Why are long chain hydrocarbons not useful?
They make good fuels because their covalent bonds store a large amount of energy, which is released when the molecules are burned (i.e., when they react with oxygen to form carbon dioxide and water). One may also ask, why are long chain hydrocarbons used in vehicles?
Longer chain hydrocarbons represent more convenient fuels because they are liquids (they are more dense, more fuel molecules per unit volume). They also provide the opportunity to “control” the burning process. Subsequently, one may also ask, why does more carbon atoms mean more energy released?
Hydrocarbons make such good fuels because they release a lot of energy when their bonds are broken (chemical bonds=stored energy).
Why are long chain hydrocarbons cracked?
What are long chain hydrocarbons used for?
A good fuel should have the following characteristics.
Supply and demand Smaller hydrocarbons are more useful as fuels than larger hydrocarbons. Since cracking converts larger hydrocarbons into smaller hydrocarbons, the supply of fuels is improved. This helps to match supply with demand.
Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are heated to vaporise them. This helps to meet the demand for the more useful fractions and to increase profit.
Shorter hydrocarbon molecules have weaker intermolecular forces and lower boiling points. They are highly volatile and therefore extremely flammable.
Smaller hydrocarbons make better fuels as they are easier to ignite. However, crude oil contains a lot of longer chain hydrocarbons. To break a longer chain hydrocarbon down into a smaller one we use a process known as cracking.
Long chain hydrocarbons have lots of intermolecular forces (think of lots of necklaces getting tangled in a jewellery box) making them difficult to separate. This gives them a high boiling point. Small molecules have very small forces of attraction between them and are easy to break by heating.
