CT: Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient. They do not flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules to meet demand – this is where cracking comes in.
Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are vaporised and passed over a hot catalyst. This breaks chemical bonds in the molecules, and forms smaller hydrocarbon molecules.
Cracking is an example of a thermal decomposition reaction.
S: http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/oils/polymersrev1.shtml (last access: 13 February 2015)
N: 1. From crack (n.), “a split, an opening,” mid-15c., earlier “a splitting sound; a fart; the sound of a trumpet” (late 14c.), probably from crack (v.).
2. Reduction in average molecular mass of the hydrocarbon constituents of a feedstock by the breakage of bonds within hydrocarbon molecules.
3. The objective is to produce lighter components which may be removed from the resulting product.
4. In petroleum refining, the two major types of cracking are thermal cracking and catalytic cracking.
5. cracking: term standardized by ISO.
S: 1. OED – http://www.etymonline.com/index.php?term=crack&allowed_in_frame=0 (last access: 13 February 2015). 2, 3, 4 & 5. TERMIUMPLUS.