3.2 Alkanes Flashcards
What is an alkane?
A saturated hydrocarbons containing C-H bonds only
What is the general formula of an alkane?
CnH2n+2
Are their bonds polar?
Why/ why not?
Non polar as carbon and hydrogen have similar electronegativities
Which intermolecular forces do they have? Why?
Only VDW forces of attraction as bonds are non-polar
Are they soluble in water?
Why?
Insoluble because hydrogen bonds in water are stronger than alkanes’ van der waals forces of attraction
How reactive are alkanes?
Very unreactive
Which reactions will alkanes undergo?
Combustion and reaction with halogens
What is crude oil? How is it formed? Is it renewable? Why?
Mixture of fractions (hydrocarbons with similar boiling points and properties)
Formed at high temperature and pressures deep below earth’s surface over millions of years —> therefore non-renewable
Name the fractions from high to low boiling points
Gases - fuel on site
Gasoline/petrol - cars
Kerosine/paraffin - jet fuel, lighting
Diesel oil - lorries/taxis
Lubricating oil/ waxes- candles, engine oil
Fuel oil - ships, power stations
Tar/bitumen - roads/roofing
What is fractional distillation/how does it work?
Crude oil heated until mostly vapourised
Passed into a fractionating tower that is cooler at the top than the bottom
Liquid fraction are piped off at the bottom
Vapours rise up the column and - via trays and bubble caps - condense when temperature < their boiling point
Shortest chain hydrocarbons condense at the top as they have the lowest boiling point
What is fracking and how is it done?
Natural gas held within shale rock
Drill into shale, force pressurised water and sand into rock to fracture it, collect gas
HCL and methanol added to break up shale and prevent corrosion
Advantages of fracking
Gas supply for many years, reduces imported gas and electricity
Disadvantage of fracking
Lots of traffic to local area, concern about amount of water used, chemical additives can pollute water supplies, can cause small earthquakes
Combust CH4 —> CO2 —> global warming
Why are alkanes cracked?
To turn a long chain alkane, which is not very economically valuable, into a shorter chain alkane( more economically valuable as can be used as a fuel) and an alkene (more reactive, starting point for many products)
What are the conditions for thermal cracking ?
700-1200K temperature
Up to 7000kPa pressure
What is the intermediate for the reaction?
Free radicals
What are the main products of thermal cracking?
Alkenes
What are the conditions for catalytic cracking?
Lower temp(720K)
Lower pressure (but above atmospheric)
Zeolite catalyst with a honeycomb structure to give a large surface area
What are the main products of catalytic cracking?
Cycloalkanes, aromatic hydrocarbons,branched alkanes
Write an equation for the combustion of propane
C3H8 + 5O2 —> 3CO2 + 4H2O
What is a fuel?
Something which releases heat energy when combusted
What are the five main fuels comprising of alkanes?
Methane, butane,propane,petrol,(about C8), paraffin (C10-C18)
What is incomplete combustion and what products are formed in the case of alkanes?
Combustion in a limited supply of oxygen
CO - carbon monoxide - poisonous
C - Carbon - particulates - soot - global dimming
Which type of hydrocarbons are most likely to undergo incomplete combustion?
Longer chains
What is the environmental impact of carbon monoxide?
It is toxic/ poisonous
What is the environmental impact of nitrogen oxides?
Form nitric acid —> acid rain, photochemical smog
What is the environmental impact of sulfur impurities/ sulfur dioxide?
Form sulphuric acid —> acid rain
What is the environmental impact of soot (carbon)
Asthma, cancer, global dimmming
What is the environmental impact of unburnt hydrocarbons?
Photochemical smog
What is the environmental impact of carbon dioxide?
Greenhouse gas —> global warming, increases global temperatures, speeds up climate change
What is the environmental impact of water vapour?
Greenhouse gas —> global warming, increases global temperatures, speeds up climate change
What are flue gases
Gases given out by power stations
Write two equations for two differ ways of desulfurising flue gases
What are catalytic converters made up of?
Ceramic honeycomb coated with platinum
What do catalytic converters catalyse (equations)?
What are greenhouse gases?
Gases which trap infrared radiation, making the earth act like a greenhouse
What is the greenhouse effect and how does it contribute to global warming?
Greenhouse gases trap infrared radiation in the atmosphere.
Atmosphere heats up —> global warming
Define carbon neutral activities
Activities that produce no net / overall carbon dioxide emissions
How are halogenoalkanes formed from alkanes
Free radical substitution reaction
What are the three stages of free radical substitution?
Initiation-breaking halogen bond to form free radical
Propagation - chain part of the reaction where products are formed but free radical remains
Termination - free radicals removed, stable products formed
What are the conditions needed for the formation of a free radical chlorine atom
Presence of UV light
Write equations for the reaction of CH4 with Cl2 to form CH3Cl
Initiation: Cl2 —> 2Cl* (in presence of UV light)
Propagation: Cl* + CH4 —> HCl + *CH3
CH3 + Cl2 —> CH3Cl + Cl
Termination:
CH3 + Cl —> CH3Cl
2Cl* —> Cl2
*CH3 + *CH3 —> CH3CH3
What is the ozone layer’s function?
Protects the earth from harmful exposure to too many UV rays
How do CFCs break the ozone layer down?
Free radical substitution
Write an equation for the overall decomposition of ozone into oxygen (O2)
2O3 —>3O2
Write free radical substitution equations to show how Cl free radicals catalyse the break down of O3
Cl2 —> 2Cl* (in presence of UV light)
Cl* + O3 —> ClO* + O2
ClO* + O3 —> 2O2 + Cl*
Overall: 2O3 —> 3O2
Outline What is free radical substitution
It’s when a molecule forms a radical. This radical now reacts with a non radical to form another radical. This new radical reacts with a non radical to form a radical. This process keeps happening until a radial reacts with a radical to form a non radical.
