3.2 - Alkanes Flashcards
What is an alkane?
A saturated hydrocarbon containing C-H bonds only.
What is the general formula of an alkane?
CnH2n+2
Does a hydrocarbon have polar or nonpolar bonds? Why/why not?
Nonpolar - carbon and hydrogen have similar electronegativities.
What intermolecular forces does a hydrocarbon contain and why?
Only Van der Waals forces of attraction - bonds are non-polar.
Are hydrocarbons soluble in water? Why?
They are insoluble because hydrogen bonds in water are stronger than alkanes’s 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?
Mixture of fractions (hydrocarbons with similar BP and properties).
Formed at high temperatures and pressures deep below earth’s surface over millions of years -> therefore non-renewable.
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 caos - condense when temperature id below their boiling point.
- Shortest chain hydrocarbons condense at the top as they have the lowest BPs.
What is frackiing and how is it done?
- Natural gas is 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 prevent corrosion.
Pros and cons of fracking?
Pros:
- Gas supply for many years.
- Reduces imported gas and electricity.
Cons:
- Lots of traffic to local area.
- Concern about the amount of water useed.
- Chemical additives can polute water supplies.
- Can cause earthquakes.
- Combust CH4 –>CO2 which leads to global warming.
Why are alkanes cracked?
To turn long chain alkane (not economically valuable) into a shorter chain alkane (more economically valuable becaus eit can be used as 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 of catalytic cracking?
- Lower temp (720K)
- Lower pressure (but above atmospheric)
- Zeolite catalyst (SiO2 and Al2O3) with a honecomb 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, prpane, pertrol, paraffin.
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?
The ones with longer chains.
Whar 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 temperature, speeds up climate change.
What is the environmental impact of water vapour?
Greenhouse gs -> global warming, increases global temperatures, speeds up climate change.
