6B: Alkanes Flashcards
Alkanes
Saturated hydrocarbons
Petroleum fraction
Mixture of hydrocarbons with a similar chain length and boiling point range
Fractional distillation
1) Oil is preheated (vaporised at 350 degrees) then passed into column
2) fractions condense at different heights
3) temperature of column decreases upwards
4) separation depends on boiling point
5) boiling point depends on size of molecules
6) the larger the molecule the large the London forces
7) similar molecules (size, bp, mass) condense together
8) small molecules condense at the top at lower temperatures
9) big molecules condense at the bottom at higher temperatures
10) hydrocarbons with lowest bp don’t condense but are drawn off as gases at top of column
- physical process involving splitting of weak London forces between molecules
Thermal cracking
1) produces a lot of alkenes
2) high temp (1000 degrees) and high pressure (70 atm)
3) alkenes used to make polymers
Catalytic cracking
Thermal decomposition
1) produces aromatic hydrocarbons and motor fuels
2) uses a zeolite catalyst (hydrates aluminosilicate) at a slight pressure and high temp (450 degrees)
- catalyst cut costs: reaction is low pressure and low temperature
3) petroleum fractions with shorter C chains (petrol and naphtha) are in more demand than larger fractions
4) to make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked
5) products of cracking are more useful and valuable than starting materials
6) smaller alkanes are used for motor fuels which burn more efficiently
- chemical process: involves splitting of strong covalent bonds therefore higher temperatures
Reforming
Processing straight chain alkanes into branched and cyclic alkanes and aromatic hydrocarbons
Burn more cleanly and more efficient combustion
Uses a catalyst: platinum stuck on aluminium oxide
Used for making motor fuels
Fuel
Releases heat energy when burnt
Alkanes as fuels
Alkanes readily burn in presence of oxygen
Combustion of alkanes is highly ectothermic, explaining their use as fuels
Larger alkanes release more energy per mole as they have more bonds to react
Incomplete combustion
Limited amount of oxygen, produces CO (toxic) or C (sooty flame)
Produces less energy per mole than complete combustion
Carbon (soot)/particulates can cause global dimming: reflection of sun’s light
Carbon monoxide is highly toxic, colourless and odourless gas, can cause death if builds up in an enclosed space due to faulty heating appliances
CO is toxic to humans as CO can form a strong bond with Hb in RBC. Stronger bond than that made with oxygen and therefore prevents oxygen attaching to Hb leads to oxygen deprivation and fatality
Pollution from combustion
Coal is high in sulfur content: large amounts of sulfur oxides emitted from power stations
Produces acid rain: destroy trees, kills fish and damages statues and buildings
Nitrogen oxides form from reaction between N2 and O2 inside car engine
High temperature and spark, high pressure in engine provide sufficient energy to break strong N2 bond to make NO and NO2
Pollutants and environmental consequences:
NO: rocks and can form smog (reacts under UV light to form a photosensitive haze - smog)
NO2: toxic and acidic and forms acid rain
CO: toxic
CO2: contributes to global warming
Unburnt hydrocarbons: contributes towards formation of smog
Soot/particulates: global dimming and respiratory problems
Catalytic converters
Three way converters remove, COx, NOx and unburned hydrocarbons from exhaust gases and turning them into N2, O2 and H2O
Converters have a ceramic honeycomb coated with a thin layer of catalyst metals: platinum, palladium and rhodium to give a large SA
