Alkanes
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/What are alkanes?
Saturated hydrocarbons.
What are alkanes used as?
Fuels and lubricants.
What is the main source of alkanes?
Crude oil
What is crude oil?
A mixture of different hydrocarbons.
General formula for alkanes?
CnH2n+2
What forms can hydrocarbons take?
Unbranched chains, branched chains or rings.
Suffix for an alkane?
-ane.
Why are alkanes almost non polar?
The electronegativities of carbon and hydrogen are similar.
What forces are present in alkanes?
Weak van der waal forces. The larger the molecule, the stronger the van der waal forces.
What are short chain hydrocarbons at room temperature?
Gases
Why do branched alkanes have lower melting points than straight chain alkanes?
They can’t pack together as closely as unbranched chains and so the van der waal forces are not so effective.
Describe the solubility of alkanes.
Insoluble in water. Water molecules are held together by hydrogen bonds which are much stronger than the van der waal forces between alkane molecules. Alkanes mix with other relatively non polar molecules.
Why are alkanes relatively unreactive?
They have strong carbon-carbon and carbon-hydrogen bonds. They don’t react with acids, bases, oxidising agents and reducing agents. They do burn and react with halogens under suitable conditions.
What is formed when an alkane burns in oxygen?
Water and carbon dioxide
What is formed when an alkane burns in limited oxygen?
Carbon monoxide and water
How is sulfuric acid formed?
Sulfur dioxide reacts with oxygen to form sulfur trioxide. Sulfur trioxide reacts with water to form sulfuric acid.
How do you convert crude oil into useful products?
Separate the mixture using fractional distillation. Heat the mixture and collect the fractions that boil over different ranges of temperature.
Describe the process of fractional distillation.
1) Crude oil is heated. 2) To vaporise the hydrocarbons. 3) The column is hotter at the bottom. 4) Vapours rise, cool and condense. 5) At their boiling points.
Where would short chain hydrocarbons be collected from in fractional distillation?
They have lower boiling points meaning they rise higher up the column before reaching their boiling point. They’re collected at the top of the column.
Describe thermal cracking.
Heat the alkane to a high temperature. Use high pressure. Free radicals are formed. High proportion of alkenes and alkanes produced. Can sometimes produce hydrogen.
Describe catalytic cracking.
High temperatures required. Slight pressures required. Zeolite catalyst used. Products are mostly branched alkanes, cycloalkanes and aromatic compounds.
What are the products of catalytic cracking used for?
Making motor fuels and aromatic hydrocarbons.
What is a zeolite catalyst?
Silicon dioxide and aluminium oxide.
Describe the structure of zeolites.
Honeycomb structure. Enormous surface area. Acidic.