alkanes Flashcards
(40 cards)
what is the general formula for alkanes?
CnHn+2
what is the general formula for ring alkane formula?
CnH2n
explain the polarity of alkanes
● alkanes are non polar
● electronegativities of carbon and hydrogen are so similar
● intermolecular forces are weak - only van der waals forces
explain boiling points of alkanes
● generally low boiling points
● weak van der waals
● increases as chain length increase due to increasing intermolecular forces
why do branched chain alkanes have lower boiling points that straight chain alkenes?
● branched chains cannot pack together closely (less surface contact) as unbranched chains so VDW forces not effective
explain solubility of alkanes
● insoluble in water
● non polar molecules dissolve in non-polar solvent
● polar molecule dissolve in polar solvent
● alkanes do mix with other non-polar liquid
● therefore, alkanes being non-polar only dissolve in non-polar solvents e.g. oil
explain reactivity of alkanes
● relatively unreactive
● strong carbon-carbon and carbon-hydrogen bonds
● they do combustion reactions (complete and incomplete)
● react with halogens under suitable conditions
how does fractional distillation work?
● crude oil is heated into a vapour in a furnace
● mixture of vapour and liquid passes into a tower that is cooler at the top than at the bottom
● vapours pass up tower via a series of trays containing bubble caps until they arrive at tray that is sufficiently cool (at a lower temp than their boiling point)
then they condense to liquid
● shorter chain hydrocarbons condense in trays nearer to the top of tower - where it is cooler - because they have lower boiling points
● thick residue that collects at base of tower - called tar or bitumen - can be used for road surfacing - as supply often exceeds demand - fraction further processed - give more valuable products
● mixture of liquids that condenses on each tray is piped off
what is a fraction?
alkanes with similar boiling points / chain lengths
what do bubble caps do?
● increase contact time
● more surface for gas to condense
what is crude oil?
● mixture of hydrocarbons
● main source of organic compounds
● formed by the break down of animal and plant remains at high pressure over millions of years
● forms so slowly - effectively non-renewable
● finite resource - used faster than its formed
● mixture of alkanes (branched and unbranched)
what is cracking?
● where long chain fractions (alkanes) are broken into shorter lengths (hydrocarbons)
● shorter chain alkanes are produced (e.g. petrol)
● some products are alkenes which are more reactive than alkanes
● cracking involves the breaking of C-C bonds in alkanes
how do you distinguish between alkenes and alkanes?
● bromine water
● stays orange when mixed with alkane
● turns colourless when mixed with alkene
what are the conditions of thermal cracking?
● heat alkanes to high temp (700 - 1200K)
● high pressure (up to 7000kPa)
● produces alkane and alkene
● produces high number of alkenes
● can also produce hydrogen
● chain can break at any point
● to avoid decomposition of C and H - process happens for 1 second
● alkenes - raw material for polymers (plastics)
what are the conditions of catalytic cracking?
● high temp (720K) - lower than thermal
● slight pressure (more than atmospheric)
● uses a zeolite catalyst (silicon dioxide and aluminium oxide)
● zeolite has honeycomb structure - higher surface area
● form of cracking produces motor fuels
● products include branched alkanes, cycloalkanes and aromatic compounds
● some hydrogen produced
what are the 2 types of combustion?
● incomplete (burned in insufficient oxygen)
● complete (burned in plentiful of oxygen)
● can use halves when balancing combustion equations
what are the products of complete combustion?
● H2O (g)
● CO2 (g)
what are the products of incomplete combustion?
● H2O (g)
● CO (g)
● C (s)
what are the environmental problems associated with carbon monoxide?
● produced by incomplete combustion of fossil fuels (coal, oil, natural gas)
● binds to haemoglobin
● odourless
● colourless
● poisonous
what are the environmental problems associated with nitrogen oxides, NOx?
● NO, NO2, N2O4
● produced in car engines at high temp
● nitrogen and oxygen react (from air/atmosphere)
● react in car engine
● when spark ignites fuel
● N2(g) + O2(g) -> 2NO(g)
● 2NO(g) + O2 (g) -> 2NO2(g)
● nitrogen dioxide reacts with oxygen and water vapour in air to form nitric acid
● which causes acid rain and photochemical smog
● 4NO2(g) + H2O(g) + O2(g) -> 4HNO3(g)
what are the environmental problems associated with sulphur dioxide?
● produced by combustion of sulphur containing fossil fuels
● not usually in car engines as sulfur is removed from petrol
● causes breathing difficulties and asthma attacks
● causes acid rain when sulfur dioxide reacts with water vapour and oxygen in air to form sulphuric acid
● SO2 + H2O + 1/2O2 -> H2SO4
what is flue gas desulphurisation?
● some chimneys (flues) contain CaO or CaCO3 to absorb SO2
● CaO and CaCO3 are bases which neutralise acidic SO2
● produces gypsum, CaSO4 which is used as plaster
● CaO + 2H2O + SO2 + 1/2O2 -> CaSO4.2H2O
● CaCO3 + 1/2O2 + SO2 -> CaSO4 + CO2
what are the environmental problems associated with carbon particulates?
● exacerbates breathing problems
● cancer
what are the environmental problems associated with unburnt hydrocarbons?
photochemical smog
