3.2 Alkanes Flashcards
what is a petroleum fraction
- a mixture of hydrocarbons with a similar chain length and boiling point range
how does fractional distillation work?
- oil is preheated then passed into column
- the fractions condense at different heights
- temp of column decreases upwards
- seperation depends on boiling point
- boiling points depend on size of molecules
- similar molecules condense together
- small molecules condense at the top at lower temps
- big molecules comsense at the bottom at higher temps
what is cracking?
- conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
what are the economic reasons for cracking
- petroleum fractions with shorter C chains are more in demand than larger fractions
- to make use of excess larger hydrocarbons and to supply demand for shorter ones
- products of cracking are more valuable than the starting materials
what are the conditions for thermal cracking
- high pressure ( 7000kPa )
- high temperature ( 400 to 900 )
what are the conditions for catalytic cracking
- slight or moderate pressure
- high temperature ( 450 )
- Zeolite catalyst
what are the products of thermal cracking
- produces mostly alkenes
- sometimes produces hydrogen
C8H18 > C6H14 + C2H4
what are the products of catalytic cracking
- produces branched and cyclic allanes and aromatic hydrocarbons
why is catalytic cracking cheaper than thermal cracking?
- saves energy as lower temperatures and pressures used
what are the products for complete combustion of alkanes
- carbon dioxide and water
what are the products for incomplete combustion of alkanes
CO and/or C and water
whats the impact of carbon soot
- causes global dimming
whats the impact of SO2 produced
- dissolves in atmospheric water and can produce acid rain
how can SO2 be removed from waste gases from furnaces
- by flue gas desulfurisation
- gases pass through scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide
- SO2 + CaO > CaSO3
how do nitrogen oxides form
- from the reaction between N2 and O2 inside the car engine
- the high temp and spark in engine provides enough energy to break strong N2 bond
give the equations for production of NO2
N2 + O2 > 2NO
N2 + 2O2 > 2NO2
give the environmental consequence for carbon monoxide
- toxic
give the environmental consequence for carbon dioxide
- contributes towards global warming
give the environmental consequence for unburnt hydrocarbons
- contributes towards formation of smog
give the environmental consequence of soot
- causes global dimming and respiratory problems
give the environmental consequence of nitrogen oxides
- toxic and can form acidic gas
- NO2 is toxic and acidic and forms acid rain
what are the roles of catalytic converters
- they remove CO, NOx and unburned hydrocarbons from exhaust gases, turning them into ‘harmless’ CO2, N2 and H20
give the equation for catalytic converters
2 CO + 2 NO > 2CO2 + N2
C8H18 + 25 NO > 8 CO2 + 12.5 N2 + 9H20
why do converters have a ceramic honeycomb coated with a thin layer of catalyst metals
- to give a large surface area
what are the main greenhouse gases
- water, carbon dioxide and methane
what is the mechanism of greenhouse effect
- UV wavelength radiation passes through the atmosphere, and heats up the Earths surface
- 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 have carbon dioxide levels risen significantly in recent years
- due to increasing burning of fossil fuels
- carbon dioxide has though to be largely responsible for
what happens when alkanes react with chlorine in the presence of UV light?
- mixture of products with halogens substituting hydrogen atoms
what happens in initation step?
- condition is UV light
- UV light supplies the energy to break the Cl-Cl bond
- it is broken in preference to the others because it is the weakest
- the bond has broken in a process called homolytic fission
what happens in homolytic fission
- a bond broken in this method will form free radicals
- free radicals do not have a charge
- each atom gets one electron from the covalent bond
what is the definition of a free radical
- a free radical is a reactive species which possesses an unpaired electron
what happens in propagation step
- the halogen free radicals are very reactive and remove a H from the methane leaving a methyl free radical
- the methyl free radicals reacts with a halogen molecules to produce the main product and another halogen free radical
what happens in the termination step?
- collision of two free radicals does not generate further free radicals
- chain is terminated
why is free radicals a chain reaction?
- because the free radical is regenerated, it can react with several more alkane molcls
