Chemistry of Crude Oil and Cracking

Crude Oil

• Crude oil is a mixture of hydrocarbons
• Hydrocarbons are made of carbon and hydrogen
• Crude oil is separated into different hydrocarbon fractions by fractional distillation
• The oil is heated until most of it turns into gas
• The gases enter a fractionating column
• The column has a temperature gradient (hot at the bottom, cooler at the top)
• Longer hydrocarbons have higher boiling points and condense at lower temperatures/near the bottom
• Shorter hydrocarbons have lower boiling points and condense at higher temperatures/near the top
• Bubble caps prevent separated liquids from mixing
• Each fraction contains a mixture of hydrocarbons with similar boiling points

Cracking

• Long-chain hydrocarbons have high boiling points and are viscous
• Shorter hydrocarbons have lower boiling points and are thinner
• Cracking is a thermal decomposition process; breaking down molecules via heating
• Cracking is used to produce shorter, more useful hydrocarbon molecules
• Cracking conditions include heat (600°C-700°C) and a catalyst (alumina or silica)
• The reaction produces shorter alkane and alkene molecules

Combustion

• Combustion is a reaction involving a substance and oxygen
• Burning hydrocarbons releases energy as heat
• Complete combustion (plenty of oxygen) produces carbon dioxide and water
• Incomplete combustion (not enough oxygen) produces carbon monoxide and soot
• Carbon monoxide is toxic and can be lethal
• Acid rain is formed when sulfur dioxide and nitrogen oxides mix with water in clouds

Alkanes

• Alkanes are saturated hydrocarbons (single bonds only)
• Alkanes have the general formula C_nH_2n+2
• The first five alkanes are methane, ethane, propane, butane, and pentane
• Alkanes combust completely when enough oxygen is present

Halogens and Alkanes

• Halogens (chlorine and bromine) react with alkanes in the presence of UV light
• This substitution reaction replaces hydrogen atoms in the alkane with halogen atoms
• The reaction produces a haloalkane and hydrogen halide

Alkenes

• Alkenes are unsaturated hydrocarbons (double bonds)
• Alkenes have the general formula C_nH_2n
• The first three alkenes are ethene, propene, and butene
• Alkenes can undergo addition reactions (halogens react with alkenes by adding across the double bond, forming dihaloalkenes)
• Alkenes undergo complete combustion in sufficient oxygen

Alcohols

• Alcohols have the -OH functional group
• The general formula for alcohols is C_nH_2n+1OH
• Alcohols with one -OH group are called monohydric alcohols
• Alcohols can be oxidised to form carboxylic acids, using an oxidising agent

Esters

• Esters are formed from alcohols and carboxylic acids
• Esters have the -COO- functional group
• Esters are usually fragrant and volatile
• The process of forming an ester with an acid catalyst needs an alcohol and a carboxylic acid to form an ester and water.

Addition Polymers

• Polymers are large molecules made of small repeating units (monomers)
• Addition polymers are formed from unsaturated monomers (alkenes)
• Addition polymerisation involves the monomers joining together, opening the double bond
• Example monomers include ethene, propene, and chloroethene
• Example polymers include poly(ethene) and poly(propene).
• Polymers are difficult or impossible to decompose and are therefore often problematic for the environment

Condensation Polymers

• Condensation polymers are formed when two monomers react together
• Condensation polymers often involve the loss of water as a byproduct
• Monomers contain groups for reacting with other molecules (e.g., alcohols, dicarboxylic acids)
• An example is Polyester.