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Chapter 16 : Reaction Chapter 16 : Reaction RatesRates

Section 2 : How Can Section 2 : How Can reaction Rates be reaction Rates be

Explained ? p. 586Explained ? p. 586

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OBJECTIVES

Write a rate law using experimental rate-versus-concentration data from a chemical reaction.

Explain the role of activation energy and collision orientation in a chemical reaction.

Describe the effect that catalysts can have on reaction rate and how this effect occurs.

Describe the role of enzymes as catalysts in living systems, and give examples

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KEY TERMS

• rate law• reaction mechanism• order• rate-determining step• intermediate• activation energy• activated complex• catalyst• catalysis• enzyme

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Kagan StructureKagan StructureNumbered Heads TogetherNumbered Heads Together

Answer the following question: Answer the following question: Explain the effect of increasing the Explain the effect of increasing the

concentration on the reaction Rate.concentration on the reaction Rate.

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Rate LawsRate Laws Rate LawRate Law : : The expression that describes the The expression that describes the

relationship between the relationship between the reaction ratereaction rate and the and the concentrations of reactantsconcentrations of reactants..

By studying rate laws , the reaction mechanism can By studying rate laws , the reaction mechanism can be guessed.be guessed.

Reaction mechanism:Reaction mechanism: the way in which a chemical the way in which a chemical reaction takes place , expressed in a series of reaction takes place , expressed in a series of chemical equations chemical equations

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Determining a General Rate Law Equation:Determining a General Rate Law Equation:

For a reaction that involves a single reactantFor a reaction that involves a single reactant : :

Rate = K [ reactant ]Rate = K [ reactant ]nn

The exponent The exponent nn is called the is called the order.order. KK : : is the rate constant. is the rate constant.

Order :Order : A way of classifying chemical reactions depending A way of classifying chemical reactions depending on the number of molecules that enter into the reaction.on the number of molecules that enter into the reaction.

Reaction orders must be found by experimentReaction orders must be found by experiment..

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Rate Law For One ReactantRate Law For One Reactant : : 2HI(2HI(gg) → H2() → H2(gg) + I2() + I2(gg).).

4 = (2)4 = (2)nn

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Kagan Structure “ Rally Coach”Kagan Structure “ Rally Coach”

With your shoulder partner , answer Practice B With your shoulder partner , answer Practice B page 587.page 587.

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Practice B (p.587):

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Rate Laws for Several Reactants:Rate Laws for Several Reactants: Example :Example : Practice : The following data relate to the reaction Practice : The following data relate to the reaction A + B C A + B C find the order with respect to each reactant.find the order with respect to each reactant. Rate = K [ A ]Rate = K [ A ]n1 n1 [ B ] [ B ]n2n2

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1.1. Find two experiments in which all but Find two experiments in which all but one one reactant’s concentration is constant.reactant’s concentration is constant.

2.2. Observe the relationship between concentration Observe the relationship between concentration change and rate change to determine the order change and rate change to determine the order for that reactant.for that reactant.

3.3. Repeat for other reactant (s).Repeat for other reactant (s).

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Select a trial where one reactant Select a trial where one reactant

concentration is held constant concentration is held constant

SO THAT IT CANCELS; SO THAT IT CANCELS;

the the kk’s will also cancel.’s will also cancel.

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To determine reaction order with respect to ATo determine reaction order with respect to A , choose 2 data , choose 2 data

points between which [B] is unchangedpoints between which [B] is unchanged.. (Rate )1(Rate )1 = ( [A] 1) n1 × ( [B] 1 ) n2 = ( [A] 1) n1 × ( [B] 1 ) n2

(Rate )3 ( [A] 3) n1 ( [B] 3 ) n2 (Rate )3 ( [A] 3) n1 ( [B] 3 ) n2

0.012 = ( 0.08 ) n10.012 = ( 0.08 ) n1

0.003 ( 0.04 ) n10.003 ( 0.04 ) n1

4 = 2 n14 = 2 n1

2 = 2 n12 = 2 n1

n1 = 2 n1 = 2

•Rate = K [ A ]n1 [ B ]n2

A + B C

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To determine reaction order with respect to B,To determine reaction order with respect to B, choose 2 data choose 2 data points between which [A] is unchanged .points between which [A] is unchanged .

