12. Alkanes Flashcards
Describe petroleum
- A mixture consisting mainly of alkane hydrocarbons
Describe a petroleum fraction
- Mixture of hydrocarbons with a similar chain length and boiling point range
Describe how fractional distillation of crude oil takes place
- Oil is pre-heated and vaporised then passed into column.
- The vapour rises, cools and condenses at different heights
- Products are siphoned off for different uses
Describe the temperature gradient in a fractioning column
- The temperature of column decreases upwards
- (Hotter at bottom, cooler at top)
What does the separation of crude oil depend on?
- The boiling point of the molecule
- Hence the size of the molecule
- The larger the molecule, the larger the van der Waals forces
Similar molecules (size, bp, mass) condense together
Small comlecules condense at the top at lower temps - big molecules condense at the bottom at higher temps
- This is a physical process involving the splitting of weak van der wall forces between molecules
- products with short carbon chains have a lower b.p. meaning they rise higher up the column before reaching their boiling point, therefore they are collected at the top of the column
Products with long carbon chains have higher boiling points meaning they don’t rise very far up the column before reaching their boiling pount
They condense and are collected at the bottom of the fractionating column
The compounds collected from the fractionating column are then broken down further via method of braking
- Fuel gas (bottled)
- petrol / gasoline - engine
- naptha - chemicals
- kerosene - jet fuel
- diesel oil - engines
- fuel oil - heating
- lubrication oil
- bitumen - road surfacing
vacuum distillation unit
- heavy residues from the fractionating column are distilled again under a vacuum
- Lowering the pressure over a liquid will lower its boiling point
- vacuum distillation allows heavier fractions to be further separated without high tempersuates which could break them down
What is fractional distiallion used for?
- To separate liquids with similar boiling points
Describe how fractional distillation works 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.
Define cracking
- Conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
High Mr alkanes –> smaller Mr alaknes + alkenes + (hydrogen)
What are the economic reasons fro cracking
The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions.
* To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked.
* The products of cracking are more valuable than the starting materials (e.g. ethene used to make poly(ethene), branched alkanes for motor fuels, etc.
The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions.
* To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked.
* The products of cracking are more valuable than the starting materials (e.g. ethene used to make poly(ethene), branched alkanes for motor fuels, etc.)
What kind of process is cracking
- Chemical process involving splitting of strong covalent bonds so requires high temps
two types of cracking
- thermal and catalytic
Describe the conditions of thermal cracking
- High pressure - 7000kPa
- High temp - 400 - 900 C
Describe the products of thermal cracking
- produces mostly alkenes e.g. ethene used for making polymers and ethanol
sometimes produces hydrogen used in the Haber Process and in margarine manufacture.
Example Equations
C8H18 C6H14 + C2H4 C12H26C10H22 +C2H4
Bonds can be broken anywhere in the molecule by C-C bond fission and C-H bond fission.
Describe the conditions of catalytic cracking
- Slight or moderate pressure
- High temp 450 C
- Zeolite catalyst to compensate for less harsh conditions
Describe the products of thermal cracking
Produces branched and cyclic alkanes and aromatic hydrocarbons
catalytic
Used for making motor fuels
Branched and cyclic hydrocarbons burn more cleanly and are used to give fuels a higher octane number
Cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used
What does fuel release when burnt
realises heart energy when burnt
Alkanes readily burn in the presence of oxygen. This combustion of alkanes is highly exothermic, explaining their use as fuels.
complete combustion
- In excess oxygen aklaknes will burn with complete combustion
- Products of complete combustion are CO2 and H2O
Incomplete combustion
- If there is a limited amount of oxygen then incomplete combustion occurs, producing CO, which is very toxic, and C producing a sooty flame
Incomplete combustion produces less energy per mole than complete combustion
Carbon (soot) can cause global dimming- reflection of the sun’s light
Sulfur containing impurities are found in petroleum fractions which produce SO2 when they are burned.
S+ O2SO2 CH3SH+ 3O2 SO2 + CO2 + 2H2O
Coal is high in sulfur content, and large amounts of sulfur dioxide are emitted from power stations.
SO2 will dissolve in atmospheric water and can produce acid rain.
,SO2 can be removed from the waste gases from furnaces (e.g. coal fired power stations) by flue gas desulfurisation. The gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction
SO2 + CaOCaSO3 calcium sulfite
The calcium sulfite which is formed can be used to make calcium sulfate for plasterboard.
Nitrogen oxides form from the reaction between N2 and O2 inside the car engine.
The high temperature and spark in the engine provides sufficient energy to break strong N2 bond
N2 +O22NO N2 +2O22NO2
Describe the environmental consequences of nitrogen oxide
- NO is toxic and can form acidic gas NO2
- NO2 is toxic and acidic and forms acid rain
Describe the environmental consequences of carbon monoxide
- Toxic
Describe the environmental consequences of carbon dioxide
- Contributes towards global warming
Describe the environmental consequences of unburnt hydrocarbons
- Contributes towards global warming
Describe the environmental consequences of soot
- Global dimming and respiratory problems
catalytic converters
These remove CO, NOx and unburned hydrocarbons (e.g. octane, C8H18) from the exhaust gases, turning them into ‘harmless’ CO2, N2 and H2O.
2CO+2NO2CO2 +N2
C8H18 + 25NO 8CO2 + 121⁄2N2 + 9H2O
Converters have a ceramic honeycomb coated with a thin layer of catalyst metals platinum, palladium, rhodium – to give a large surface area.
Carbon dioxide (CO2), methane (CH4) and water vapour (H2O) are all greenhouse gases. Water is the main greenhouse gas (but is natural), followed by carbon dioxide and methane.
Explain the mechanism of 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 CO2 absorb infrared radiation so the IR radiation does not escape from the atmosphere.
This energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed.
why is nitrogen unreacive
triple covalent bond
