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?
Nonpolar- carbon and hydrogen have similar electronegativities
Which intermolecular forces do they have?
Only Van de Waals forces of attraction - bonds are non polar
Are they soluble in water? Why?
Insoluble because hydrogen bonds in water are stronger than Van de Waals forces of attraction
How reactive are alkanes?
Very unreactive
Which reactions will alkanes undergo?
Combustion and reactions 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 temperatures and pressures deep below the earths surface over millions of years —> therefore non-renewable
Name the fractions from high to low boiling points
Gases - fuels on sight
Gasoline/ petroleum/ naphtha- cars
Kerosene/ paraffin - jet fuel / lighting
Diesel fuel - lorries / taxis
Lubricating oils/ waxes - candles/ engine oil
Fuel oil- ships/ power stations
Tar / bitumen- roads / roofing
What is fractional distillation and how does it work?
Crude oil heated til mostly vaporised
Passed into a fractioning tower that is cooler at the top than at the bottom
Liquid fractions 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 points
What is fracking and how is it done?
Natural gas held within a shale rock
Drill into shale, force pressurised water and sand in the rock to fracture it, collect gas
HCl and methanol added to break up shale and prevent corrosion
Pros/cons of fracking
Advantages - gas supply for many years, reduces imported gas and electricity
Disadvantages - lots of traffic to local area, concern about amount of water used, chemical additives can pollute water supplies, can cause small earthquakes, combustion of CH3 —> CO2 —> global warming
Why are alkanes cracked?
To turn a long chain alkane, which is not very economically valuable, into a shorter chain alkane chain (more economically valuable as used as fuels) 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 temperature (720K)
Lower pressure (but above atmospheric)
Zeolite catalyst (SiO2 and Al2O3) with a honey comb 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 5 main fuels comprising of alkanes ?
Methane, Butane, propane, petrol (about C8) and 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 CO
Toxic/poisonous
What is the environmental impact of NO3
Form nitric acid —> acid rain
Photochemical smog
What is the environmental impact of Sulphur impurities / sulphur dioxide
Form sulphuric acid —> acid rain
What is the environmental impact of Soot (Carbon)
Asthma, cancer, global dimming
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 different equations for two different ways of desulphurising flue gases
Ca (s) + 2H2O (l) + SO2 (g) + 1/2O2 (g) —> CaSO4.2H2O
Or
CaCO3 (s) + SO2 (g) + 1/2 O2 (g) —> CaSO4 + CO2 (g)
What are catalytic converters made of
Ceramic honeycomb coated with platinum palladium and rhodium
What do catalytic converters catalyse
They exhaust these products from car exhausts:
2CO (g) + 2NO (g) —> N2 (g) + 2CO2 (g)
Hydrocarbons + NO —> N2 + CO2 + H2O
What are greenhouse gases
Gases which trap infrared radiation, which make the earth act like a greenhouse
What is the greenhouse effect and how does contribute to global warming
Trap infrared radiation in the atmosphere, atmosphere heats up —> global warming
Define carbon neutral activities
Activities which have 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 radicals
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 of chlorine
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 —> CH2Cl + Cl•
Termination
•CH3 + Cl• —> CH3Cl
2Cl• —> Cl2
•CH3 + •CH3 —> CH3CH3
What is the ozone layers function
Protect the earth from too much harmful UV radiation
How do CFCs break down the ozone layer
Free radical substitution
Write an equation for the overall decomposition of the ozone layer
2O3 —> 3O2
Write free radical substitution reactions to show how Cl• catalyse the breakdown of O3
Cl2 —> 2Cl• (in the presence of UV light)
Cl• + O3 —> ClO• + O2
ClO• + O3 —> 2O2 + Cl•
Overall: 2O3 —> 3O2
