15.5 Uses of Hydrocarbons

Sources of hydrocarbons

• Crude oil is the main source of hydrocarbons, it is found trapped in layers beneath the surface of the earth. Crude oil is a complex mixture of hydrocarbons –  alkanes,  cycloalkanes  and  aromatic  compounds. [ Note: Aromatic compounds are compounds which have a benzene ring attached to it ]
• These substances can be separated by a technique called fractional distillation. Fractional distillation is the separation of compounds by their boiling points.
• Different hydrocarbons have different molar masses, and so they have different boiling points
• Crude oil enters the bottom of the fractionating column as liquid and The liquids(less volatile hydrocarbons) are drawn off at the bottom while less volatile ones rise up the column. They condense at different levels as the temperature gradually falls and are collected as liquids.
• The most volatile short-chain hydrocarbons, which are methane and butane, leave the top as gases

Catalytic cracking

• After the hydrocarbons are separated, oil companies found that hydrocarbons from lighter fractions(such as gasoline) are in higher demand compared to the ones from heavier fractions
• Therefore something must be done to convert those heavier hydrocarbons into the more useful, lighter ones. This is done by cracking.
• Cracking breaks less useful hydrocarbons(normally long-chain) into smaller and more useful ones. Alkenes are normally produced as they are more industrially useful. This is because starting from alkenes, a wide range of organic compounds can be produced
• The larger hydrocarbons are fed into a chamber which contains no oxygen, so combustion does not take The larger hydrocarbons are heated at high temperature(about 500 °C) and passed over zeolite catalyst.
• Cracking of a hydrocarbon is not unique, different molecules of hydrocarbons can be broken in different ways, giving rise to different products. One possible reaction of involving C15H32 is as follow:

Environmental consequences of using hydrocarbons

• Alkanes are often used as fuels. This is because the combustion of alkanes is an exothermic process and produces a lot heat Alkanes are also readily available and relatively cheap. The main uses of alkanes as fuels are:
  -To generate
  -To heat domestic houses and cook
  -To provide energy needed in industrial
  -To provide power for ships, aeroplanes, trains, lorries, buses, cars and motorbikes.

• However, the combustion of alkanes can produce some poisonous gases which can act as pollutants

• One such example is carbon monoxide, CO which arises from the incomplete combustion of alkane. Carbon monoxide is a toxic gas that will bind readily to the haemoglobin molecules in the blood. This causes oxygen cannot be transported to cells and the victim will die

• The carbon dioxide, CO2 produced from the complete combustion of alkane can also act as a greenhouse gas. The increasing amount of these greenhouse gases enhances global warming.

• Burning fuels in car engines will also oxidise the nitrogen gas in air to produce oxides of nitrogen(nitrogen monoxide, NO or nitrogen dioxide, NO2). These oxides of nitrogen is believed to contribute in the formation of acid (For more details, refer Chapter 13)

• Catalytic converters can be installed to remove carbon monoxide and the oxides of nitrogen. More details about catalytic converters can be found in Chapter 13.