(Rate )1 (Rate )1 = ( [A] 1 ) = ( [A] 1 ) n1 n1 × ( [B]1 ) × ( [B]1 ) n2 n2

(Rate )2 ( [A] 2) (Rate )2 ( [A] 2) n1n1 ( [B] 2) ( [B] 2) n2 n2

0.012 = ( 0.06 ) 0.012 = ( 0.06 ) n2 n2 0.006 ( 0.03 ) 0.006 ( 0.03 ) n2 n2

2 = 2 n22 = 2 n2

n2 = 1n2 = 1

Rate = K [ A ]Rate = K [ A ]22 [ B ] [ B ]11

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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)

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

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

11 0.080.08 0.0340.034 2.2 x 102.2 x 10-4-4

22 0.080.08 0.0170.017 1.1 x 101.1 x 10-4-4

33 0.160.16 0.0170.017 2.2 x 102.2 x 10-4-4

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

13.2

rate = k [S2O82-][I-]

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Kagan StructureKagan StructureNumbered Heads TogetherNumbered Heads Together

Exp. #Exp. # [NH[NH44++]]oo [NO[NO22¯]¯]oo Initial Rate (M·sInitial Rate (M·s-1-1))

11 0.1000.100 0.00500.0050 1.35 x 101.35 x 10-7-7

22 0.1000.100 0.01000.0100 2.70 x 102.70 x 10-7-7

33 0.2000.200 0.01000.0100 5.40 x 105.40 x 10-7-7

For the following reaction , Determine the rate law and calculate k.

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Rate determining stepRate determining step : : in a multi step chemical reaction is the in a multi step chemical reaction is the step that has the lowest velocity.step that has the lowest velocity.

Intermediate Intermediate :: a substance that forms in a middle stage of a a substance that forms in a middle stage of a reaction and is considered a stepping stone between reactants reaction and is considered a stepping stone between reactants and products.and products.

In the above reaction HOBr is considered as intermediate.

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The experimental rate law for the reaction between NO2 and CO to produce NO and CO2 is rate = k[NO2]2. The reaction is believed to occur via two steps:

Step 1: NO2 + NO2 NO + NO3

Step 2: NO3 + CO NO2 + CO2

• What is the equation for the overall reaction?

NO2+ CO NO + CO2

• What is the intermediate?

NO3

• What can you say about the relative rates of steps 1 and 2?

rate = k[NO2]2 is the rate law for step 1 so step 1 must be slower than step 2

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Zero orderZero order

The change in concentration of reactant has The change in concentration of reactant has no effect on the rate.no effect on the rate.

These are not very common. These are not very common.

General form of rate equation: General form of rate equation:

Rate = kRate = k

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First orderFirst order

DoublingDoubling concentration of reactant concentration of reactant doubles doubles rate. rate.

General form of rate equation: General form of rate equation:

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

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Second orderSecond order

When concentration of When concentration of reactant is doubledreactant is doubled Rate is quadrupled ( increase 4 times) Rate is quadrupled ( increase 4 times) when concentration of when concentration of reactant is tripledreactant is tripled, the , the

rate rate increases by a factor of 9increases by a factor of 9 These are common, particularly in gas-phase These are common, particularly in gas-phase

reactions.reactions. General form of rate equation: General form of rate equation:

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

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Fractional orders are rare!Fractional orders are rare!

Ex. Ex. rate = rate = kk[A][A] mm[B][B]

nn

If If m = 0m = 0 ; reaction is ; reaction is zero orderzero order with respect to A with respect to A If If m = 1m = 1 ; reaction is ; reaction is 1st order1st order with respect to A with respect to AIf If m = 2m = 2 ; reaction is ; reaction is 2nd order2nd order with respect to A with respect to AIf If n = 0n = 0 ; reaction is ; reaction is zero orderzero order with respect to B with respect to BIf If n = 1n = 1 ; reaction is ; reaction is 1st order1st order with respect to B with respect to BIf If n = 2n = 2 ; reaction is ; reaction is 2nd order2nd order with respect to B with respect to B

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Example 1Example 1

Rate Rate == k k[[AA]] [[BB]]

•In this case, the order of reaction with respect to both A and B is 1.• The overall order of reaction is 2 it is found by adding up the individual orders.

Note:  Where the order is 1 with respect to one of the reactants, the "1" isn't written into the equation. [A] means [A]1.

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Kagan Structure “ Rally Coach”Kagan Structure “ Rally Coach”

With your shoulder partner answer the Rally With your shoulder partner answer the Rally Coach sheet.Coach sheet.

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Reaction Pathways and Activation Energy:Reaction Pathways and Activation Energy: For two For two moleculesmolecules to react they to react they must collide violentlymust collide violently

to overcome the repulsion force between their to overcome the repulsion force between their electrons,electrons,

Violent collisions happen only Violent collisions happen only when the colliding when the colliding molecules have large amount of energy.molecules have large amount of energy.

