Alkanes Flashcards
as chem
What type of hydrocarbons are alkanes?
saturated hydrocarbons
single c-c bonds
What is fractional distillation used for?
To seperate crude oil which has a mixture of different length alkane hydrocarbons
Summarise steps of fractional distillation..
- The vaporised oil enters the column and rises through the trays. The longest hydrocarbons don’t vaporise and run down to the bottom.
The column has a
temperature gradient. It is cooler at the top.
As the vapour rises parts of the mixture condense at different temperatures.
This is because there are different chain lengths and hence different boiling points.
The fractions are drawn off at different levels.
What do short hydrocarbons come off as in fractional distillation?
Gases as they won’t condense sp they come off as a gas at the top of the column.
What is cracking ?
Conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C – C bonds.
What are reasons for cracking?
shorter hydrocarbons are more in demand then longer hydrocarbons.
makes use of excess larger hydrocarbons
Products of cracking are more valuable than starting materials.
What are the 2 main types cracking ?
Thermal cracking
Catalytic cracking
Describe thermal cracking..
Conditions:
High pressure= 7000kPa
High temperature= 400-900
products:
mostly alkenes (which can be used to make polymers)
How does fractional distillation occur in the laboratory?
- Heat the flask, with a Bunsen burner or electric mantle
- This causes vapours of all the components in the mixture to be produced.
- Vapours pass up the fractionating column.
- The vapour of the substance with the lower boiling point reaches the top of the fractionating column first.
- The thermometer should be at or below the boiling point of the most volatile substance.
- The vapours with higher boiling points condense back into the flask.
- Only the most volatile vapour passes into the condenser.
- The condenser cools the vapours and condenses to a liquid and is collected
Describe catalytic cracking..
Conditions:
moderate pressure
Temperature= 450*
Zeolite catalyst
Products:
aromatic hydrocarbons (useful for fuel and vehicles)
Describe complete combustion meaning
Alkanes completely combust in excess oxygen
products: CO2 and H20
Describe incomplete combustion meaning
If there is limited amount of oxygen incomplete combustion occurs.
Products:
CO - (toxic)
C-soot
CO+H20
C+H20
What is the issue with carbon monoxide? (CO)
carbon monoxide is poisonous as it binds to haemoglobin in blood and prevents oxygen from bonding.
What is the issue with soot? (C)
Soot can cause global dimming. reflection of the sun’s light.
Soot can cause breathing problems.
What is the issue with sulfur dioxide as a product of combustion ?
Sulfur containing impurities are found in petroleum fractions which produce SO2 when their burned
So2 will dissolve in atmospheric water and can produce acid rain.
How can so2 be removed from waste gases and what can be made from the product?
Through flue gas desulfurisation/ wet scrubbing
This is when the gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction.
SO2 +CaO–> CaSO3
CaSO3 (calcium sulfite) can be used to make calcium sulphate for plasterboard.
What is the issue with nitrogen oxides as a product of combustion?
Nitrogen oxides form from the reaction between N2 and O2 in a car engine.
the high temperature and spark in engine provides energy to break strong n2 bonds.
Products:
N2 +O2–>NO
N2+ 2O2–>NO2
Summary of pollutants and their environmental consequences..
Nitrogen oxides) NO toxic and can form acidic gas NO2.
NO2 is toxic and acidic and can form acid rain.
Carbon monoxide) toxic
Carbon dioxide) contributes towards global warming
Unburnt hydrocarbons) contributes towards formation of smog
soot) global dimming and respiratory problems.
What is the use of catalytic converters and what are they?
Converters have a ceramic honeycomb coated with a thin layer of catalyst metals ( platinum, palladium, rhodium) to give large surface area.
They remove CO (carbon monoxide) and unburned hydrocarbons from exhaust gases by turning them into harmless CO2 N2 AND H20
What is the greenhouse effect?
- UV wavelength radiation passes through the atmosphere to the Earth’s surface and heats up Earth’s surface.
The Earth radiates out infrared long wavelength radiation.
The C=O Bonds in CO, absorb infrared radiation so the IR radiation does not escape from the atmosphere.
This energy is transfer absorb infrared radiation se the here by collisions so the atmosphere is warmed
What mechanism occurs for alkanes?
Free-radical substitution
What are the 3 stages of free-radical substitution and when can it occur?
FR substitution can only occur in the presence of UV light - alkanes are able to react with chlorine. Products are made where the halogens substitutes hydrogen atoms.
Initiation
Propagation
Termination
Describe initiation
Radicals are produced normally using visible light or ultra violet. Called photochemical reactions.
Describe propagation
When a radical reacts with a non radical molecule. New radicals are created which then go on to react with other non-radicals. This is why it’s called a chain reaction.
Describe termination
When 2 radicals react they form a non-radical molecule. This ends the chain reaction. This is why it’s called termination.
What can alkanes produce?
Halogenoalkanes
