Carbon and It’S Compounds VII: Octane & Petrol – SS1 Chemistry Lesson Note (Third Term)

The octane number of petrol measures how smoothly it burns. It is the percentage of branched-chain 2,2,4-trimethylpentane (iso-octane) in a mixture with straight-chain heptane. Petrol with a high octane number causes little knocking, while petrol with a low octane number causes more. Super petrol has an octane number closer to 100 than regular petrol.

Behavioural objectives

By the end of the lesson, students should be able to:

• explain octane number and knocking;
• state the octane numbers of heptane and 2,2,4-trimethylpentane and interpret an octane rating;
• distinguish super petrol from regular petrol;
• describe how synthetic petrol is made from coal and from coke;
• name some petrochemicals and the products made from them;
• state the problems of petroleum chemistry and how they are solved.

What is octane number?

The octane number (octane rating) of petrol is a measure of the proportion of branched-chain hydrocarbons to straight-chain hydrocarbons in a given blend of gasoline (petrol).

Gasoline is composed of C₇ to C₉ hydrocarbons, that is, heptane, octane and nonane. These hydrocarbons are present as straight-chain or branched-chain isomers.

What is knocking?

Straight-chain hydrocarbons (for example n-heptane) burn too rapidly in the car engine. This causes irregular motion of the pistons, which results in a rattling noise. The rattling noise is known as knocking.

Petrol containing a higher percentage of straight-chain hydrocarbons causes more knocking than petrol containing a higher percentage of branched-chain hydrocarbons.

How is the octane number scale set?

A straight-chain alkane like heptane is assigned an octane number of 0. A highly branched-chain alkane like 2,2,4-trimethylpentane is assigned an octane number of 100, and it burns very smoothly in engines. The quality of any petrol is therefore rated according to its octane number, that is, the percentage of 2,2,4-trimethylpentane in a mixture of it with heptane.

How is an octane rating read?

Octane number is a measure of the performance of a fuel in engines. The rating is given as the percentage of iso-octane (2,2,4-trimethylpentane) to straight-chain hydrocarbon present.

For example, gasoline with an octane rating of 94 contains 94% iso-octane and 6% straight-chain heptane. Similarly, a fuel with an octane number of 50 has a performance equivalent to a 50-50 mixture of heptane and 2,2,4-trimethylpentane.

When low-grade petrol is used in some automobile engines, the engine tends to knock. The difference in the grade of petrol is therefore a difference in octane numbers. Motor car engines are known as petrol engines.

What are the grades of petrol?

Petrol can be graded as:

• Super or extra
• Regular or ordinary

Petrol graded as super or extra has an octane number closer to 100 than petrol graded as regular or ordinary.

What is synthetic petrol?

Synthetic petrol is made from materials such as coal, coke and hydrogen, which do not occur in crude petroleum. It can be obtained from two sources.

Synthetic petrol from coal

When powdered coal is heated with hydrogen in the presence of Fe or Sn as catalyst at 500°C and 20 atmospheric pressure, it is converted into an oily mixture of hydrocarbons. The mixture is separated by distillation into a petrol fraction boiling at 200°C and a heavy oil residue. The residue can be treated further with fresh coal to obtain more petrol.

Synthetic petrol from coke

When steam is passed over heated coke at 1000°C, a mixture containing equal volumes of CO and hydrogen is obtained. The mixture is known as water gas.

C + H₂O → CO + H₂

The water gas can be hydrogenated to a mixture of hydrocarbons by adding hydrogen and passing it over finely divided nickel as catalyst at 200°C. About half of the product is petrol. The less volatile fraction is used as fuel for diesel engines.

What are petrochemicals?

Petrochemicals are substances manufactured from the by-products of petroleum. These include plastics, synthetic rubber and fibres.

Petroleum and natural gas are now used in increasing amounts to produce many organic compounds. Examples include ethanol, ethane, propane-1,2,3-triol, benzene and toluene. These small-molecule organic compounds are in turn used to make large-molecule organic compounds such as plastics, synthetic rubber, insecticides, detergents and synthetic fibres like nylon and Dacron.

What are the problems of petroleum chemistry and their solutions?

Problems

1. The quantity of petrol produced is too small to meet the demand of the world.
2. The quality of the petrol produced is too low and may cause knocking in the petrol engine.

Solutions

These problems can be solved in two ways.

1. Increase the amount of petrol produced by breaking down larger hydrocarbon molecules into smaller ones (cracking or pyrolysis).
   C₁₀H₂₂ → H₂ + C + C₅H₁₂ + CH₃-CH=CH-CH₃ (but-2-ene)
2. Add tetraethyl lead (known as an anti-knock compound) to prevent knocking and so increase the octane number.

Note: Cracking, catalytic reforming and the use of additives (tetraethyl lead) are used to improve the yield and quality of petrol.

Evaluation

1. What is octane number?
2. What is knocking, and which type of hydrocarbon causes it more?
3. State the octane numbers of heptane and 2,2,4-trimethylpentane.
4. Describe how synthetic petrol is made from coal.
5. State two ways of improving the yield and quality of petrol.

Assignment

1. Oil deposits in Nigeria are
   (a) on land and offshore (b) only offshore (c) mainly imported (d) downstream
2. Fractional distillation involves the following processes
   (a) Boiling (b) Boiling and condensation (c) Boiling, evaporation and condensation (d) Condensation and collection
3. Which is the odd one out of the following?
   (a) petroleum ether, petroleum gases, kerosene (b) Gas oil and diesel lubricating oil (c) Petroleum ether and bitumen (d) Haematite and asphalt
4. Which of the petrol samples are likely to cause knocking?
   (a) octane (b) 2,2,3,3-tetramethylbutane (c) 2,2,3-trimethylpentane (d) 2,2,3-trimethylpentane
5. Nigeria earns money from
   (a) petroleum gas and liquids only (b) petroleum liquids and solids only (c) petroleum liquids like petrol and kerosene oil (d) petroleum gases, liquids and solids
6. (a) Distinguish between cracking and reforming. Of what importance are the two processes in the petroleum industry?
   (b) State the problem associated with oil-producing areas.