1 Chapter 25Alkanes and Cracking 25.1Alkanes 25.2The petrochemical industry 25.3Cracking in...
-
Upload
christiana-lite -
Category
Documents
-
view
216 -
download
1
Transcript of 1 Chapter 25Alkanes and Cracking 25.1Alkanes 25.2The petrochemical industry 25.3Cracking in...
1
Chapter 25 Alkanes and Cracking
25.1 Alkanes
25.2 The petrochemical industry
25.3 Cracking in petrochemical industry
25.4 Cracking in laboratory
25.5 Household gaseous fuels
CONTENTS OF CHAPTER 25
2
25.1 ALKANES
INTRODUCING ALKANES
Petroleum and natural gas contain lots of hydrocarbons, most of
which are alkanes.
ALKANES are hydrocarbons with the general formula CnH2n+2.
25.1 ALKANES
Figure 25.1 Space-filling models of methane, ethane, propane and butane.
3
PHYSICAL PROPERTIES OF ALKANES
Table 25.1 Physical properties of some straight-chain alkanes.
25.1 ALKANES
4
Melting point, boiling point and density
Figure 25.2
Some useful alkanes (or mixtures of
alkanes):
(a) Butane gas (liquefied) in a gas
lighter
(b) Baby oil
(c) Candle wax.
25.1 ALKANES
5
Viscosity
The viscosity of liquid alkanes increases with a greater
number of carbon atoms.
Solubility
Alkanes are insoluble in water. On the other hand, alkanes
are soluble in many non-aqueous solvents (e.g.
methylbenzene).
CHEMICAL PROPERTIES OF ALKANES
Alkanes are saturated hydrocarbons. They are quite unreactive.
25.1 ALKANES
6
Figure 25.3
Sodium reacts with air but not with
alkanes. That is why sodium is
stored under paraffin oil (a mixture
of liquid alkanes).
25.1 ALKANES
7
Combustion
Alkanes are often used as fuels.
The general equation for the complete combustion of alkanes
(or other hydrocarbons) is:
CxHy + (x + ) O2 xCO2 + H2O
For example,
CH4(g) + 2O2(g) CO2(g) + 2H2O(l)
y
4
y
2
25.1 ALKANES
8
Figure 25.4
(a) LPG (consisting of lower alkanes)
burns with a non-sooty blue flame.
25.1 ALKANES
(b) A candle (consisting of higher
alkanes) burns with a sooty
yellow/orange flame.
9
A25.1
2C4H10(g) + 13O2(g) 8CO2(g) + 10H2O(l)
Decomposition by heat
On strong heating, higher alkanes decompose into simpler
hydrocarbons.
Reaction with halogens
In diffuse sunlight, hexane reacts with bromine solution (in 1,1,1-
trichloroethane). This is indicated by the disappearance of the
red-orange colour of bromine.
25.1 ALKANES
10
sunlight
hexane + bromine in 1,1,1-trichloroethane
red-orange colour of bromine is discharged
red-orange colour of bromine is not discharged in darkness
(a) (b)
Figure 25.5 Hexane reacts with bromine in sunlight but not in the dark.
25.1 ALKANES
11
25.1 ALKANES
12
CH3Br + Br2 CH2Br2 + HBr
dibromomethane
CH2Br2 + Br2 CHBr3 + HBr
tribromomethane
CHBr3 + Br2 CBr4 + HBr
tetrabromomethane
The above reactions are examples of a type of reaction called
substitution reaction.
A SUBSTITUTION REACTION is a chemical change in which an
atom (or a group of atoms) of an organic molecule is replaced by
another atom (or group of atoms).
25.1 ALKANES
13
Substitution reactions are typical reactions of alkanes.
HALOGENATION of an alkane is the substitution of a hydrogen
atom in the alkane molecule by a halogen atom.
A25.2
Chloromethane, dichloromethane, trichloromethane,
tetrachloromethane and hydrogen chloride.
25.1 ALKANES
14
25.1 ALKANES
Chemical properties of hexane. Other alkanes behave similarly.
15
CHARACTERISTICS OF A HOMOLOGOUS SERIES —
A BRIEF SUMMARY
(1) All members of a series can be represented by the same
general formula.
