cracking and fractional distillation of alkanes

describe the process of fractional distillation:

• crude oil vaporised

• vapour passed into a column - hot at bottom and cool at top (temp gradient)

• as the vapour rises it cools

• molecules condense at different fractions as they have different bpts

• larger the molecules, the lower down the column they condense

what is thermal decomposition? give an example:

breaking down compounds by heating into simpler compounds - e.g. cracking

what is cracking?

breaking up large, less useful hydrocarbons into smaller, more useful hydrocarbons (thermal decomposition of alkanes)

why do we crack?

• petroleum fractions w/ shorter C chains are in higher demand than larger fractions

• longer hydrocarbons are cracked to make use of XS larger hydrocarbons and supply demand for shorter ones

• products are more valuable than the starting materials

give a use of shorter alkanes:

fuels

give a use of alkenes:

making plastics

describe the conditions and products of thermal cracking:

• high temp (900^oC)

• high pressure (70 atm)

• no catalyst

• produces alkenes

describe the conditions and products of catalytic cracking:

• high temperature (450^oC), but lower than thermal cracking

• slight pressure (1-2 atm)

• zeolite catalyst

• produces motor fuels, aromatic hydrocarbons, cyclic hydrocarbons and branched alkanes