Presentation by

Department of Chemistry,J. M. PATEL ARTS, COMMERCE AND SCIENCE COLLEGE,

BHANDARA – 441 904 (M.S.)

“CATALYSIS”

Subject : CHEMISTRY

CH 102: PHYSICAL CHEMISTRY

SEMESTER I

Dr. Girdharilal Balkishan TiwariAssistant Professor

12/19/2015

Points to be covered

INTRODUCTION

HOMOGENEOUS CATALYSIS

HETEROGENEOUS CATALYSIS

ACTION OF CATALYTIC PROMOTERS

INHIBITORS

ENZYME CATALYSIS

AUTOCATALYSIS

KINETICS OF ENZYME CATALYSED REACTIONS – MICHAELIS-MENTEN EQUATION

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CATALYSIS

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INTRODUCTION

CATALYSTSubstances, which accelerate the rate of a chemical reaction and

themselves remain chemically and quantitatively unchanged after the reaction,

are known as catalysts.

For example, MnO2 acts as a catalyst for the following reaction

The phenomenon of increase in the rate of a reaction that results from the

addition of a catalyst is known as catalysis.

In Greek, kata = wholly, lein = to loosen

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According to intermediate complex theory, a catalyst participates in a chemicalreaction by forming temporary bonds with the reactant resulting in an intermediatecomplex which decomposes to yield product and the catalyst.It is believed that the catalyst provides an alternative pathway or reactionmechanism by reducing the activation energy between reactants and productsand hence lowering the potential energy barrier, and the reaction rate isincreased . Catalyst does not affect the energy difference ∆H between the productsand reactants. It is clear from Arrhenius equation, lower the value of activation energy(Ea) faster will be the rate of a reaction (Note: Arrhenius equation is K = A e-Ea/RT)

∆H

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CHARACTERISTICS OF CATALYSIS

1. Catalyst remains chemically and quantitatively unchanged after the reaction.However, it may undergo a physical change.

2. A small amount of the catalyst can catalyse a large amount of reactants. Forexample, 1 g of metallic platinum is sufficient to decompose 108 liters of H2O2.

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3. The activity of a solid catalyst is enhanced with increase in its surfacearea. Thus, finely divided nickel is a better catalyst than lumps of nickel.

4. Catalyst cannot initiate a reaction. In most cases, it accelerates thereaction already in progress.

while, some catalysts are required in relatively large amount to be effective. Forexample, In Friedel-Crafts reaction, anhydrous AlCl3 catalyst is required to theextent of 30% of the mass of benzene.

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5. Catalyst is specific in its action. while a particular catalyst can be used forone reaction, it will not necessarily work for another reaction. example,decomposition of KClO3 is catalyzed by MnO2 but not by Platinum. Sometimes,for the same substrate different catalyst yield different products.

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6. Maximum activity of a catalyst is obtained at a particular temperature called asoptimum temperature.

7. Catalyst does not change the equilibrium constant of a reaction. It helps inattaining the equilibrium faster, that is, it catalyses the forward as well thebackward reactions to the same extent so that the equilibrium state remainssame but is reached earlier.

8. Catalyst does not change the enthalpy of reaction (∆H), i.e. it does not affectthe energy difference ∆H between the products and reactants.

9. A catalyst does not alter Gibbs energy, ∆G of a reaction. It catalyses thespontaneous reactions but does not catalyse non-spontaneous reactions.

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HOMOGENEOUS AND HETEROGENEOUS CATALYSIS

1. Homogeneous catalysis:When the reactants and the catalyst are in the same phase (i. e. liquid or gas)and are evenly distributed throughout, the process..

Homogeneous catalysis in liquid phaseHomogeneous catalysis in gas phase

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2. Heterogeneous catalysis:Catalytic process in which the reactants and the catalyst are in different phases .The catalyst is usually a solid, and the reactants are either gases or liquids.It is the most important type of catalysis in industrial chemistry.

It is also used in catalytic converters in automobiles.

A. With gaseous reactants B. With liquid reactants

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At high temperatures inside a car’s engine, N2(g) and O2(g) react to form

NO(g) (Nitric oxide).

