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Ho Chi Minh City University of Technology

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PHYSICAL CHEMISTRY IIChapter 1 - Introduction

Dr. Ngo Thanh AnEmail: [email protected]

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References

Peter Atkins and Julio de Paula, Physical chemistry, 8 th Ed.1

Robert G. Mortimer, Physical chemistry, 3 rd Ed.2

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3

1. Concepts

Does a reaction take place?

How fast does a reaction proceed?

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Initial Rates Rates measured at the beginning of the reaction, which is

dependent on the initial concentrations of reactants. Instantaneous Rates

Rates measured at any point during the reaction. Average Rates

An overall rate measured over a period or time interval.

1. Concepts

Reaction rate is the change in the concentration of a reactant or a product with time

(M /s).

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The detail pictures of how a given reaction occurs atmolecular level

It consists of a set of elementary steps that shows probable

reactions involving molecular species including reactionintermediates . The sum of these elementary steps yields the overall

balanced equation for the reaction.

1. Concepts

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For example, the overall reaction:2A + B C + D

may involves the following elementary steps in its mechanism: Step-1: A + B X; Step-2: X + A Y; Step-3: Y C + D Overall reaction: 2A + B C + D;

1. Concepts

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Molecularity is the number of molecular species that react inan elementary process.

Rate Law for Elementary Processes :

Elementary Reactions Molecularity Rate Law

A product Unimolecular Rate = k [A] 2A product Bimolecular Rate = k [A] 2

A + B product Bimolecular Rate = k [A][B] 2A + B product Termolecular Rate = k [A] 2[B]

1. Concepts

The molecularity of a process tells how many molecules are involved in the process.

The rate law for an elementary step is written directly from that step.

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Elementary step: NO + NO N 2O 2

Elementary step: N 2O 2 + O 2 2NO 2 Overall reaction: 2NO + O 2 2NO 2

+

In t e rmed ia t e s are species that appear in a reaction mechanismbut not in the overall balanced equation.

An intermediate is always formed in an early elementary step and

consumed in a later elementary step.

1. Concepts

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The rate determining step is the slowest elementary step in amechanism, and the rate law for this step is the rate law for theoverall reaction.

The (determined) rate law is, rate = k [NO2] [F

2], for the reaction:

2 NO 2 (g) + F 2 (g) 2 NO 2F (g),and a two-step mechanism is proposed:

i NO 2 (g) + F 2 (g) NO 2F (g) + F (g)ii NO 2 (g) + F (g) NO 2F (g)

Which is the rate determining step?A n s w e r :The rate for step i is rate = k [NO 2] [F2], which is the rate law, thissuggests that step i is the rate-determining or the s-l-o-w step.

1. Concepts

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dt

dN.

V

1W i + N i : product

- N i : reactant

If V = const, theni

i

N

C V

Thereforei

V const

dC W

dt

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Rate of change of concentration with time

2. Rate of chemical reaction

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2 Fe 3+(aq) + Sn 2+ 2 Fe 2+(aq) + Sn 4+(aq)

t = 38.5 s [Fe 2+] = 0.0010 M

t = 38.5 s [Fe 2+] = (0.0010 0) M


Rate of formation of Fe 2+ = = = 2.6x10 -5 M s -1 [Fe 2+]

t

0.0010 M38.5 s

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Rates of reactions can be determined by monitoring thechange in concentration of either reactants or products as afunction of time. [A] vs t


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A B

rate = -

[A]

t

rate =[B]

t

time


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In this reaction, theconcentration of butylchloride, C4H9Cl , was

measured at varioustimes, t.

C 4H9Cl (aq ) + H 2O (l ) C4H9OH (aq ) + HCl (aq ) [C 4H9Cl] M


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The average rate of thereaction over each intervalis the change in

concentration divided by thechange in time:

C 4H9Cl(aq ) + H 2O (l ) C 4H9OH (aq ) + HCl (aq ) Average Rate, M/s


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Note that the average ratedecreases as the reactionproceeds.

This is because as thereaction goes forward,there are fewer collisionsbetween reactantmolecules.

C 4H9Cl(aq ) + H 2O (l ) C 4H9OH (aq ) + HCl (aq )


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The reaction slows downwith time because theconcentration of the

reactants decreases.