(2) Each member differs from the next by a – CH2 – group.
(3) Members show a gradual change of physical properties with
increasing relative molecular mass.
(4) All members have similar chemical properties, though higher
members are less reactive.
(5) All members can usually be obtained by the same general
methods.
25.1 ALKANES
16
25.2 THE PETROCHEMICAL INDUSTRY
25.2 THE PETROCHEMICAL INDUSTRY
Today, petroleum is the main source of a wide range of chemicals
(petrochemicals). The industry that separates petroleum into fracti
ons and changes them into other compounds is called the petroch
emical industry.
17
Figure 25.6 Petrochemical industry consists of many plants, occupying a large area.
25.2 THE PETROCHEMICAL INDUSTRY
18
Petrochemical industry is divided into two main parts:
(1) Oil refining
This separates crude oil into fractions by fractional distillation.
(2) Conversion processes
Some of the oil fractions undergo further processes to make
many useful products.
Cracking Heavy fractions (those with high boiling point
ranges) may be cracked.
CRACKING is the process of breaking down large molecules
(usually long-chain organic molecules) into smaller ones.
25.2 THE PETROCHEMICAL INDUSTRY
19
Gasification of oil fraction In this process, an oil fraction
(e.g. naphtha) is changed into a gaseous fuel.
Hong Kong town gas is currently made by this method.
Conversion of alkenes Alkenes, formed in cracking
processes, are useful starting materials for making a great
variety of organic chemicals.
25.2 THE PETROCHEMICAL INDUSTRY
20
Figure 25.7 A variety of petroleum products.
25.2 THE PETROCHEMICAL INDUSTRY
21
25.3 CRACKING IN PETROCHEMICAL INDUSTRY
25.3 CRACKING IN PETROCHEMICAL
INDUSTRY
SUPPLY AND DEMAND OF OIL FRACTIONS
22
Figure 25.8 Comparison of supply and demand of different oil fractions.
25.3 CRACKING IN PETROCHEMICAL INDUSTRY
23
Heavy oils (in less demand) can be cracked to provide more
petrol or kerosene (in greater demand).
CRACKING OF OIL
Cracking refers to the process of heating organic compounds in
the absence of air.
Usually a catalyst of aluminium oxide mixed with silicon(IV)
oxide is also added. (The process is thus called catalytic
cracking.)
25.3 CRACKING IN PETROCHEMICAL INDUSTRY
24
Figure 25.9 Catalysts can speed up reactions.
25.3 CRACKING IN PETROCHEMICAL INDUSTRY
25
large molecules in
heated catalyst
small molecules out
Figure 25.10 A catalytic cracker at an oil refinery.
25.3 CRACKING IN PETROCHEMICAL INDUSTRY
26
25.3 CRACKING IN PETROCHEMICAL INDUSTRY
Manganese(IV) oxide can catalyse the decomposition of hydrogen
peroxide into oxygen and water. (Oxygen can relight a glowing splint.)
27
Heavy fractions such as fuel oil are usually cracked to
produce petrol. Use CH3(CH2)8CH3 as an example.
25.3 CRACKING IN PETROCHEMICAL INDUSTRY
28
Figure 25.11
A decane molecule may be cracked at various points along the chain. Here are two of the
many possible ways.
25.3 CRACKING IN PETROCHEMICAL INDUSTRY
29
A25.3
More and more petrol is required as fuel for the ever-increasing
number of motor vehicles.
Importance of cracking
Cracking is very important in the petroleum industry for two
reasons:
To produce extra petrol Heavy fractions in less demand
can be cracked to produce extra petrol.
To produce alkenes Cracking always produces alkenes.
Alkenes (especially ethene and propene) can be used to
make many useful organic chemicals.
25.3 CRACKING IN PETROCHEMICAL INDUSTRY
30
25.4 CRACKING IN LABORATORY
25.4 CRACKING IN LABORATORY
Liquid paraffin is a mixture of alkanes.
Molecules in the paraffin vapour break down on a hot porcelai
n surface. The products are lower alkanes and alkenes.
31broken pieces of unglazed porcelainrocksil soaked
with liquid paraffin
strong heat
gaseous products obtained from cracking
water
Figure 25.13 Cracking liquid paraffin in the laboratory.
