CM1201 Lecture 4 2016 Pemble

7/24/2019 CM1201 Lecture 4 2016 Pemble

1/21

1

k= Ae

-Ea/RT

k= A exp[-Ea/RT]

lnk= lnA -Ea/RT

Kinetics 4 Arrhenius and

Catalysis

The Key Equation


2/21

2

Kinetics 4 - Aims

How catalysts work

Catalysis in biology, in the atmosphereand in industry


3/21

3

Catalysis

A catalyst is a substance that changes the

speed (rate) of a chemical reaction without

undergoing any permanent change itself

Catalysts are very common; most reactions in

the human body, the atmosphere, the oceansor in industrial chemistry occur with the help

of catalysts


4/21

4

Catalysis

Catalysts are not used up during a

reaction but may be changed physicallyegsolid catalysts sometimes crumble

Only small amounts are needed

Catalysts do not affect the amount of

product formed; only the rate of

formation


5/21

5

Examples of catalysts

1. Biological: protein molecules called enzymes

2. Atmospheric: radicals affecting the ozone

concentration

3. Industrial: Pt metal used in the petroleum industry

Catalysts are either homogeneous (in the same phase

as the reactant) or heterogeneous (in a different

phase from the reactant)


6/21

6

How do catalysts work?

Arrhenius equation: k= Ae-Ea/RT

The rate coefficient kis determined by the activation

energy (Ea) and the frequency factor (A)

A catalyst can affect the rate of a chemical reaction

by altering the value of Eaor A

Most dramatic catalytic effects come from lowering

the Eavalue

Generally a catalyst lowers the overall activation

energy for a chemical reaction- care needed here!


7/21

7


Chemical reactions involve bond breaking

and bond making

Bond breaking is an endothermic process

(requires energyamount Ea)

Catalysts provide an alternative pathway forthe bond-breaking process, which have lower

Eas


8/21

8


0

1

2

3

4

5

6

7

8

9

0 5 10 15 20

Reaction Pathway

Energy(kJm

ol-1)

Uncatalysed rxn

Catalysed rxn


9/21

9

How do catalysts work 2?

Highlighting the alternative pathways


10/21

10

Semantic argument?

Does a catalyst really work by decreasing

the activation energy of the reaction?

OR

Does a catalyst work by providing a new

route for the reaction that has a lower

activation energy?


11/21

11

Biological Catalysts

All the systems in the human body must occur at

carefully controlled rates in order to maintain

human life

Enzymes are an example of biological catalysts

that are necessary for the reactions in the human

body to occur at suitable rates

Most enzymes are large protein molecules

Very selective in the reactions they catalyse


12/21

12


Decomposition of H2O2is an important biological process

H2O2is strongly oxidising and can be very harmful

Many enzymes can catalyse the decomposition of H2O2

into H2O and O2 egbromide ion (Br-(aq))

Br-(aq) reacts with H2O2in acidic solution, forming

aqueous bromine and water:

2Br-(aq) + H2O2(aq) + 2H+ Br2(aq) + 2H2O(l)

H2O2also reacts with the Br2(aq) generated

Br2(aq) + H2O2(aq) 2Br-(aq) + 2H++ O2(g)


13/21

13


The overall reaction is

2H2O2(aq) 2H2O(l) + O2(g)

H2O2is completely decomposed and Br-(aq) is the

catalyst

It speeds up the reaction without itself undergoing

any net change

Br2is a reaction intermediate because it is first

formed and then consumed


14/21

14


Catalase is another enzyme that will speed

up the decomposition of H2O2

The enzyme is a large molecule but the

reaction is catalysed at a very specific

region/ location in the enzyme called the

active site

The substances that undergo reaction at

this site are called substrates


15/21

15


Combination of the

enzyme and the substrate

is called the

enzyme-substratecomplex


16/21

16


The substrate weakly interacts with the active

site. A product is formed. The initial weak

bond formed is easily broken and the enzyme

is regenerated

Enzymes are sensitive to both pH and

temperature. Most are killed off at

temperatures > 60

Enzymes are homogeneous catalysts

https://www.youtube.com/watch?v=CZD5xsOKres
https://www.youtube.com/watch?v=CZD5xsOKreshttps://www.youtube.com/watch?v=CZD5xsOKres


17/21

17

Atmospheric catalysts

O3in our upper atmosphere is very important as

it helps to filter out harmful UV radiation

Chlorofluorcarbons (CFCs) are inert compounds

which can rise to the upper atmosphere where

they are then broken down by light (photolysis) to

make Cl radicals

These Cl radicals can react with ozone causing

its destruction


18/21

18


1. CF2Cl2(g) + light CF2Cl(g) + Cl(g)

2. Cl(g) + O3(g) ClO(g) + O2(g)

This ClO can then react with O atoms to give

Cl and O2

The Cl radical is therefore regenerated and the

cycle is known as a catalytic cycle


19/21

19


The rate at which ozone is destroyed

increases as the quantity of Cl atoms

increase

The greater amount of CFCs that

diffuse in to the stratosphere, the faster

the destruction of the ozone layer


20/21

20

Industrial catalysts

Often HETEROGENEOUSin nature

The octane number (ON) of a gasoline is a

measure of its resistance to knocking: high

ON burn more smoothly and more effective

fuels

Example: High octane ratings in fuels are

related to : (i) length of the hydrocarbon chain

(ii) branched structure


21/21

21

Industrial catalysts

Pentane has an octane number, ON = 62,

whereas 2-methyl butane hasON= 93

CH3

CH2

CH2

CH2

CH3

CH3

C(H)CH2

CH3

The rate of the isomerization between the twois increased in the presence of Pt-metal

CH3

Catalysis- a good summary of the things we need to know:

https://www.youtube.com/watch?v=KYD5LNVWne8
https://www.youtube.com/watch?v=KYD5LNVWne8https://www.youtube.com/watch?v=KYD5LNVWne8

Documents

CM1201 Lecture 4 2016 Pemble