C 4H9Cl(aq ) + H 2O (l ) C 4H9OH (aq ) + HCl (aq )


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Br2 (aq ) + HCOOH (aq ) 2Br- (aq ) + 2H+ (aq ) + CO2 (g)

time

393 nmlight

Detector

[Br2] a Absorption

3 9 3 n m

Br2 (aq )

20


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In this reaction, the ratio ofC

4H

9Cl to C

4H

9OH is 1:1.

Thus, the rate ofdisappearance of C 4H9Cl isthe same as the rate ofappearance of C 4H9OH .

C4H9Cl(aq ) + H 2O (l ) C4H9OH (aq ) + HCl (aq )

Rate = - [C4H9Cl]t =[C4H9OH]

t


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To generalize, for the reaction

a A + bB c C + d D

Reactants (decrease) Products (increase)


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General reaction:

a A + bB c C + d D

dt

Dd

d dt

C d

cdt

Bd

bdt

Ad

aW

][1][1][1][1


Write down the rate of reaction:

CH 4 (g ) + 2O 2 (g ) CO 2 (g ) + 2H 2O (g )

W = -d[CH4]

dt = -

d[O 2]d t

12

=d[H2O]

dt 12

=d[CO2]

dt

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Rate of conversion related to the advancement of thereaction, .

dt d

V 1 = reaction of rate

V = solution volume


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Examine the following reaction2 N 2O 5 (g) 4 NO 2 (g) + O 2 (g)

N2O 5 NO 2 O 2Initial n 0 0

Change -2 +4 +

Final n - 2 4


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Each reaction has its own equation that gives its rate as a

function of reactant concentrations.This is called its Rate LawTo determine the rate law we measure the rate at differentstarting concentrations.


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Compare Experiments 1 and 2:when [NH 4+] doubles , the initial rate doubles .


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Likewise, compare Experiments 5 and 6:when [NO 2-] doubles, the initial rate doubles.


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This equation is called the rate law , and k is the rate constant .


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A rate law shows the relationship between the reaction rate andthe concentrations of reactants. For gas-phase reactants use P A instead of [A].

k is a constant that has a specific value for each reaction. The value of k is determined experimentally.

Constant

is relative here -

k is unique for each reactionk changes with T

3. Rate law

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The concentrations of the products do not appear in the rate

law because the reaction rate is being studied underconditions where the reverse reaction does not contribute tothe overall rate.

The value of the exponent n must be determined byexperiment; it cannot be written from the balanced equation.

3. Rate law

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An expression or equation that relates the rate of reaction tothe concentrations of reactants at constant temperature.

For the reaction:R1 + R 2 + R 3 Products

Rate = k [R1] x [R2]y [R3]z

Where k = rate constant ; x , y , and z are the rate orders withrespect to individual reactants. Rate orders are determinedexperimentally.

3. Rate law

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This reaction isFirst-order in [NH 4+]First-order in [NO 2

] The overall reaction order can be found by adding the

exponents on the reactants in the rate law. This reaction is second-order overall .

3. Rate law

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Determine the rate law and calculate the rate constant for the

following reaction from the following data:S 2O 82- (aq ) + 3I - (aq ) 2SO 42- (aq ) + I 3- (aq )

Experiment [S 2O 82-] [I-]Initial Rate

(M /s)

1 0.08 0.034 2.2 x 10-4

2 0.08 0.017 1.1 x 10 -4

3 0.16 0.017 2.2 x 10 -4

rate = k [S 2O 82-] x [I-]y

Double [I-], rate doubles (experiment 1 & 2)Double [S 2O 82-], rate doubles (experiment 2 & 3)

y = 1 x = 1

k =rate

[S 2O 82-][I-]=

2.2 x 10 -4 M /s

(0.08 M )(0.034 M )= 0.08/ M s

rate = k [S 2O 82-][I-]

3. Rate law

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4. Factors that affect reaction rate

Concentration of Reactants As the concentration of reactants increases, so does the

likelihood that reactant molecules will collide. Temperature

At higher temperatures, reactant molecules have morekinetic energy, move faster, and collide more often andwith greater energy.

Catalysts Speed reaction by changing mechanism.

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Molecules must collide with the correct orientation and with

enough energy to cause bond breakage and formation.


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