Organic 2: Alkanes
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What is an alkane?
A saturated hydrocarbon Contains only carbon-carbon and carbon-hydrogen single bonds
What are alkanes used for?
Fuels and lubricants
What is the main source of alkanes?
Crude oil
What is the general formula of alkanes?
CnH2n+2
What is the general formula of ring alkanes?
CnH2n
What is the bond angle for alkanes?
109.5 (tetrahedral)
Describe the polarity of alkanes.
1) Alkanes are almost non-polar 2) Because there is only a small difference in electronegativities for carbon and hydrogen 3) The only intermolecular forces are weak van Der Waal forces
Describe boiling points of alkanes.
Longer alkanes have higher melting and boiling points due to stronger van Der Waal forces between molecules Shorter alkanes have less, weaker van Der Waal forces so have lower melting and boiling points
What states are short chain alkanes and long chain alkanes at room temperature?
Short chain alkanes are gases at room temperature Long chain alkanes are solids or liquids at room temperature
How does the melting and boiling point of branched alkanes differ from straight alkanes with the same number of carbon atoms?
Branched alkanes have lower melting points than straight chains Because they are less compact, so the van Der Waal forces are not as effective
Are alkanes soluble in water, and why?
Alkanes are insoluble in water Because water molecules are held together by strong hydrogen bonds Which are much stronger than the weak van Der Waal forces that hold alkanes together
What substances can alkanes mix with?
Other non-polar liquids
What substances can or can’t alkanes react with?
Can react with: 1) Halogens (under appropriate conditions) 2) Oxygen (during combustion) Can’t react with: 1) Acids or bases 2) Reducing agents or oxidising agents
What is crude oil, or petroleum?
A mixture of branched and unbranched hydrocarbons, mainly alkanes
Why does crude oil contain impurities?
It often contains small amounts of other compounds from the original plant or animal that it was formed from E.g. some might contain sulfur, so would release sulfur dioxide when burnt
How is crude oil separated into useful products for industry?
By fractional distillation
Describe the process of fractional distillation.
1) The crude oil is heated in a furnace 2) A mixture of liquid and vapour passes into a fractionating tower with a temperature gradient (cooler at the top, hotter at the bottom) 3) Vapours pass up the tower and condense at a lower temperature than their boiling point, and are then piped off at that fraction 4) Shorter chain hydrocarbons condense nearer the top, because they have lower boiling points 5) Thick residue at the bottom of the tower (tar, or bitumen) is made of long hydrocarbon chains so they have very high boiling points
Why are shorter chain hydrocarbons more valuable?
They are in higher demand as they are more useful in industry For example, as petrol
How are longer chain hydrocarbons broken into smaller chain hydrocarbons?
Industrial cracking
Why are alkenes useful in industry?
1) As chemical feedstock (they supply industries with starting materials to make different products) 2) Can be converted into other products like polymers
What temperature is used for thermal cracking?
700K-1200K (High temperature) 7000kPa (High pressure)
Is a catalyst used in thermal cracking?
No
What are the products of thermal cracking?
Alkenes Hydrogen gas
How does thermal cracking produce alkenes?
1) C-C bonds break so that one electron from the covalent pair goes to each carbon atom 2) The fragments produced are free radicals 3) Free radicals are highly reactive so can react in a variety of ways to form shorter chain molecules 4) There are not enough hydrogen atoms to produce two alkanes 5) So one of the new chains forms a C=C bond