Combustion, cracking and improving fuels

Starter

Answers

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 combustion Short chained alkanes make a great fuel – clean, form carbon dioxide and water Methane – natural gas, used in our domestic supply LPG – bbq, patio heaters etc used in camping Octane – present in petrol Complete combustion means there is a plentiful supply of oxygen

Incomplete combustion Limited 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 fuels Short chain alkanes for use in polymer production Cracking is used to break down long- chained saturated hydrocarbons to form a mixture of shorter-chained alkanes and alkenes

Catalytic cracking First 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 alkanes Unbranched 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 hydrocarbons Aliphatic hydrocarbons can be converted into cyclic or aromatic hydrocarbons in a process sometimes known as reforming. Examples of this reaction include:

Improving fuels Read 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?

Plenary Complete the questions on page 121

Homework Read page and make notes Answer the questions on page 123. Please give full answers to the questions.