Only molecules with high kinetic energy are likely Only molecules with high kinetic energy are likely to react.to react.

The other molecules must wait until the collision give The other molecules must wait until the collision give them enough kinetic energy.them enough kinetic energy.

Activation energy ( Ea )Activation energy ( Ea ) : : The minimum energy The minimum energy required to start a chemical reaction.required to start a chemical reaction.

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ExampleExample

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Activation Energy Diagrams Model Reaction Activation Energy Diagrams Model Reaction Progress:Progress:

For two molecules to react they must have For two molecules to react they must have enough enough Kinetic EnergyKinetic Energy to change chemically. to change chemically.

With a combined With a combined K.E equal to the activation energy , K.E equal to the activation energy , the molecules reach a state where there is a 50 : 50the molecules reach a state where there is a 50 : 50 chance of either returning to the initial state without chance of either returning to the initial state without reacting ,or react to produce products .reacting ,or react to produce products .

This point is called This point is called Activation complexActivation complex or transition or transition state of the reaction.state of the reaction.

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Reaction Pathways and Activation Energy:Reaction Pathways and Activation Energy: Activation complex :Activation complex : a molecule in an unstable state intermediate a molecule in an unstable state intermediate

to the reactants and the products in the chemical reaction.to the reactants and the products in the chemical reaction.

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Not All Collisions Result in Reaction:Not All Collisions Result in Reaction: Collision between the reacting molecules is necessary but not enough Collision between the reacting molecules is necessary but not enough

((why?) why?) because molecules must collide with enough energy to because molecules must collide with enough energy to overcome the activation energy barrier also the collision must be with overcome the activation energy barrier also the collision must be with correct orientation..correct orientation..

Unless the collision brings the correct atoms close together and in the Unless the collision brings the correct atoms close together and in the proper orientation , the molecules will not react , even if the activation proper orientation , the molecules will not react , even if the activation energy is enough.energy is enough.

Collision between the reacting molecules must have two conditions Collision between the reacting molecules must have two conditions to be successful to be successful

1. Molecules must collide with enough energy to overcome the activation 1. Molecules must collide with enough energy to overcome the activation energy barrier energy barrier

2. Also the collision must be with correct orientation..2. Also the collision must be with correct orientation..

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Numbered Heads TogetherNumbered Heads Together

Answer the following question/Answer the following question/ State the conditions that makes collision State the conditions that makes collision

between the reacting molecules successfulbetween the reacting molecules successful

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Molecular Collisions

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Successful Collision:Successful Collision:

In the above reaction if a chlorine molecule Cl2 collides with the oxygen end of nitrogen monoxide NO the reaction will occur , But if the chlorine molecule collide with the nitrogen end the reaction will not occur.

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Possible Collision Orientations for the Reaction of HPossible Collision Orientations for the Reaction of H2 2

and Iand I22

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Catalysts Increase Reaction Rate:Catalysts Increase Reaction Rate:

Catalyst Catalyst : : a substance that changes the rate of a substance that changes the rate of the reaction without being consumed or the reaction without being consumed or changed.changed.

Catalysis Catalysis :: the acceleration of a chemical the acceleration of a chemical reaction by a catalyst.reaction by a catalyst.

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Example :Example : the decomposition of hydrogen the decomposition of hydrogen peroxide Hperoxide H22OO22

•Two catalysts can be used potassium iodide KI or manganese dioxide MnO2 .

• Catalysis :is widely used in chemical industry specially in petrochemicals to save enormous amount of energy.

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Catalysts Lower the Activation EnergyCatalysts Lower the Activation Energy:: How catalysis works?How catalysis works? By making different pathway By making different pathway

available between the reactants and the products available between the reactants and the products with lower with lower activation energyactivation energy than uncatalyzed than uncatalyzed reactionreaction..

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ExampleExample : :The decomposition of hydrogen The decomposition of hydrogen

peroxide Hperoxide H22OO22 • It can be catalyzed by two different catalysts KI • second catalyst manganese dioxide MnO2.

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Enzymes Are Catalysts found In Nature:Enzymes Are Catalysts found In Nature:

Enzyme Enzyme : : a type of protein that speeds metabolic a type of protein that speeds metabolic reactions in plants and animals without being reactions in plants and animals without being permanently changed or destroyed.permanently changed or destroyed.

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Section 2 Review Questions (p. 595):Section 2 Review Questions (p. 595):