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
Intermolecular forces? Why?
Only van der Waals forces of attraction - bonds are
non-polar
Solubility 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 and 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 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 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
Pros of fracking?
- Gas supply for many years
- reduces imported gas and electricity
Cons 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, with 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?
700-1200 K temperature
Up to 7000 kPa pressure
What is the intermediate for the thermal cracking 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 (SiO2 and Al2O3) with a honeycomb structure to give a large SA
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 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 catalytic converters made up of?
Ceramic honeycomb coated with platinum, palladium and rhodium (Pt, Pd and Rh) metals
What do catalytic converters catalyse (equations)?
They catalyse these reactions of 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, 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 CO2 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 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
decomposition of O3
Cl2 → 2Cl• (in presence of UV light)
Cl• + O3→ ClO• + O2
ClO• + O3 → 2O2 + Cl•
Overall: 2O3→ 3O2
