Alkanes and Petroleum Processing

Question-and-answer flashcards covering properties of alkanes, fractional distillation, fuel improvement processes, combustion, and environmental considerations.

What are alkanes?

Saturated hydrocarbons containing only single covalent bonds; general formula CₙH₂ₙ₊₂.

What type of bonding is present in alkanes?

Strong sigma (σ) bonds between C–C and C–H atoms formed by overlap of s and/or p orbitals.

Why are alkanes relatively unreactive?

They possess strong C–C and C–H bonds and are non-polar molecules.

Which reactions do alkanes commonly undergo?

Combustion and free-radical substitution.

How are alkanes found in nature?

Primarily in crude oil (petroleum), formed from fossilised organisms over millions of years.

What is petroleum?

A complex mixture of alkanes (and some other hydrocarbons) of varying chain lengths that must be separated and processed.

How is crude oil separated into useful fractions?

By fractional distillation in a fractionating column based on differing boiling points.

Describe the main steps of fractional distillation of crude oil.

Crude oil is vaporised (~350 °C), vapour enters a hot-bottom/cool-top column, hydrocarbons condense at heights corresponding to their boiling points.

How does chain length affect condensation height in fractional distillation?

Shorter-chain hydrocarbons condense higher up; longer-chain hydrocarbons condense lower down.

List the main fractions obtained from crude oil from top to bottom.

Gases, petrol (gasoline), naphtha, kerosene, diesel, lubricating oils & waxes, fuel oil, bitumen.

What problems do straight-chain alkanes have as fuels?

Lower octane numbers, less power output, and can cause engine knocking.

Name the three key industrial processes used to improve fuel quality.

Cracking, reforming, and isomerisation.

What is cracking?

The breaking of long-chain alkanes into shorter, more useful hydrocarbons such as petrol and alkenes.

Differentiate thermal cracking and catalytic cracking.

Thermal: high T & P; Catalytic: moderate T (~450 °C), low P, zeolite catalyst.

Why is catalytic cracking important?

Produces branched alkanes, aromatics, and alkenes used for high-octane fuels and polymer feedstocks under milder conditions.

What is reforming?

Conversion of straight-chain alkanes into branched or aromatic hydrocarbons to raise octane number using a platinum catalyst.

What is isomerisation?

Rearrangement of straight-chain alkanes into branched-chain isomers with higher octane ratings.

What are the products of complete combustion of alkanes?

Carbon dioxide (CO₂) and water (H₂O).

What are the products of incomplete combustion of alkanes?

Carbon monoxide (CO) or carbon (soot) plus water (H₂O).

List five environmental issues associated with alkane combustion.

CO₂ (global warming), CO (toxic), NOₓ (acid rain, respiratory issues), unburned hydrocarbons (smog), soot (respiratory problems, global dimming).

What do catalytic converters do and how?

Using a platinum/rhodium catalyst, they convert CO, NOₓ, and unburned hydrocarbons in exhaust gases into less harmful CO₂, N₂, and H₂O.