When released into atmosphere, NO(g) rapidly combines with O2(g) to form NO2(g).

NO2(g) and other gases such as CO(g) emitted by automobiles, and unburned

hydrocarbons, make automobile exhaust a major source of air pollution.

Most new cars are equipped with catalytic converters.

An efficient catalytic converter serves two purposes:

1. It oxidizes CO(g) and unburned hydrocarbons to CO2(g) and H2O(g)

2. It converts NO(g) and NO2(g) to N2(g) and O2(g).

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Hot exhaust gases into which air has been injected are passed through the firstchamber of one converter to accelerate the complete burning ofhydrocarbons and to decrease CO(g) emissions.

Because high temperatures increase NO production, however, a secondchamber containing a different catalyst (a transition metal or a transitionmetal oxide such as CuO or Cr2O3 and operating at a lower temperaturedissociates NO into N2 and O2 and the exhaust is discharged through the tailpipe.

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ACTION OF CATALYTIC PROMOTERS or ACTIVATORSSubstances that enhance activity of a catalyst. Act as catalyst for the catalyst.

MECHANISM OF ACTION:2.Change in catalyst spacing:

The lattice spacing is increased thus,enhancing the space between the catalystparticles. The bonds between themolecules of the reactants (For e.g. H2)are weakened and cleaved. This makesthe reaction go faster.

For e.g., In Haber’s process for manufacture ofNH3(g), Mo(s) acts as promoter for Fe(s) catalyst.

1. Increase of active sites:No. of peaks and cracks oncatalyst surface is increased onaddition of promoter to thecatalyst. This increasesconcentration of reactants andrate of reaction.

increasedspacingLattice

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INHIBITORS OR CATALYTIC POISONS:Substances that decrease the activity of a catalyst or make the catalyst inactive.Arsenius oxide and HCN are two of the most powerful catalytic poisons. Some examples of catalytic poisonsa. Arsenius oxide poisons catalytic activity of platinized asbestos in the manufacture of

H2SO4 by contact process.

Pt

Poisoning of platinum catalyst by CO

Pt Pt Pt Pt Pt

b. Platinum catalyst used in the oxidation of H2 is poisoned by CO.

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AUTO CATALYSIS

When one of the products formed during the course of reaction itself act asa catalyst for that reaction the phenomenon is called as Autocatalysis.In normal reaction, the rate of reaction decreases with the passage of time.In autocatalysis, as the reaction proceeds, concentration of catalytic productincreases and so the rate of reaction increases.

In the titration of H2C2O4 with acidified KMnO4, the reaction is slow in thebeginning but becomes fast as the reaction progresses. MnSO4 or Mn2+ ionsproduced during the reaction acts as autocatalyst for the reaction. As theconcentration of Mn2+ ions increases with time, the rate of reaction increaseswith time. Hence, the time required for decolourisation of first drop ofKMnO4 is much higher and it goes on decreasing with time.

Examples:

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ENZYME CATALYSIS Enzymes are complex nitrogenous organic compounds produced by living

plants and animals which facilitate the reactions, but unlike other catalystsare not consumed in reactions.

They are typically proteins or RNA. Numerous reactions that occur in the bodies of animals and plants to maintain the

life process are catalysed by enzymes. The enzymes are, thus, termed as biochemical catalysts and the phenomenon is

known as biochemical catalysis. The first enzyme was synthesised in the laboratory in 1969.

In stomach, the pepsin convertsproteins into peptides while in intestine,the pancreatic trypsin convertsproteins into amino acids by hydrolysis.Conversion of milk into curd is anenzymatic reaction brought about bylacto bacilli enzyme present in curd.

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1. One molecule of an enzyme may transform one million molecules of thereactant per minute.

2. Each enzyme is specific for a given reaction, i.e., one catalyst cannot catalysemore than one reaction.For example, the enzyme urease catalyses the hydrolysis of urea only.

