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.