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10/10/2012 Laju Reaksi - by Nuniek UNESA 1 Reaction Rates Reaction Rates By : Nuniek Herdyastuti Oktober- 2012 Combustion Combustion Corrosive Corrosive Before reaction Before reaction After 10 s After 10 s 4 Definition of Reaction Rate Thermodynamics – does a reaction take place? Kinetics – how fast does a reaction proceed? Reaction rate is the change in the concentration of a reactant or a product with time (M/s). A B rate = - [A] t rate = [B] t [A] = change in concentration of A over time period t [B] = change in concentration of B over time period t
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• 10/10/2012

Laju Reaksi - by Nuniek UNESA 1

Reaction RatesReaction Rates

By : Nuniek HerdyastutiOktober - 2012

CombustionCombustion CorrosiveCorrosive

Before reactionBefore reaction After 10 sAfter 10 s

4

Definition of Reaction Rate

Thermodynamics does a reaction take place?

Kinetics how fast does a reaction proceed?

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

A B

rate = -[A]t

rate = [B]t

[A] = change in concentration of A overtime period t

[B] = change in concentration of B overtime period t

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A B

rate = -[A]t

rate = [B]t

time

6

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

time

393 nmlight

Detector

[Br2] a Absorption

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

average rate = -[Br2]

t= -

[Br2]final [Br2]initial

tfinal - tinitial

slope oftangent

slope oftangent

slope oftangent

Faktor yang Mempengaruhi Laju Reaksi

KonsentrasiMolekul - molekul harus bertumbukan agar terjadi reaksidalam konteks ini laju reaksi proporsional dengankonsentrasi reaktanLuas permukaan zatMakin luas permukaan, maka tumbukan makin banyak, sehingga reaksi makin cepatSuhumolekul harus bertumbukan dengan energi yang cukupuntuk bereaksiPenambahan katalis

Katalis dapat menurunkan energi aktivasi (energi minimumyang diperlukan agar suatu reaksi kimia dapatberlangsung. Penambahan katalis akan mempercepatreaksi

• 10/10/2012

Laju Reaksi - by Nuniek UNESA 3

Effect of reactant concentrations on rate of reactionsodium thiosulfate, Na2S2O3, in acidic solution and sodium arsenite, Na3AsO3

laju = k [Br2]

k = laju

[Br2]= konstanta laju

= 3,50 x 10-3 s-1

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The Rate LawThe rate law expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers.

a A + b B c C + d D

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

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

rate = k [F2][ClO2]

Rate laws are always determined experimentally

Reaction order is always defined in terms of reactant (not product) concentrations

The order of a reactant is not related to the stoichiometriccoefficient of the reactant in the balanced chemical equation

1

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Laju Reaksi - by Nuniek UNESA 4

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

rate = k [F2]x[ClO2]y

[F2] meningkat dua kali dan [ClO2] konstan

Laju meningkat dua kali x = 1

[ClO2] meningkat empat kali dan [F2] konstan

Laju meningkat empat kali y = 1

rate = k [F2][ClO2]

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Menentukan hukum laju dan menghitung konstanta lajureaksi dari data berikut ini:S2O82- (aq) + 3I- (aq) 2SO42- (aq) + I3- (aq)

Percobaan [S2O82-] [I-]Laju Awal

(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

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

[I-] meningkat dua kali, laju menjadi dua kali (percobaan 1 & 2)

y = 1

[S2O82-] meningkat dua kali, laju menjadi dua kali (percobaan 2 & 3)

x = 1

k = laju

[S2O82-][I-]=

2,2 x 10-4 M/s

(0,08 M)(0,034 M)= 0,08/M s

laju = k [S2O82-][I-]

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Menentukan Orde Reaksi dengan Percobaan

Jika diketahui reaksi : O2 (g) + 2 NO(g) 2 NO2 (g)Hukum laju untuk reaksi ini adalah : laju = k [O2]m[NO]n

Tentukan persamaan laju reaksinya !

