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

Octane number is the percentage of branched-chain 2,2,4-trimethylpentane (iso-octane) in a mixture with straight-chain heptane that burns as smoothly as the petrol being tested.

• High octane number: petrol burns smoothly and causes little knocking.
• Low octane number: petrol burns too quickly and causes more knocking.
• 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:

1. explain octane number and knocking;
2. state the octane numbers of heptane and 2,2,4-trimethylpentane and interpret an octane rating;
3. distinguish super petrol from regular petrol;
4. describe how synthetic petrol is made from coal and from coke;
5. name some petrochemicals and the products made from them;
6. 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).

• Composition of gasoline: it is made mainly of C₇ to C₉ hydrocarbons, that is, heptane, octane and nonane.
• Isomers: these hydrocarbons are present as straight-chain or branched-chain isomers.
• Use: the octane number measures the performance of a fuel in an engine.
• Petrol engines: motor car engines are known as petrol engines.

What are straight-chain and branched-chain hydrocarbons?

• Straight-chain hydrocarbon: the carbon atoms join in one unbroken line, as in n-heptane.
• Branched-chain hydrocarbon: the carbon chain carries side groups, as in 2,2,4-trimethylpentane.

What is knocking?

Knocking is the rattling noise made by a petrol engine when the fuel burns too rapidly.

• Cause: straight-chain hydrocarbons such as n-heptane burn too rapidly in the car engine.
• Effect: the pistons move irregularly, which produces the rattling noise.
• Link to composition: petrol with more straight-chain hydrocarbons knocks more than petrol with more branched-chain hydrocarbons.
• Link to grade: low-grade petrol makes some engines knock, so the grades differ in octane number.

How is the octane number scale set?

The scale gives heptane a value of 0 and 2,2,4-trimethylpentane a value of 100.

• Heptane: a straight-chain alkane, given an octane number of 0.
• 2,2,4-trimethylpentane: a highly branched alkane, given an octane number of 100, and it burns very smoothly in engines.
• Rating of any petrol: the percentage of 2,2,4-trimethylpentane in a mixture of it with heptane.

How is an octane rating read?

An octane rating is read as the percentage of iso-octane in a mixture with heptane.

• Octane number 94: the fuel performs like a mixture of 94% iso-octane and 6% straight-chain heptane.
• Octane number 50: the fuel performs like a 50-50 mixture of heptane and 2,2,4-trimethylpentane.
• Octane number 0: the fuel performs like pure heptane and knocks most.
• Octane number 100: the fuel performs like pure iso-octane and burns most smoothly.

To find the heptane percentage from a rating:

1. Take the octane number as the percentage of iso-octane.
2. Subtract it from 100 to get the percentage of heptane.
3. Example: octane number 80 means 80% iso-octane and 20% heptane.

What are the grades of petrol?

There are two grades of petrol: super (extra) and regular (ordinary).

What is synthetic petrol?

Synthetic petrol is petrol made from materials such as coal, coke and hydrogen, which do not occur in crude petroleum.

It is obtained from two sources: coal and coke.

How is synthetic petrol made from coal?

1. Powdered coal is heated with hydrogen at 500°C and 20 atmospheric pressure.
2. Iron (Fe) or tin (Sn) is present as catalyst.
3. The coal changes into an oily mixture of hydrocarbons.
4. The mixture is separated by distillation into a petrol fraction boiling at 200°C and a heavy oil residue.
5. The residue is treated further with fresh coal to obtain more petrol.

How is synthetic petrol made from coke?

1. Steam is passed over heated coke at 1000°C.
2. A mixture of equal volumes of carbon monoxide and hydrogen forms. It is known as water gas.
3. Water gas is mixed with more hydrogen and passed over finely divided nickel catalyst at 200°C.
4. It is hydrogenated to a mixture of hydrocarbons.
5. About half of the product is petrol. The less volatile fraction is used as fuel for diesel engines.

C + H₂O → CO + H₂

What are petrochemicals?

Petrochemicals are substances manufactured from the by-products of petroleum and natural gas.

• Small-molecule compounds: ethanol, ethane, propane-1,2,3-triol, benzene and toluene.
• Plastics: large-molecule compounds made from the small ones.
• Synthetic rubber: used in place of natural rubber.
• Insecticides and detergents: made from petroleum-based organic compounds.
• Synthetic fibres: such as nylon and Dacron.

What are the problems of petroleum chemistry and their solutions?

What are the 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.

How are the problems solved?

The problems are solved by cracking, catalytic reforming and the use of additives such as tetraethyl lead.

• Cracking (pyrolysis): breaks larger hydrocarbon molecules into smaller ones and so increases the amount of petrol.
• Catalytic reforming: rearranges straight-chain hydrocarbons into branched-chain ones and so raises the octane number.
• Tetraethyl lead: an anti-knock compound that prevents knocking and increases the octane number.

Example of cracking:

C₁₀H₂₂ → H₂ + C + C₅H₁₂ + CH₃-CH=CH-CH₃ (but-2-ene)

• Note on lead: leaded petrol is now largely replaced because lead compounds pollute the air.

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

Answer all parts of this question.

1. A petrol sample has an octane number of 85. State the percentage of iso-octane and of heptane in the equivalent mixture.
2. Explain why this sample knocks less than a sample with an octane number of 60.
3. Write the equation for the reaction of steam with heated coke, and name the gas mixture formed.
4. State the difference between cracking and catalytic reforming.