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.
EXAMINATION QUESTIONS
SECTION A: OBJECTIVE QUESTIONS
Instruction: Choose the correct answer from the four options provided.
1. Which statement best explains why catalytic cracking is preferred over thermal cracking in modern refineries?
(a) It requires higher temperatures
(b) It gives better control of product distribution
(c) It produces only alkanes
(d) It eliminates the need for catalysts
2. The primary feedstock for catalytic reforming is:
(a) Methane
(b) Naphtha (C₆–C₁₀)
(c) Diesel
(d) Bitumen
3. Aromatization of cyclohexane involves:
(a) Hydrogenation
(b) Polymerization
(c) Dehydrogenation
(d) Halogenation
4. A catalyst commonly used in catalytic cracking is:
(a) Platinum
(b) Silica-alumina
(c) Iron filings
(d) Copper oxide
5. Which hydrocarbon contributes most to engine knocking?
(a) Iso-octane
(b) Benzene
(c) n-Heptane
(d) Cyclohexane
6. One barrel of crude oil is equivalent to:
(a) 100 litres
(b) 120 litres
(c) 159 litres
(d) 200 litres
7. Reforming mainly improves petrol by increasing its:
(a) Sulphur content
(b) Density
(c) Octane rating
(d) Water content
8. Which petroleum fraction is mainly used in road construction?
(a) Diesel
(b) Petrol
(c) Bitumen
(d) Lubricating oil
9. Natural gas consists predominantly of:
(a) Ethane
(b) Propane
(c) Methane
(d) Butane
10. Which process converts straight-chain hydrocarbons into branched-chain hydrocarbons?
(a) Distillation
(b) Reforming
(c) Combustion
(d) Polymerization
11. Which factor primarily determines the octane number of petrol?
(a) Sulphur concentration
(b) Carbon percentage
(c) Resistance to knocking
(d) Flash point
12. Which hydrocarbon has an octane number of 100?
(a) Hexane
(b) Methane
(c) n-Heptane
(d) 2,2,4-Trimethylpentane
13. Fractional distillation separates crude oil mainly based on differences in:
(a) Molecular formula
(b) Density
(c) Boiling point
(d) Solubility
14. Which product is NOT a petrochemical?
(a) Plastic
(b) Fertilizer
(c) Glass
(d) Synthetic rubber
15. Which method is least useful in locating oil-bearing rocks?
(a) Core drilling
(b) Aerial photography
(c) Rock-layer mapping
(d) Neutralization
16. Thermal cracking mainly produces:
(a) Heavier hydrocarbons
(b) Smaller alkanes and alkenes
(c) Alcohols
(d) Aromatic compounds only
17. Branched-chain hydrocarbons generally possess:
(a) Lower octane numbers
(b) Greater knocking tendency
(c) Higher octane numbers
(d) Lower combustion efficiency
18. Which additive has historically been used to improve octane rating?
(a) Sodium chloride
(b) Tetraethyl lead (IV)
(c) Calcium carbonate
(d) Potassium nitrate
19. Which of the following is classified as a fossil fuel?
(a) Hydrogen
(b) Coal
(c) Oxygen
(d) Water
20. The principal objective of cracking in petroleum refining is to:
(a) Produce heavier hydrocarbons
(b) Increase petrol yield
(c) Remove sulphur completely
(d) Produce oxygenated fuels
---
ANSWERS TO SECTION A
1. (b)
2. (b)
3. (c)
4. (b)
5. (c)
6. (c)
7. (c)
8. (c)
9. (c)
10. (b)
11. (c)
12. (d)
13. (c)
14. (c)
15. (d)
16. (b)
17. (c)
18. (b)
19. (b)
20. (b)
---
SECTION B: THEORY QUESTIONS
1.
(a) Define hydrocarbon.
(b) State the general molecular formula of hydrocarbons.
(c) Give five examples of hydrocarbons.
2.
(a) Classify hydrocarbons into their two major groups.
(b) State three characteristics of aliphatic hydrocarbons.
(c) Give four examples of aromatic hydrocarbons.
3.
(a) Define crude oil (petroleum).
(b) State four characteristics of crude oil.
(c) Explain why crude oil is regarded as a fossil fuel.
4.
(a) List the three natural sources of hydrocarbons.
(b) State the physical state of each source.
(c) Explain why these sources are called fossil fuels.
5.
(a) Define fractional distillation.
(b) State the principle behind fractional distillation.
(c) Draw and label a fractional distillation column for crude oil.
6.
(a) List six fractions obtained from crude oil.
(b) State one use of each fraction.
(c) Arrange the fractions in order of increasing boiling point.
7.
(a) State four methods used in crude oil exploration.
(b) Explain any two methods.
(c) State two advantages of proper oil exploration.
8.
(a) Define cracking.
(b) State the two types of cracking.
(c) Mention four benefits of cracking.
9.
(a) Define thermal cracking.
(b) Define catalytic cracking.
