6. Alkanes Flashcards
What are some uses of alkanes?
Fuels
lubricants
Starting materials for other compounds
What kind of hydrocarbons are alkanes?
Saturated hydrocarbons
What is the main source of alkanes?
Crude oil
What is the general formula for alkanes?
CnH2n+2
What shapes can alkanes take?
Unbranched and branched chains or rings
What general formula do ring alkanes have?
CnH2n (the end hydrogen a are not required)
Unbranched chains are often called ‘straight’ chains but the C-C-C angle is…
109.5 degrees
When naming alkanes find the longest unbranched…
Chain (for meth-/eth-/prop-/but- etc) (remember, you can go round corners for longest chain)
Which alkanes have no isomers?
Methane, ethane and propane
What is the polarity of the alkanes like?
Alkanes are almost non-polar because the electronegativity of carbon and hydrogenated so similar.
What intermolecular forces act between alkanes?
van der Waals forces, the larger the molecule the stronger the van der Waals forces
What are the boiling points of alkanes like?
The larger the molecule the stronger the van der Waals forces, the higher the boiling point.
Which alkanes are gases?
The shorter chain alkanes e.g 1-4 carbons
Which alkanes are liquids?
Alkanes with carbons 5-17
Which alkanes are solids?
Alkanes with 18+ carbons
Are alkanes soluble in water?
No, the hydrogen bonds in water are stronger than the van der Waals forces between the alkanes. They do mix with non-polar liquids.
How reactive are alkanes?
Alkanes are relatively unreactive. They do not react with acids, bases oxidising agents and reducing agents but they do burn in oxygen to form carbon dioxide and water (or in incomplete combustion to form carbon monoxide or carbon).
What is the worlds main source of organic chemicals?
Crude oil
Is crude oil renewable?
No
What is crude oil a mixture of?
Mostly alkanes.
How do we convert crude oil into useful products?
We have to separate the mixture by heating it and collecting fractions.
What does each fraction contain?
A mixture of hydrocarbons of similar chain length and therefore properties.
What is the process of separating the alkanes in crude oil called? Where is it done?
Fractional distillation
A fractionating tower
Give a short explanation of fractional distillation
The crude oil is heated in a furnace.
A mixture of liquid and vapour passes into a tower that is cooler at the top than at the bottom.
The vapours pass up the tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool. Then they condense into a liquid.
Each collected tray is piped off.
Where do shorter chain hydrocarbons condense in a fractionating tower?
In trays nearer to the top of the tower where it is cooler because they have a lower boiling point.
What collects at the base of a fractionating tower?
A thick residue called tar or bitumen. It is used for road surfacing but this fraction is often further processed to give more valuable products.
Which fraction is in high demand?
The naphtha fraction for petrol and the chemical industry