Unit 3: Section 2 - Alkanes and Halogenoalkanes Flashcards
What is an alkane?
A saturated hydrocarbon containing C-H bonds only
What is the general formula of an alkane?
CnH2n+2
Are alkane’s bonds polar? Why/why not?
Non polar - carbon and hydrogen have similar electronegativities
Which intermolecular forces do alkenes have? Why?
Only van der Waals forces of attraction - bonds are non-polar
Are alkanes 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 temperatures and pressures deep below earth’s surface over millions of years -> therefore non-renewable
Name the fractions from high to low boiling point
- Gases - fuel on site
- Gasoline/petrol/naphtha - cars
- Kerosene/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 vaporised
- Passed into a fractionating tower that is cooler at the top than 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 with shale rock
- Drill into shale, force pressurised water and sand into rock to fracture it, collect gas
- HCl and methanol added to beak up shale and prevent corrosion
Pros/cons of fracking?
Advantages - gas supply for many year, 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, 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 7000 kPa pressure
What is the intermediate for the thermal cracking?
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 (SiO2 and Al2O3) 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 warming
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
What are the equations for 2 different ways of desulfurising flue gases?
CaO(s) + 2H2O(l) + SO2(g) + 1/2O2(g) -> CaSO4.2H2O(s)
OR
CaCO3(s) + SO2(g) + 1/2O2(g) -> CaSO4(s) + CO2(g)
What are catalytic converters made up of?
Ceramic honeycomb coated with platinum, palladium and rhodium
What do catalytic converters catalyse (equations)?
These reactions of products from car exhausts
* 2CO(g) + 2NO(g) -> N2(g) + 2CO2(g)
* Hydrocarbons + NO -> N2 + C)2 + H2O
What are greenhouse gases?
Gases which trap infrared radiation
What is the greenhouse effect an how does it contribute to global warming?
- Greenhouses gases trap infrared radiation in the atmosphere
- Atmosphere heats up
What is the definition of 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 3 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 radicals remain
- Termination - free radicals removed, stable products formed
What are the conditions needed for the formation of a free radical chlorine atom?
UV light
What is the ozone’s layer function?
Protects the earth from harmful exposure to too many UV rays
How do CFCs break down the ozone layer?
Free radical substitution
What is the equation for the overall decomposition of ozone into oxygen?
2O3 -> 3O2
What are the free radical substitution equations that show how Cl free radicals catalyse the break down of O3?
Cl2 -> 2Cl⋅ (in the presence of UV)
Cl⋅ + O3 -> ClO⋅ + O2
ClO⋅ + O3 -> 2O2 + Cl⋅
Are halogenoalkanes soluble in water?
Insoluble as C-H bonds are non-polar, not compensated for enough by C-X bond polarity
Do halogenoalkanes have a polar bond? Why?
Yes polar, as halogen has a higher electronegativity than C (halogen is δ-, carbon is δ+)
What intermolecular forces do halogenoalkanes have? Why?
Permanent dipole-dipole and van der waals forces of attraction
C-X bond polarity creates permanent dipoles
When would halogenoalkanes have higher boiling points?
- Increase carbon chain length
- Halogen further down group 7
How would the mass of a halogenoalkane compare with the mass of an alkane of the same chain length
Greater as mass of halogen > mass of H
What is the most important factor in determining a halogenoalkane’s reactivity?
Carbon-halogen bond enthalpy
What is the order of reactivity of halogenoalkanes?
Although C-F is the most polar bond, the bond enthalpy of C-X decreases down the group, so reactivity increases down the group
What is a nucleophile?
A negatively charged ion/δ- atom with a lone pair of electrons which can be donated to an electron deficient atom
What is nucleophilic substitution?
A reaction where a nucleophile donates a lone pair of electrons to δ+ C atom, δ- atom leaves molecule (replaced by nucleophiles)
What are CFCs?
Chloro-fluoro-carbons - halogenoalkanes containing C, F and Cl only (no H)
What is the problem with CFCs?
Although unreactive under normal conditions, they catalyse the breakdown of ozone in the atmosphere via free radical substitution
What are CFCs being replaced with?
- HCFCs (hydrogen, chlorine, fluorine, carbon)
- HFCs (hydrogen, fluorine, carbon)
What are the conditions/reactants needed for the elimination reaction of halogenoalkanes?
NaOH or KOH dissolved in ethanol (no water present)
Heated
What is formed from the elimination reaction of halogenoalkanes?
An alkene, water and halogen ion
