Hydrocarbons as fuels

Combustion, cracking and improving fuels

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Learning outcomes• Describe the combustion of alkanes.

Explain the incomplete combustion of alkanes.

Describe catalytic cracking to obtain more useful alkanes and alkenes from long-chain hydrocarbons.

Describe further processing to form branched alkanes and cyclic hydrocarbons.

Complete combustionShort chained alkanes make a great fuel –

clean, form carbon dioxide and waterMethane – natural gas, used in our domestic

supplyLPG – bbq, patio heaters etc used in campingOctane – present in petrol

Complete combustion means there is a plentiful supply of oxygen

Incomplete combustionLimited supply of oxygen

Forms carbon monoxide instead of carbon dioxide

Poisonous, prevents haemoglobin in the blood bonding with oxygen and so the bodies tissues can be starved of oxygen. This can be fatal.

Cracking After fractional distillation we are

left with a surplus of long chained hydrocarbons

Short chain hydrocarbons are in high demand:Short chain alkanes for use as fuelsShort chain alkanes for use in

polymer productionCracking is used to break down

long-chained saturated hydrocarbons to form a mixture of shorter-chained alkanes and alkenes

Catalytic crackingFirst used in the mid-1930s

Initially catalysts used were aluminium oxide and silicon dioxide

Today however, the catalysts used are zeolites at around 450˚C

Example

Long-chained alkanes are broken randomly

Above is one possible cracking reaction of dodecane

Producing branched alkanesUnbranched alkanes can be converted into

branched alkanes by a process referred to as isomerisation.

An example of an isomerisation reaction of pentane is

Producing cyclic hydrocarbonsAliphatic hydrocarbons can be converted into

cyclic or aromatic hydrocarbons in a process sometimes known as reforming.

Examples of this reaction include:

Improving fuelsRead the paragraph on page 121.

Why are high octane fuels desirable?What are branched and cyclic alkanes used

for?Hydrogen is a by-product of some of these

reactions, what is it used for?

PlenaryComplete the questions on page 121

HomeworkRead page 122-123 and make notes

Answer the questions on page 123. Please give full answers to the questions.