3. The rate of an enzyme reaction become maximum at a definite temperature,called the optimum temperature. On either side of the optimum temperature, theenzyme activity decreases. The optimum temperature range for enzymatic activityis 298-310 K. Human body temperature being 310 K is suited for enzyme-catalysedreactions.4. The rate of an enzyme-catalysed reaction is maximum at a particular pH called

optimum pH, which is between pH values 5-7.5. The enzymatic activity is increased in the presence of certain substances, known

as co-enzymes.6. The inhibitors interact with the active functional groups on the enzyme surface

and often reduce or completely destroy the catalytic activity of enzymes.

CHARACTERISTICS OF ENZYME CATALYSIS

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MECHANISM OF ENZYME CATALYSIS

There are a number of cavities present on the surface of enzymes. These cavitiesare of characteristic shape and possess active groups like -NH2, -COOH, -SH, -OH,etc. These are actually the active centres on the surface of enzyme particles.Molecules of the substrate, which have complementary shape, fit into thesecavities just like a key fits into a lock.

Step II: Decomposition of the enzymesubstrate (ES) complex to formproduct (P)

Step I: Binding of enzyme (E) tosubstrate (S) forms enzymesubstrate complex (ES)

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KINETICS OF ENZYME CATALYSED REACTIONS - MICHAELIS-MENTEN EQUATION

In 1913, Michaelis and Menten proposed a mechanism for the kinetics of enzymecatalyzed reaction. It involves two stepsStep I: The enzyme (E) reacts with the substrate (S) to form enzyme-substratecomplex (ES). This is a reversible process and equilibrium is established rapidly.

Step II: The ES complex undergoes dissociates to give product (P) and enzyme (E)

Second step is the slow and rate determining step.Thus, Rate of reaction = - d[S] = d[P] = K2[ES] .....(3)

dt dtBut, [ES] is not experimentally measurable quantity.

Solve for intermediate [ES], d[ES] = K1[E][S] – K-1[ES] – K2[ES]dt

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Use of steady state approximation for [ES], We get, d[ES] = 0dt

d[ES] = K1[E][S] – K-1[ES] – K2[ES] = 0 …..(4)dt

Instead of solving for [ES] in terms of Free enzyme [E], solve for [ES] interms of Total enzyme [E]0.

Total enzyme [E]0 = Free enzyme [E] + Bound enzyme [ES]

[E] = [E]0 – [ES] …..(5)Replace [E] = [E]0 – [ES] from equation (5) in equation (4)

d[ES] = K1{[E]0 –[ES]} [S] – K-1[ES] – K2[ES] = 0dt

d[ES] = K1[E]0 [S] – K1[ES] [S] – K-1[ES] – K2[ES] = 0dt

Rearrange [ES] terms to one side of the equationK1 [ES] [S] + K-1 [ES] + K2 [ES] = K1 [E]0 [S][ES] {K1 [S] + K-1 + K2} = K1 [E]0 [S]

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Dividing the numerator and denominator by K1[ES] = K1 [E]0 [S] …..(6)K-1 + K2 + K1[S]

[ES] = K1 [E]0 [S] ÷ K1 = K1 [E]0 [S]{ K-1 + K2 + K1 [S]} ÷ K1 K1 { K-1 + K2 + K1 [S]} ÷ K1

[ES] = K1 [E]0 [S] = [E]0 [S] …..(7)K1 { K-1 + K2 + K1 [S]} { K-1 + K2} + {K1 [S]}

K1 K1 K1

Introduce new term Km (Michaelis-Menten constant) in equation (7)

Where, Km = K-1 + K2

K1

[ES] = [E]0 [S] …..(8) ( Since, K-1 + K2 = Km )[S] + Km K1

Substitute [ES] into equation (3)Rate of product formation = d[P] = K2[ES] = K2 [E]0 [S] …..(8)

dt [S] + Km

Michaelis-Menten equation

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When [S] >> Km, then neglecting Km as compared to [S]

Rate of product formation = K2 [E]0 [S] = K2 [E]0 This is called Vmax

[S] + Km (small and is neglected)

Maximal rate = Vmax = K2 [E]0

When [S] = Km, thenRate of product formation = K2 [E]0 [S] = K2 [E]0 [S] = 1 K2 [E]0 Half maximal rate

[S] + [S] 2[S] 2

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Thank you

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