C12 Flashcards
what are alkanes
saturated hydrocarbons
contain only single C-C and C-H bonds
main source = crude oil
used as fuels and lubricants
physical properties of alkanes
- polarity
- bp
- solubility
polarity
- alkanes are almost non polar because the electronegativities of carbon and hydrogen are so similar
- as a result the only IM forces between their molecules are weak VdW
- the larger the molecule, the stronger the VdW
bp
- increasing VdW –> bp increases as chain length increases
- short chains = gas at room temp
- 5C = pentane = liquid
- 18C = solid
alkanes with branched chains have lower melting points than straight chain alkanes with the same no. C atoms
- this is because they cannot pack together as closely so VdW are not as effective
solubility
- insoluble in water
- water molecules are held together by hydrogen bonds which are much stronger than the VdW forces between alkane molecules
–> alkanes = non polar
how alkanes react
- relatively unreactive
- strong C-C and C-H
- do not react with acids, bases, oxidising/reducing agents
- they DO burn, and react with haloalkanes under suitable conditions
how was crude oil formed
millions of years ago, by the breakdown of plants and animal remains, mainly plankton, at the high pressures and temperatures in mud deep below the earth’s surface
crude oil is a mixture. what type of molecules is crude oil made of and why does its composition vary
mixture of mostly alkanes, branched and unbranched
crude oils from different sources have different compositions
what impurities does crude oil contain and where do they come from
e.g. sulfur, from original plant and animal matter
- sulfur forms sulfur dioxide when crude oil is burnt
- this causes by acid-rain
—> SO2 reacts in O2 high in atmosphere –> SO3
—> reacts with H2O —> H2SO4
why is it possible to separate crude oil by fractional distillation
- the bp of hydrocarbons depends on the size of its molecule
- the larger the molecule the higher the bp
- crude oil is a mixture of long and short chain hydrocarbons, so can be separated into fractions based on their different bps
what is a ‘fraction’
a mixture of hydrocarbons with similar chain length and therefore similar properties e.g. bp
what is the purpose of bubble caps in fractionating column
- force oil vapour to bubble through liquid on the tray
- this causes the vapour to cool as it flows upwards and to condense into liquids
fractions with smaller hydrocarbons and lower bp…
- have lower viscosities so are runny liquids
- are flammable so they ignite easily
- burn with clean flames, producing little smoke
what process is fractional distillation
a physical process
- no covalent bonds within the molecule are broken
- it is the VdW forces between molecules that are broken during vaporisation and reform on condensing
what is ‘fracking’ and how does it work
- there are resources of natural gas within shale rock
- gas can be extracted by drilling into the shale and forcing pressurised water mixed with sand into the shale
- causes soft shale to fracture, releasing the trapped gas that flows to the surface
fractional distillation vs distillation
fractional distillation: separates a mixture into several components with different ranges of boiling temperatures
distillation: separates all the volatile components of a mixture from the non-volatile ones
why are ‘suitably large’ hydrocarbons cracked
smaller chain molecules = more in demand + more economically valuable
what is cracking used to produce
petrol (short chain molecules)
alkenes
what are alkenes are used for
- chemical feedstock
–> starting materials for different products
converted into many compounds e.g. polymers, paints, drugs
thermal cracking
- homolytic fission/ free radical
- involves heating alkanes to high temp
700-1200K - under high pressure
7000kPa
C-C bonds break so 1 e- from each pair goes to each atom
- so reactive intermediate = C free radicals- highly reactive
- not enough H atoms to produce 2 alkanes, so 1 alkene produced
–> thermal produces high proportion of ALKENES - to avoid too much decomposition, alkanes = kept in conditions for 1 second
catalytic cracking
- carbocation/ heterolytic fission
- takes place at lower temperatures
720 K - lower pressure (but more than atm)
- uses a ZEOLITE catalyst
—> consisting of silicon dioxide and aluminium oxide (aluminosilicates) - zeolites have a honeycomb structure with a huge sa
- also acidic
- normally produces motor fuels.
- products mostly BRANCHED ALKANES, CYCLOALKANES AND AROMATIC COMPOUNDS
what is a fuel
releases heat energy when burnt/ undergo combustion
why are alkanes good fuels
store a large amount of energy for a small amount of weight
what is incomplete combustion and why can this be a problem for the combustion of alkanes
- occurs when fuels are burnt in a limited supply of oxygen
- CO is produced- a toxic gas
occurs most often with longer chain hydrocarbons, which require more oxygen to burn completely
list the pollutants produced when hydrocarbon-based fuels are burnt
*carbon monoxide
- incomplete combustion
- binds with Hb irreversibly
–> breathlessness + death
*nitrogen oxides
- enough energy for O2 and N2 to combine (to break N triple bond N)
- occurs in a petrol engine at the high temps present, when the sparks ignite the fuel
- these oxides react with water vapour and O2 in air to form nitric acid
*sulfur dioxide
- sulfur containing impurities present in crude oil
- combine with H2O and O2 in air –> sulfuric acid
–> acid rain
*carbon particles (particulates)
- incomplete combusting w limited O2
- exacerbate/ athsma cause cancer
*unburnt hydrocarbons
- greenhouse gas
- contribute to photochemical smog which can cause respiratory irritation and difficulty breathing
*CO2
- combustion
- greenhouse gas
- traps IR, earths atmosphere heats up
*H2O vapour
- greenhouse gas
what is photochemical smog
chemical reaction of sunlight, nitrogen oxides and volatile organic compounds in the atmosphere
–> leads to PM and ground level ozone
flue gas desulfurisation
- the process of removing sulfur dioxide from flue gases produced by powerstations
- a slurry of calcium oxide (lime) and water
- or calcium carbonate (limestone)
= sprayed into flue gas
–> reacts to produce calcium sulfite
–> further oxidised to calcium sulfate (gypsum)
gypsum = saleable product used to mke builder’s plaster(board)
describe a catalytic converter
- the internal combustion engine produces most of the pollutants produced in combustion
- catalytic converters reduce the output of carbon monoxide, nitrogen oxides and unburnt hydrocarbons in the exhaust gas mixture
- honeycomb structure made of ceramic material coated with platinum and rhodium metals (as the catalysts)
- honeycomb= large surface area
- so small amount of metal is necessary
as the polluting gases pass over the catalyst, they react with each other to form less harmful products
2CO + 2NO –> N2 + 2CO2
what is the greenhouse effect
the long wavelength energy that the earth radiates as IR is trapped by gases in the atmosphere, causing the earth’s surface to heat up
what does ‘carbon neutral’ mean
no net CO2 emissions are produced
describe fractional distillation of crude oil
to convert crude oil into useful products the mixture must be separated
- this is done by heating the oil and collecting the fractions that boil over different ranges of temperatures
- each fraction = mixture of hydrocarbons with similar chain length and therefore similar properties
- crude oil = heated in furnace
- mixture of liquid and vapour enter the fractionating tower that is cooler at top
–> temperature gradient due to heater - vapours pass up tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool, then condense into liquid
- mixture of liquids that condense = piped off
- shorter chain hydrocarbons condense nearer top, as lower bp
- thick residue at base = tar/ bitumen
formation of haloalkanes
see notes
explain how the CIP priority rules can be used to deduce the full IUPAC name of a compound
- consider groups joined to RH C of the C=C bond
- consider atomic number of atoms attached
—> first atom joined to bond - consider groups joined to LH carbon of C=C
- consider atomic number of atoms attached
higher atomic number takes priority
highest priority group on opposite/ same
