3.2 Alkanes Flashcards
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
What is fracking?
Extracting natural gas trapped within shale rock
Why is fracking used?
Natural gas supplied by the north sea is decreasing rapidly
Many areas of the UK has natural gas trapped
How is gas extracted in fracking?
Drill into the shale
Force pressurised water mixed with sand causing the soft shale to break up
Add Hydrochloric acid to help break up the shale
Add methanol to prevent corrosion in the system
This releases trapped gas flowing to the surface
What pollutants are produced in combustion?
Carbon dioxide Carbon monoxide Carbon Sulfur dioxide Nitrates of oxides Carbon particulates Unburnt hydrocarbons
What is produced in complete combustion? Cause?
Carbon dioxide CO2
Global warming
What is produced in incomplete and very incomplete combustion? Causes?
Carbon monoxide CO
Poisonous
Carbon C or soot
Global dimming
What is incomplete combustion?
A lack of oxygen in the reaction
What do carbon particulates cause?
Cancer
It can exacerbate asthma
Where does sulfur come from if present in combustion equations?
It is contained within some parts of crude oil as it is from the sea creatures many years ago
It is an impurity in the crude oil
What is produced as a result of sulfur?
Sulfuric acid H2SO4
Which is acid rain
How is sulfuric acid produced?
S + O2 —> SO2
2SO2 + O2 —> 2SO3
SO3 + H2O —> H2SO4
How do we prevent sulfuric acid or acid rain being produced?
Flue gas desulfurisation
What can be used as a neutraliser in flue gas desulfurisation?
Calcium oxide
Calcium carbonate
Why is there opposition to fracking?
Concern about amount of water used
Scared about chemicals polluting the water
Occasional earthquakes
Burning the natural gas itself produces CO2 contributing to global warming
What can calcium sulfate be used for?
Plasterboard
What else can be formed in combustion?
Oxides of Nitrogen
What are the equations for how smog is formed?
N2 + O2 —> NO
N2 + 2O2 —> 2NO2
What is the effect of smog?
It is bad for the lungs
How does N2 and O2 react?
Because of the heat in car engines at around 2500 degrees they have enough energy
What are used to prevent pollutants?
Catalytic converters
Where are catalytic converters used? What do they prevent?
Cars - just behind the engine
Carbon monoxide, nitrates of oxygen and unburnt hydrocarbons
What is the structure of a catalytic converter?
Honeycomb structure (large surface area) made from a ceramic material
Coated in platinum, palladium, rhodium or iridium which are catalysts
What does the large surface area of the honeycomb structure of the catalytic converter lead to?
Little of the expensive metal goes a long way
What are some reactions that take place in catalytic converters?
Producing CO2 instead of CO
Producing Nitrogen from Nitrates of oxygen
What is carbon neutral? Example?
Something that produces no overall carbon dioxide emissions
Plants take in CO2
Converted into biodiesel
The CO2 produced is just what was taken in
Therefore no overall change
What is the last measure for preventing acid rain?
CaO + H2SO4 —> CaSO4 + H2O
What kind of reaction is flue gas desulfurisation? What are the reactions?
Neutralisation
SO2 + CaO + 1/2 O2 —> CaSO4
SO2 + CaCO3 + 1/2 O2 —> CaSO4 + CO2
