Organic 1 - Chapter 12: Alkanes Flashcards
What is an alkane?
Saturated hydrocarbon with only C-C and C-H bonds
What are alkanes used for and why?
Fuels, lubricants and feedstock due to being unreactive
What is the general formula for a chain alkane?
CnH2n+2
What is the general formula for a ring alkane?
CnH2n
Describe the physical properties of alkanes
Mention polarity, boiling point, solubility and reactivity
Polarity: Almost non-polar as electronegativity of carbon and hydrogen are similar
MP + BP: Bigger chain -> more Van der waals forces -> Higher MP+ BP
Solubility: Insoluble in water as hydrogen bonds in water are stronger than vdw forces between alkane molecules, can mix in other non-polar liquids
Reactivity: Relatively unreactive due to strong C-C and C-H bonds. They do burn and react with halogens under suitable conditions
Describe the steps for the fractional distillation of crude oil
Crude oil heated in furnace
Mixture of liquid + vapour pass into fractionating tower which is cooler at top than bottom
Vapours pass up tower via a series of trays containing bubble caps until they arrive at a tray where they can condense to liquid
The mixture of liquids that condense on each tray is piped off
Shorter chain hydrocarbons condense near top of tower as it is cooler
Thick residue collects at the base (bitumen) often processed to give more valuable products as supply > demand
What is broken during fractional distillation?
Vdw forces between the molecules break during vaporisation and reform on condensing
NOT COVALENT BONDS
What is fracking?
Natural gas is trapped within shale rock:
Drill into the shale
Force pressurised water with chemicals mixed with sand into the shale. Chemicals such as HCl and methanol added to break shale and prevent corrosion
Shale fractures -> trapped gas flows to surface
Why are people against fracking?
People do not like the infrastructure of wells and associated ‘traffic’ around them
Concern about the amount of water used
Concern about chemical additives polluting water supply
Fracking occasionally causes small earthquakes
Burning natural gas creates CO₂ which causes global warming
What are the benefits of cracking?
Shorter more useful chains produced
Some products are alkenes which are more reactive than alkanes
What are the conditions required for thermal cracking?
700-1200K
7000kPa
What are the main products of thermal cracking?
High proportion of alkenes
Alkanes and hydrogen can also be produced
Why are hydrocarbons not subjected to thermal cracking for long durations?
To avoid too much decomposition, ultimately to just carbon and hydrogen
How does thermal cracking work?
C-C bonds break
1 electron from the pair in the covalent bond goes to each carbon
This produces 2 shorter chains with a free radical at the end
They react to form shorter chain molecules
There is not enough hydrogen atoms to produce 2 alkanes, therefore one of the new chains must be an alkene
What are the conditions required for catalytic cracking?
720K
Just above atmospheric pressure
Zeolite catalyst
What are the main products of catalytic cracking?
Alkanes
Cycloalkanes
Aromatic compounds
These are then separated via fractional distillation
What are the products of complete combustion?
Carbon dioxide
Water
What are the products of incomplete combustion?
Water
[ Carbon monoxide
Soot (Carbon)
Products will depend on oxygen availability which you can calculate with a chemical equation ]
What are some pollutants that may be produced from burning fuel derived from crude oil?
CO: Carbon monoxide
NOₓ: Nitrogen oxides
SO₂: Sulfur dioxide
CO₂: Carbon dioxide
H₂O: Water vapour
Carbon particulates
Unburnt hydrocarbons
How is carbon monoxide produced from combustion of crude oil and why is it a pollutant?
Produced by incomplete combustion
It is poisonous
How are nitrous oxides produced from combustion of crude oil and why is it a pollutant?
Produced when there is enough energy for nitrogen and oxygen in the air to combine
Happens in petrol engines due to high temperatures
Nitrous oxides cause photochemical smog
They can also react with water vapour and oxygen in the air to form nitric acid contributing to acid rain. However. nitric acid itself does not cause photochemical smog
How is sulfur dioxide produced from combustion of crude oil and why is it a pollutant?
Sulfur-containing impurities present in crude oil. This combines with water vapour and oxygen in the air to form sulfuric acid
SO₂(g) + ½O₂(g) + H₂O(l) -> H₂SO₄(l)
This causes acid rain
How is carbon dioxide produced from combustion of crude oil and why is it a pollutant?
Produced from the complete combustion of hydrocarbons
It is a greenhouse gas
How is water vapour produced from combustion of crude oil and why is it a pollutant?
A product of combustion
It is a greenhouse gas
How are carbon particulates produced from combustion of crude oil and why is it a pollutant?
A product of incomplete combustion
It can cause global dimming, cancer and exacerbate asthma
How are unburnt hydrocarbons produced from combustion of crude oil and why is it a pollutant?
May be produced from incomplete combustion
It is a significant greenhouse gas and contributes to photochemical smog
What methods are used for flue gas desulfurisation?
Needs better explaination
A slurry of calcium oxide (lime) and water is sprayed onto the flue gas to form calcium sulfate
Calcium carbonate can be used instead of calcium oxide which also forms CO₂
What is a catalytic converter made of?
Honeycomb made of ceramic coated with platinum and rhodium, which are the catalysts
What is the purpose of a catalytic converter’s honeycomb shape?
Large surface area. Reactions between pollutants can only happen on the surface of the catalyst
Why does infrared radiation get trapped by greenhouse gases?
It has a longer wavelength than visible light, which passes through
Why are CFCs a problem in the atmosphere?
C-Cl bond breaks homolytically (when it splits into 2 free radicals) in the presence of UV radiation and reacts with ozone. This causes a chain reaction that damages the ozone layer