(c) State five differences between thermal cracking and catalytic cracking.
10.
(a) State the conditions required for catalytic cracking.
(b) Name the catalyst used in catalytic cracking.
(c) Explain why low pressure is preferred during catalytic cracking.
11.
(a) State four reasons catalytic cracking is preferred to thermal cracking.
(b) Explain how catalytic cracking improves petrol production.
(c) Mention two industrial products obtained during cracking.
12.
(a) Define reforming.
(b) State the feedstock used in reforming.
(c) Explain the importance of reforming in petroleum refining.
13.
(a) Define aromatization.
(b) Write the chemical equation for the conversion of cyclohexane to benzene.
(c) State two industrial importance of aromatization.
14.
(a) Differentiate between cracking and reforming.
(b) State three similarities between cracking and reforming.
(c) Mention three industrial applications of both processes.
15.
(a) Define octane number.
(b) Explain engine knocking.
(c) State four factors that increase the octane number of petrol.
16.
(a) State the octane number assigned to n-heptane.
(b) State the octane number assigned to 2,2,4-trimethylpentane.
(c) Explain why branched-chain hydrocarbons have higher octane ratings than straight-chain hydrocarbons.
17.
(a) Define petrochemicals.
(b) List ten products manufactured from petrochemicals.
(c) State five importance of petrochemicals to the economy.
18.
(a) State four importance of crude oil.
(b) State four importance of petrochemicals.
(c) Explain how crude oil contributes to national economic development.
19.
(a) Calculate the octane number of a fuel equivalent to a mixture containing 70% iso-octane and 30% n-heptane.
(b) Calculate the octane number of a fuel equivalent to a mixture containing 82% iso-octane and 18% n-heptane.
(c) Explain the significance of high octane numbers.
20.
(a) State five additives used to improve petrol quality.
(b) Explain how ethanol and methanol improve petrol performance.
(c) Discuss the environmental effects of tetraethyl lead as a petrol additive.
---
MARKING GUIDE FOR SECTION B
1. (a) Hydrocarbons are organic compounds containing only carbon and hydrogen.
(b) General formula: CₓHᵧ.
(c) Examples: Methane, Ethane, Ethene, Ethyne, Benzene.
2. (a) Aliphatic hydrocarbons and Aromatic hydrocarbons.
(b) Any three correct characteristics.
(c) Benzene, Toluene, Xylene, Naphthalene.
3. (a) Crude oil is a dark, sticky, viscous liquid containing many hydrocarbons.
(b) Any four characteristics.
(c) It is formed from ancient plants and animals over millions of years.
4. (a) Natural gas, crude oil and coal.
(b) Gas, liquid and solid respectively.
(c) They are formed from the remains of living organisms.
5. (a) Separation of crude oil into fractions based on boiling points.
(b) Difference in boiling points.
(c) Correctly labelled diagram.
6. (a) Petroleum gas, petrol, kerosene, diesel, lubricating oil and bitumen.
(b) One correct use each.
(c) Petroleum gas → Petrol → Kerosene → Diesel → Lubricating oil → Bitumen.
7. (a) Aerial photography, examining surface rocks, core drilling and rock-layer mapping.
(b) Any two explained correctly.
(c) Any two valid advantages.
8. (a) Cracking is the breakdown of large hydrocarbon molecules into smaller ones.
(b) Thermal cracking and catalytic cracking.
(c) Any four benefits.
9. (a) Correct definition.
(b) Correct definition.
(c) Any five valid differences.
10. (a) 450–550°C, low pressure and silica-alumina catalyst.
(b) Silica-alumina catalyst.
(c) Proper explanation.
11. (a) Any four correct reasons.
(b) Correct explanation.
(c) Examples: Plastics, detergents, synthetic rubber.
12. (a) Rearrangement of hydrocarbon molecules into branched or cyclic compounds.
(b) Naphtha (C₆–C₁₀).
(c) Improves octane rating.
13. (a) Formation of aromatic hydrocarbons by dehydrogenation.
(b) C₆H₁₂ → C₆H₆ + 3H₂.
(c) Any two correct importance.
14. (a) Five correct differences.
(b) Any three similarities.
(c) Any three applications.
15. (a) Definition of octane number.
(b) Explanation of knocking.
(c) Branched hydrocarbons, aromatic hydrocarbons, ethanol and methanol.
16. (a) 0.
(b) 100.
(c) Correct explanation.
17. (a) Definition of petrochemicals.
(b) Any ten correct products.
(c) Any five correct importance.
18. (a) Any four importance.
(b) Any four importance.
(c) Correct explanation.
19. (a) 70.
(b) 82.
(c) High octane fuel resists knocking and improves engine performance.
20. (a) Ethanol, methanol, tetraethyl lead, MTBE and ETBE (or any other valid additives).
(b) Correct explanation.
(c) Lead pollution, environmental contamination and health hazards.
Comments
Post a Comment