reactants, each raised to the first power. raised to the first power.

Simple Type: ASimple Type: A→B →B – rate = k[A]rate = k[A]22

Complex Type: A + B→CComplex Type: A + B→C– rate = k[A][B]rate = k[A][B]

Second-order ReactionsSecond-order Reactions

For AFor A→B, the following expression is →B, the following expression is used:used:

1

[A]=

1

[A]0

+ kt

Half-life of a Second-order Half-life of a Second-order ReactionReaction

Equation for half-lifeEquation for half-life

What is the difference between this What is the difference between this equation and the equation for half-equation and the equation for half-life of first-order reactions?life of first-order reactions?

t½ =1

k[A]0

Zero-order ReactionsZero-order Reactions

Very rare reactionsVery rare reactions Usually occur on metallic surfacesUsually occur on metallic surfaces Half-life Equation:Half-life Equation:

Reaction rate is described by:Reaction rate is described by:– Rate = kRate = k– Why?Why?

t½ =[A]0

2k

SummarySummary

Activation Energy and Temperature Activation Energy and Temperature Dependence of Rate ConstantsDependence of Rate Constants

The Collision Theory of The Collision Theory of Chemical KineticsChemical Kinetics

Gas molecules frequently collide with Gas molecules frequently collide with one anotherone another

Expect that the rate of a reaction is Expect that the rate of a reaction is equivalent to the number of collisionsequivalent to the number of collisions

Reaction rate is dependent on Reaction rate is dependent on concentrationconcentration


Activation Energy Activation Energy (E(Eaa)-)- the minimum the minimum amount of energy amount of energy required to initiate a required to initiate a chemical reaction.chemical reaction.

Activated Complex Activated Complex (Transition State)-(Transition State)- a a temporary species temporary species formed by the formed by the reactant molecules as reactant molecules as a result of the collision a result of the collision before they form the before they form the product. product.


What does this have to do with What does this have to do with temperature?temperature?– High energy moleculesHigh energy molecules– High temperaturesHigh temperatures– Increased product formationIncreased product formation


Factors that affect rateFactors that affect rate– 1.1.– 2.2.– 3.3.

The Arrhenius EquationThe Arrhenius Equation

Relation between activation energy Relation between activation energy and temperature. and temperature.

lnk = (Elnk = (Eaa/R)/R) xx (1/T) + lnA(1/T) + lnA

Rate Constants and Rate Constants and TemperatureTemperature

lnKlnK11 = = EEaa xx (T(T11 – T – T22) )

lnKlnK2 2 R (T R (T11TT22))

Activation Energy, Reaction Rates Activation Energy, Reaction Rates and Temperatureand Temperature

As stated earlier, for a reaction to As stated earlier, for a reaction to take place, molecules must posses take place, molecules must posses enough kinetic energy.enough kinetic energy.

Kinetic energy must be higher than Kinetic energy must be higher than EEaa..

Each reaction takes place at a Each reaction takes place at a specific temperature……but what specific temperature……but what happens if we adjust this temp.?happens if we adjust this temp.?

Activation Energy, Reaction Rates Activation Energy, Reaction Rates and Temperatureand Temperature

Increasing Temperature leads to:Increasing Temperature leads to:– Molecules reach high ke fasterMolecules reach high ke faster– Number of molecules with high enough Number of molecules with high enough

ke increaseske increases– Reaction rate increasesReaction rate increases

CatalystsCatalysts

A A catalystcatalyst is defined by the ability of is defined by the ability of a substance to do each of the a substance to do each of the following:following:– Catalysts increase the rate of reaction.Catalysts increase the rate of reaction.– Catalysts are not consumed by the Catalysts are not consumed by the

reaction.reaction.– A small quantity of catalyst should be A small quantity of catalyst should be

able to affect the rate of reaction for a able to affect the rate of reaction for a large amount of reactant.large amount of reactant.

– Catalysts do not change the equilibrium Catalysts do not change the equilibrium constant for the reaction.constant for the reaction.

CatalystsCatalysts

Heterogeneous catalyst-Heterogeneous catalyst- the the reactants and the catalyst are in reactants and the catalyst are in different phases. catalyst = soliddifferent phases. catalyst = solidreactants = liquid/gasreactants = liquid/gas

Homogeneous catalyst-Homogeneous catalystcatalyst catalyst and reactants are in the same phase, and reactants are in the same phase, usually liquid.usually liquid.

CatalystsCatalysts

Enzyme CatalystsEnzyme Catalysts

The End!!!!!!!The End!!!!!!!

chemical kinetics chapter 13 13.1-13.6. chemical kinetics in learning chemical kinetics, you will...

Documents

reaction slide

average rate slide

rate calculations average

rate expression

tor rate

rate calculations br

o g rate

chemical reaction