CM1201 Lecture 4 2016 Pemble
-
Upload
lucy-byrne -
Category
Documents
-
view
218 -
download
0
Transcript of 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
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
2/21
2
Kinetics 4 - Aims
How catalysts work
Catalysis in biology, in the atmosphereand in industry
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
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
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
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
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
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)
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
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!
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
7/21
7
How do catalysts work?
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
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
8/21
8
How do catalysts work?
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
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
9/21
9
How do catalysts work 2?
Highlighting the alternative pathways
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
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?
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
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
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
12/21
12
Biological Catalysts
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)
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
13/21
13
Biological Catalysts
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
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
14/21
14
Biological Catalysts
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
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
15/21
15
Biological Catalysts
Combination of the
enzyme and the substrate
is called the
enzyme-substratecomplex
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
16/21
16
Biological Catalysts
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 -
7/24/2019 CM1201 Lecture 4 2016 Pemble
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
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
18/21
18
Atmospheric catalysts
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
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
19/21
19
Atmospheric catalysts
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
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
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
-
7/24/2019 CM1201 Lecture 4 2016 Pemble
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