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PercobaanKonsntrasi Reaktan awal

(mol / L) Laju awal(mol/L . s)O2 NO1 1,10 x 10-2 1,30 x 10-2 3,21 x 10-32 2,10 x 10-2 1,30 x 10-2 6,40 x 10-33 1,10 x 10-2 2,60 x 10-2 12,8 x 10-34 3,30 x 10-2 1,30 x 10-2 9,60 x 10-35 1,10 x 10-2 3,90 x 10-2 28,8 x 10-3

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First Order Rate Law

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

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Second Order Reactions

A product

rate = - [A]t = k [A]

2

1[A] =

1[A]0

+ kt

- d[A][A]2 = k dt

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Zero Order Reactions

A product rate = -[A]

t

= k [A]0 = k

[A]

t= k-

[A] = [A]0 - kt

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t = t when [A] = [A]0/2

ln[A]0

[A]0/2

k=t

ln 2

k= 0.693

k=

Half Life of a Reaction (t)

The half-life, t, is the time required for the concentration of a reactant to decrease to half of its initial concentration

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Summary of the Kinetics of Zero Order, First Orderand Second Order Reactions

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TEORI TUMBUKAN1. Reaksi kimia terjadi sebagai hasil dari

tumbukan antara molekul-molekul yang bereaksi

2. Agar dapat bereaksi, molekul-molekulyang bertumbukan harus memilikia. arah (orientasi) yang tepatb. energi kinetik totalnya sama dengan

atau lebih besar daripada energi aktivasi,Ea, yaitu energi minimum yang dibutuhkanuntuk memulai suatu reaksi kimia.

Spesi yang terbentuk sementara olehmolekul-molekul reaktan akibat tumbukansebelum nantinya akan membentuk produkdisebut keadaan transisi atau kompleks teraktifkanDalam keadaan transisi, ikatan-ikatan yang lama sedang mengalami proses pemutusan danikatan-ikatan yang baru mulai terbentuk. Energi yang dibutuhkan untuk mencapai keadaantransisi disebut energi aktivasi

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Exothermic Reaction Endothermic Reaction

The activation energy (Ea ) is the minimum amount of energy required to initiate a chemical reaction.

A + B AB C + D++

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Temperature Dependence of the Rate Constant

Ea is the activation energy (J/mol)

R is the gas constant (8.314 J/K mol)

T is the absolute temperature

A is the frequency factor

(Arrhenius equation)

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Reaction Mechanisms

The overall progress of a chemical reaction can be represented at the molecular level by a series of simple elementary steps or elementary reactions

The sequence of elementary steps that leads to product formation is the reaction mechanism.

2NO (g) + O2 (g) 2NO2 (g)

N2O2 is detected during the reaction!

Elementary step: NO + NO N2O2Elementary step: N2O2 + O2 2NO2

Overall reaction: 2NO + O2 2NO2

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Elementary step: NO + NO N2O2

Elementary step: N2O2 + O2 2NO2

Overall reaction: 2NO + O2 2NO2

+

Intermediates are species that appear in a reaction mechanism but not in the overall balanced equation.

An intermediate is always formed in an early elementary step and consumed in a later elementary step

The molecularity of a reaction is the number of molecules reacting in an elementary step

Unimolecular reaction elementary step with 1 molecule

Bimolecular reaction elementary step with 2 molecules

Termolecular reaction elementary step with 3 molecules

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Unimolecular reaction A products rate = k [A]

Bimolecular reaction A + B products rate = k [A][B]

Bimolecular reaction A + A products rate = k [A]2

Rate Laws and Elementary Steps

Writing plausible reaction mechanisms:

The sum of the elementary steps must give the overall balanced equation for the reaction.

The rate-determining step should predict the same rate law that is determined experimentally.

The rate-determining step is the slowest step in the sequence of steps leading to product formation.

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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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A catalyst is a substance that increases the rate of a chemical reaction without itself being consumed

k = A . exp( -Ea / RT ) Ea k

ratecatalyzed > rateuncatalyzed

Ea < Ea

Uncatalyzed Catalyzed

Katalis adalah zat yang meningkatkan laju reaksitanpa ikut terpakai dalam reaksi

Katalis bekerja dengan cara mengubah mekanisme.Mekanisme yang baru ini mempunyai energi aktivasiyang rendah

Katalis mempercepat reaksi baik ke arah kirimaupun kanan dan tidak dapat meningkatkan hasilsetimbang akhirnya, namun dapat mempercepattercapainya keadaan setimbang akhir

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In heterogeneous catalysis, the reactants and the catalysts are in different phases.

In homogeneous catalysis, the reactants and the catalysts are dispersed in a single phase, usually liquid.

Haber synthesis of ammonia

Ostwald process for the production of nitric acid

Catalytic converters

Acid catalysis

Base catalysis

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N2 (g) + 3H2 (g) 2NH3 (g)Fe/Al2O3/K2O

catalyst

Haber Process

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Enzyme Catalysis

See You !!!!!!

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