3Demand for different fractions The lighter, low boiling fractions – made up of small hydrocarbon molecules.Can be sold in a higher price (of higher economic value).More demanded by us.
4Demand for different fractions Heavier, high boiling fractions – made up of large hydrocarbon molecules.Supply is more than demand.Cracking is a process in which large hydrocarbon molecules are broken down into small molecules.A way to meet our demand for petrol.
5Cracking of petroleumWhen hydrocarbon molecules are heated, they move and vibrate faster. Their more frequent collisions lead to the breaking up of large molecules into smaller ones.
6Methods of crackingThermal cracking – heating hydrocarbons up to 500oC and 30 atmospheres.Cracking cracking – vapours of large hydrocarbons are passed over red hot catalyst (aluminium oxide and silicon(IV) oxide) at 500o C and 1 atmosphere.
7CatalystA substance that can speed up the rate of reaction, but itself is not used up at the end of reaction (remains unchanged at the end of the reaction).Compare with enzyme!Enzymes would be denatured.
9Cracking in the laboratory Broken porous pot (unglazed porcelain pieces) or alumina (aluminium oxide) – used as catalyst.Products of cracking (differing from case to case). Why?Carbon soot, hydrogen, small alkanes and small alkenes.
10Importance of cracking Producing extra petrolHigh boiling fractions of C13 to C25 are cracked into small molecules which are fuels for motor-cars.As a source of alkenes (raw materials for plastics)Alkenes – by products of cracking and raw materials for making plastics, polymers and detergents.
11Town gas Made from the reforming of naphtha. Naphtha vapour and steam were passed over red-hot nickel catalyst at 700oC.C6H H2O → 5CO + CH H2C6H H2O → 2CO2 + 4CO + 15H2An odorant is added to alert people if there is a leakage of town gas.
12Composition of town gas Hydrogen (49%)Methane (28.5%)Carbon dioxide (19.5%)Carbon monoxide (3%)