Carbon And It’S Compounds VII – Octane & Petrol SS1 Chemistry Lesson Note

OCTANE NUMBER

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

Gasoline is composed of C7 – C9 hydrocarbons i.e. heptane, octane and nonane. These hydrocarbons are present in their straight-chain or branched-chain isomers. It has been shown that straight chain hydrocarbons (e.g. n-heptane) burn too rapidly in the car engine thus, causing irregular motion of the pistons which results in 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

A straight chain alkane like heptanes is assigned an octane number of O while a highly branched chain alkane like 2,2,4 trimethylpentane is assigned an octane number of 100 which burns very smoothly in engines. Therefore the quality of any petrol is rated according to its octane number, i.e. the percentage of heptanes to 2,2,4 – trimethylpentane in a mixture.

CH3 – CH 2– CH 2– CH 2– CH 2– CH2 – CH3

Heptane                 octane number = 0

CH3        CH3        2,2,4 – trimethylpentane (iso-octane 

CH3        C        CH2        C    CH3    octane number = 100

CH3         H

Octane number is a measure of the performance of the fuel in engines and the rating is given as the percentage of issue-octane2,2,4 – trimethyl pentane) to straight chain hydrocarbon present. For example, gasoline with an octane rating of 94 is understood to contain 94% iso-octane and 6% straight-chainptane. 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, there is a tendency for the engine to knock. The difference in the grade of petrol is, therefore, a difference in their octane numbers. 

The motor car engines are known as petrol engines.

GRADE OF PETROL

Petrol can be graded as:

(a) Super or extra        

(b) Regular or Ordinary

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

SYNTHETIC PETROL

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

1. From Coal: When powdered coal is heated with hydrogen in the presence of Fe or Sn as catalyst at 5000C 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 2000C and heavy oil residue which can be further treated with fresh coal to obtain more petrol.
2. From Coke: When steam is passed over heated Coke at 1000C, a mixture containing equal volumes of CO and hydrogen known as water gas is obtained.

C + H2O > CO + H2

3. Water gas

The water gas can be hydrogenated to a mixture of hydrocarbons by adding hydrogen and passing it over finely divided nickel as catalysts at 2000C. About half of the product is petrol, the less volatile fraction being used as fuel for diesel engines.

PETROCHEMICALS

Petrochemicals are substances that are 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 inorganic compounds. Examples include ethanol, ethane, propane 1, 2, 3 – triol, benzene and toluene. These small molecules organic compounds are in turn used to make large–molecule organic compounds like plastic, synthetic rubber, insecticides, detergents and synthetic fibres like nylon and Dacron.

PROBLEMS OF PETROLEUM CHEMISTRY

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

SOLUTION TO THE PROBLEMS

These problems can be solved by increasing the amount of petrol produced through

(a)  Breaking down of larger hydrocarbon molecules into smaller hydrocarbon molecules (Pyrolysis)

C10H22     H2 + C  C5H12 + CH3 – CH = CH – CH3

But – 2 – ene (iso butene)

(b) By the addition of lead–tetraethyl (known as ‘anti-knock’ compounds) to prevent knocking and thereby increase the octane number.

Note; Cracking, catalytic reforming and use of additives (tetraethyl lead) are used to improve 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-tetramethyl butane (c) 2,2,3-trimethyl pentane (d) 2,2,3-trimethyl pentane.

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.