3.2 Alkanes
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What is the general formula of an alkane?
CnH2n+2
What is the functional group of the alkanes?
C-C Carbon carbon single bonds
What are saturated hydrocarbons?
Only contains c-c single bonds, contains carbon and hydrogen only
What are some properties of alkanes?
Differs by CH2 Insoluble in water Unreactive Almost non-polar: Electronegativity difference between carbon and hydrogen is 0.4
What is the significance of different chain lengths?
As the carbon chain increases in length the boiling point increases
Why does the boiling points increase with the chain length?
There is more surface area for intermolecular forces of attraction therefore the bonds require more energy to overcome
What are intermolecular forces of attraction also called?
Van der Waals
What are properties of branched chain alkanes?
Lower melting/ boiling point than straight chains
Why do branched alkanes have lower boiling points?
They cannot pack together as closely as unbranched chains so the van der waal forces are not as effective
What is used to separate crude oil?
Fractional distillation
What is the process of fractional distillation?
Crude oil is heated in a furnace A mixture of liquid and vapour passes into the fractionating column It is hotter at the bottom and cooler at the top The vapours pass up the column via a series of trays until they reach one sufficiently cool The vapour condenses on a tray Shorter hydrocarbons condense at the top because they have lower boiling points
What is left over in a fractionating column?
A thick residue called tar or bitumen Used for road surfacing
What is the issue with tar?
Supply often exceeds demand It is often processed further to give more valuable products
What is cracking?
A process where you break up a large hydrocarbons molecules into smaller more useful molecules
What are the two types of cracking?
Catalytic Thermal
How does catalytic cracking take place?
Using the catalyst zeolite Temp: 500 degrees Pressure: 1 atmosphere
What is produced in catalytic cracking?
High percentage of hydrocarbons between 5 and 10 Branched, aromatic and cycloalkanes
What is zeolite made from?
Aluminoscilicates Lattices of Aluminium, silicon and oxygen
What is special about zeolite?
Honeycomb structure creating a large surface area It has a ionic intermediate - acidic
What is used in catalytic cracking in a laboratory?
Mineral wool soaked in light paraffin Aluminium oxide catalyst
What is produced in catalytic cracking in a laboratory? How can you tell?
Mostly gases - chain lengths less than C5 The mixture will decolourise bromine solution (alkene test)
How does thermal cracking take place?
High temperatures: 450-750 degrees High pressure: 70 atmospheres
What does thermal cracking produce?
High proportion of alkenes
What happens in thermal cracking?
Carbon carbon bonds are broke so each carbon ends up with a single electron i.e free radicals are formed