9.2 Hydrocarbons Flashcards
Alkanes
Alkanes are a group of saturated hydrocarbons.
The term saturated means that they only have single carbon-carbon bonds, there are no double bonds.
The general formula of the alkanes is CnH2n+2.
They are colourless compounds which have a gradual change in their physical properties as the number of carbon atoms in the chain increases.
Alkanes are generally unreactive compounds but they do undergo combustion reactions, can be cracked into smaller molecules and can react with halogens in the presence of light.
Methane is an alkane and is the major component of natural gas.
Examples of alkanes
methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane.
Alkenes
Alkenes are a homologous series of hydrocarbons. They’re more reactive than alkanes. All alkenes contain the functional group C=C, a double covalent bond between two of the carbon atoms in their chain.
Alkenes are known as unsaturated because they contain a C=C double bond. This can open up to become a single bond, allowing the two carbon atoms to bond with other atoms.
Examples of alkenes
ethene, propene, butene, pentene, hexene, heptene, octene, nonene, decene, and undecene.
Bromine and alkenes
Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond.
The reaction between bromine and ethene is an example of an addition reaction.
The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms across the C=C double bond.
Distinguishing between alkanes and alkenes
Halogens can be used to test if a molecule is unsaturated (i.e. contains a double bond).
Br2(aq) is an orange-yellow solution, called bromine water.
The unknown compound is shaken with the bromine water.
If the compound is unsaturated, an addition reaction will take place and the coloured solution will become clear.
Combustion of alkanes
Alkanes and alkenes undergo combustion in the presence of air
Complete combustion occurs to form water and carbon dioxide gas
Hydrocarbon + oxygen ——- carbon dioxide + water
Gasoline is largely composed of isomers of octane, C8H18 , which requires large amounts of oxygen to combust fully.
The efficiency of car engines does not usually enable all the gasoline to burn, so car exhaust will contain small amounts of unburnt hydrocarbons as well as other products such as carbon monoxide and soot which lead to environmental problems.
The carbon dioxide produced is a major contributor to global warming and the replacement of combustion engines with electric vehicles is a major on-going challenge for all countries.
Combustion of alkenes
These compounds undergo complete and incomplete combustion, but because of the higher carbon to hydrogen ratio they tend to undergo incomplete combustion, producing a smoky flame in air.
