SSS 1: Hydrocarbon
Hydrocarbons are organic compounds containing hydrogen and carbon only.
All hydrocarbons have the general molecular formula of CxHy
Examples of hydrocarbons include methane[CH4], ethene[C2H4], ethyl[C2H2], benzene[C6H6], etc
Hydrocarbons are classified into two main groups according to their structure.
1. Aliphatic hydrocarbons
2. Aromatic hydrocarbons
Sources of hydrocarbons
The natural sources of hydrocarbons are:
1. Natural gas
2. Crude oil (Petroleum)
3. Coal
They are known as fossil fuels.
•Natural gas is a gaseous fuel
•Petroleum is a dark viscous liquid fuel
•Coal is a solid fuel
Crude oil (Petroleum)
Crude oil is a dark and sticky viscous liquid found in the underground. It contains hydrocarbons such as alkanes, alkenes, etc. Natural gas is found together with crude oil. Natural gas consists mainly of methane. Crude oil is measured in barrels.
1 barrel = 159 litres
Fractional distillation of crude oil
Fractional distillation is the process used to separate crude oil into its fractions.
Fractions of crude oil
The fractions of crude oil are:
1. Petroleum gas
2. Petrol
3. Kerosene
4. Diesel
5. Lubricating oil
6. Bitumen
Crude oil exploration and drilling
Oil-bearing rock can be located by:
1. Aerial photography
2. Examining surface rock
3. Core drilling
4. Mapping the earth's rock layers
Uses of crude oil
1. Petroleum gas: Used as domestic fuel
2. Petrol: Used as fuel in motor vehicles
3. Kerosene: Used as fuel for heating and lighting
4. Diesel: Used as fuel in diesel engine
5. Lubricating oil: Used in making cream and hair-care products. Also as a lubricant
6. Bitumen: Used in road construction
Cracking and reforming
Cracking: Cracking is the process whereby a heavier hydrocarbon molecule is split into two or more lighter hydrocarbon molecules.
Types of cracking
The two types of cracking are:
1. Thermal cracking
2. Catalytic cracking
Thermal cracking
This is the process whereby large hydrocarbon molecules are heated at high temperatures to give smaller and more useful hydrocarbon molecules
During thermal cracking, high temperatures and sometimes pressure are applied to initiate the decomposition reactions. e.g.
C16H34 → C8H18 + C8H16
(alkane) (alkene)
C10H22 → C8H18 + C2H4
(alkane) (alkene)
Catalytic cracking
This is the process whereby large hydrocarbon molecules are heated at high temperatures in the presence of a catalyst to give smaller molecules.
It can be adjusted such that desirable products of certain chain lengths are produced while thermal cracking is not particularly effective because there is poor control over the cracking patterns to yield alkane of desirable chain length
Conditions for catalytic cracking
1. Finely divided silica-alumina catalyst
2. Temperature of 450° to 550°C
3. Low pressure.
Catalytic cracking is preferred to thermal cracking because:
1. The process is more controllable
2. The process yields more petrol
3. It produces high-quality petrol
Benefits of cracking
1. It increases the yield of petrol
2. It provides petrol with good octane number rating
3. By-products are used in making detergent, plastics, synthetic rubber, etc
Reforming
Reforming is the rearrangement of atoms in the molecules of petroleum fraction to obtain branched and cyclic hydrocarbons.
The feedstock for this process is the light fraction, naphtha with a carbon range of C6 to C10.
Reforming is an isomerization process that takes place in the presence of a catalyst such as oxides of silicon and aluminium. e.g.
Hexane → 2-Methylpentane
Aromatization: This is the process whereby cyclic alkanes undergo dehydrogenation to form aromatic hydrocarbons. e.g.
Cyclohexane → Benzene
Differences between cracking and reforming
Octane number
Octane number, also called octane value or octane rating is the rating of the performance of a petrol in an internal combustion engine. Poor quality fuel tends to cause knocking. The octane number of a fuel can be increased by the presence of branched-chain hydrocarbons and aromatic compounds.
The octane number of any fuel is the percentage of iso-octane blended with heptane.
The grade or quality of petrol is measured according to a scale called the octane-number system.
Petrol containing a higher percentage of straight-chain hydrocarbons e.g. heptane causes more knocking than that containing a higher percentage of branched-chain ones e.g. 2, 2, 4-trimethylpentane (Iso-octane). Octane number is highest in petrol containing a high proportion of
2,2, 4-trimethylpentane
A straight-chain alkane such as heptane, is assigned an octane number of zero(0), while 2,2, 4-trimethylpentane is assigned an octane number of 100.
A given fuel with an octane number of 100, means that the knocking tendency of the fuel is low.
Example: The octane number of a fuel whose performance is the same as that of a mixture of 55 g of 2,2,4- trimethyl pentane and 45 g of n-heptane is (a) 45 (b) 55 (c) 100 (d) 25.
The answer is B
The following substances when added to some petrol can improve their performance:
1. Methanol
2. Ethanol
3. Tetraethyl lead(IV)- [Pb(C2H5)4]
Petrochemicals
Petrochemicals are substances that are manufactured from the by-products of petroleum.
Petrochemical products
Products made from petrochemicals include the following items:
1. Plastics
2. Fertilizers
3. Detergents
4. Pesticides
5. Drugs
6. Cosmetics
7. Food additives
8. Paints
9. Textile materials
10. Synthetic rubber etc
The importance of crude oil and petrochemicals:
1. Provision of employment
2. Sources of revenue for individuals and country
3. Sources of energy
4. Provision of raw materials for the industry
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