Hydrocarbons Flashcards
What are alkanes?
Saturated hydrocarbons with the formula CnH2n+2.
Why are alkanes generally unreactive?
They are non-polar and do not contain multiple bonds.
What are the three reactions for alkanes?
Combustion, halogenation and cracking.
What happens in a complete combustion reaction?
Alkanes act as fuels and burn with oxygen exothermically to produce CO2 and H2O.
What happens in incomplete combustion reactions?
Insufficient oxygen is present and carbon monoxide and soot is produced as well as CO2 and H2O.
What are the conditions for free radical substitution?
UV light and gaseous phase reactants.
Describe the steps of free radical substitution
- Initiation- UV light provides energy to break bonds homolytically, producing two radicals
- Propagation- Uses a radical and a reactant that forms a radical as a product, the reaction continues
- Termination- The chain reaction will continue until two radicals meet
What does cracking require?
High temperature and a catalyst to produce a more useful alkene.
What are alkenes?
Unsaturated hydrocarbons with the formula CnH2n.
Why are alkenes reactive?
As they contain a double bond.
What is a reaction for alkenes?
Electrophilic addition.
What is an electrophile?
An electron deficient species that can accept a lone pair of electrons.
What is an addition reaction?
A reaction in which reagents combine to give one product.
What is the test for the presence of alkenes?
Bromine water turns from water orange to colourless in the presence of alkenes (electrophilic substitution).
What are the advantages to using fossil fuels?
The variety of fuels available means that each use can be matched to an appropriate form of fuel.
Fossil fuels are available at all times, unlike wind and solar.
What are the disadvantages to using fossil fuels?
Non-renewable since formation takes millions of years.
Combustion produces greenhouse gases.
Sulphur fossil fuels cause acid rain.
Carbon monoxide formed with incomplete combustion can be toxic.
What are greenhouses gases?
Those that cause an increase in the earths temperature.
What is a fossil fuel?
One that is derived from organisms that lived long ago.
What is a radical?
A species with an unpaired electron.
Why does electrophilic addition sometimes produce two products?
As one has a more stable carbo cation than the other product.
What is the structure of an alkene double bond?
A sigma and pi bond.
What is a pi bond?
It is formed from the sideways overlap of a p-orbital electron on each carbon.
What is the only intermolecular force present in hydrocarbons?
Induced dipole-induced dipole.
What is the relationship between a hydrocarbons surface area and its melting and boiling point?
The smaller the surface area, the lower the VDW forces and therefore the lower the melting and boiling point. Small hydrocarbons are gases, medium hydrocarbons are liquids and large hydrocarbons are solids at room temperature.
What is the relationships between a hydrocarbons chain length and its boiling point?
The boiling temperature of hydrocarbons increase when chain length is increased. The more branches an isomer has, the more like a sphere it is and the lower the boiling temperature.
What is polymerisation?
The joining of a very large number of monomers to make a polymer.
What polymerisation reaction do alkenes undergo?
Addition polymerisation, where the double bond is used to join the monomers and nothing is eliminated.
What is ethene polymerised to?
Polythene.
What is polythene used for?
It is un reactive and flexible, so is used to make plastic bags.
What does the addition of a catalyst do to polythene?
A catalyst can be used to make polythene with straight chains, so the polymer has a high density and melting temperature, so can be used in high temperatures where rigidity is needed.
What is polypropene used for?
It is rigid and used in food containers and kitchen equipment.
What is polychloroethene used for?
It is flexible so is used for covering electrical cables and pipes.
What is polyphenylethene used for?
It is hard so is used in many household items needing strength and rigidity. It can be made into an insulator by creating holes in the structure.
