1 Chapter 25Alkanes and Cracking 25.1Alkanes 25.2The petrochemical industry 25.3Cracking in...

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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

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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.

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PHYSICAL PROPERTIES OF ALKANES

Table 25.1 Physical properties of some straight-chain alkanes.

25.1 ALKANES

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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

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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

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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

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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.

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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

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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

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25.1 ALKANES

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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

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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

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25.1 ALKANES

Chemical properties of hexane. Other alkanes behave similarly.

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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

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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.

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Figure 25.6 Petrochemical industry consists of many plants, occupying a large area.


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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.


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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.


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Figure 25.7 A variety of petroleum products.


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25.3 CRACKING IN PETROCHEMICAL INDUSTRY

25.3 CRACKING IN PETROCHEMICAL

INDUSTRY

SUPPLY AND DEMAND OF OIL FRACTIONS

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Figure 25.8 Comparison of supply and demand of different oil fractions.


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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.)


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Figure 25.9 Catalysts can speed up reactions.


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large molecules in

heated catalyst

small molecules out

Figure 25.10 A catalytic cracker at an oil refinery.


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Manganese(IV) oxide can catalyse the decomposition of hydrogen

peroxide into oxygen and water. (Oxygen can relight a glowing splint.)

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Heavy fractions such as fuel oil are usually cracked to

produce petrol. Use CH3(CH2)8CH3 as an example.


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Figure 25.11

A decane molecule may be cracked at various points along the chain. Here are two of the

many possible ways.


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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.


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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.


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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.


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Cracking medicinal paraffin and testing for flammability of product.

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One purpose of cracking heavy oil fractions is to break alkane

molecules by heat into smaller molecules, which are more

flammable.


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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.

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Figure 25.15

A LPG tank.


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HONG KONG TOWN GAS

Figure 25.16

The town gas plant at Tai Po

Industrial Estate in Hong

Kong.


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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)


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Composition of town gas

Table 25.2 Typical composition of Hong Kong town gas.


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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.


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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

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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.

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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.

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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.

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