25.4 CRACKING IN LABORATORY
32
A25.4
(a) An oil fraction.
(b) The first few cm3 of gas is mainly air expelled from inside the
apparatus.
(c) The delivery tube should be removed from water before
stopping to heat. This is to prevent sucking back of water
which may crack the hot reaction tube.
25.4 CRACKING IN LABORATORY
33
25.4 CRACKING IN LABORATORY
Cracking medicinal paraffin and testing for flammability of product.
34
One purpose of cracking heavy oil fractions is to break alkane
molecules by heat into smaller molecules, which are more
flammable.
25.4 CRACKING IN LABORATORY
35
25.5 HOUSEHOLD GASEOUS FUELS
25.5 HOUSEHOLD GASEOUS FUELS
LPG AND TOWN GAS
In Hong Kong, the commonest domestic fuels are LPG (Liquefied
Petroleum Gas) and town gas. LPG is a mixture of mainly propan
e and butane liquefied under pressure.
36
Figure 25.15
A LPG tank.
25.5 HOUSEHOLD GASEOUS FUELS
37
HONG KONG TOWN GAS
Figure 25.16
The town gas plant at Tai Po
Industrial Estate in Hong
Kong.
25.5 HOUSEHOLD GASEOUS FUELS
38
How is town gas made in Hong Kong?
The raw material
Naphtha (used for Hong Kong town gas production) is a mixture
of C5 to C10 alkanes, mainly pentane and hexane.
Steam reforming of town gas
Liquid naphtha is heated. The vapour produced is mixed with
steam and passed over a hot nickel catalyst bed (at 700oC).
Hydrogen, methane, carbon dioxide and carbon monoxide are
formed. The following are typical reactions:
C5H12(g) + 5H2O(g) 5CO(g) + 11H2(g)
2CO(g) + 2H2(g) CO2(g) + CH4(g)
25.5 HOUSEHOLD GASEOUS FUELS
39
Composition of town gas
Table 25.2 Typical composition of Hong Kong town gas.
25.5 HOUSEHOLD GASEOUS FUELS
40
A25.5
(a) LPG: a fraction from refining of petroleum;
HK town gas: steam reforming of naphtha.
(b) LPG: mainly propane and butane;
Hong Kong town gas: mainly hydrogen and methane.
25.5 HOUSEHOLD GASEOUS FUELS
41
SUMMARY
1. On ascending the alkane series from methane to higher mem
bers, there is an increase in melting point, boiling point, densi
ty and viscosity (for liquid members).
2. Alkanes are quite unreactive. However, they can react with h
alogens (in sunlight) and burn in air.
3. A substitution reaction is a chemical change in which an atom
(or a group of atoms) of an organic molecule is replaced by a
nother atom (or group of atoms).
Example: CH4 + Br2 CH3Br + HBr
SUMMARY
42
SUMMARY
4. All alkanes have similar chemical properties, but alkanes with
larger molecules react more slowly.
5. There are certain characteristics of a homologous series. Ple
ase refer to p. 51.
6. There is a greater demand than supply for the following oil fra
ctions:
Petrol
Kerosene
Gas oil
7. Cracking is the process of breaking down large molecules (us
ually long-chain organic molecules) into smaller ones.
43
SUMMARY
8. Cracking heavy oils produces lighter oil fractions. That is, cra
cking long-chain alkanes produces short-chain alkanes. Alke
nes are also formed.
9. Cracking is important in the petrochemical industry for two re
asons:
It produces extra petrol (as motor vehicle fuel)
It produces alkenes (as starting materials to make
a great variety of organic chemicals)
10. We can crack liquid paraffin using a simple laboratory set-up.
Liquid paraffin requires a wick to burn. After cracking, the gas
eous products are flammable.
44
SUMMARY
11. Hong Kong town gas is made by the steam reforming of naph
tha. Two typical reactions in the process:
C5H12(g) + 5H2O(g) 5CO(g) + 11H2(g);
2CO(g) + 2H2(g) CO2(g) + CH4(g)
12. Hong Kong town gas contains mainly hydrogen (~49%) and
methane (~29%). It is poisonous because it contains carbon
monoxide (3%) as well.