3.3.2 Alkanes Flashcards
Alkane
- Saturated hydrocarbon containing C-H bonds only
Crude oil
- Mixture of diff hydrocarbons
Steps in fractional distillation
- Vapourised + fed to column
- Rise, cool + condense
- Products siphoned off
Short carbon chains
- Lower bp, rise higher, collected at top
Long carbon chains
- Higher bp, collected at bottom.
How are hydrocarbons broken down further?
- Cracking
2 types of cracking
- Thermal
- Catalytic
Why cracking?
- Forms smaller more useful molecules
Thermal cracking produces:
- Alkanes + alkenes
Thermal cracking requires
- 1000℃ + 70atm
Catalytic cracking produces:
- Aromatic compounds w/ c rings
Catalytic cracking requires
- 450℃ + atmospheric pressure
- ZEOLITE CATALYST
Needed for complete combustion
- Oxygen
Products of complete combustion
- CO2 + H2O
Incomplete combustion
- Not enough oxygen
Products of incomplete combustion
- CO + H2O + C(soot)
Catalytic converter used for
- Remove toxic gases by converting to more stable products
What gases are removed w/ catalytic converter?
- Nitrogenous oxides (NOx)
- CO
What catalyst is in catalytic converter
- Rhodium
- Platinum
- Palladium
What are the products of catalytic converters?
- N2, H2O, CO2
Carbon particulates
- Small fragments of unburned hcarbons
Problems of carbon particulates
- Respiratory problems
Sulfur impurities problems
- Acid rain / acidification of water
- H2SO4
How can sulfur impurities be removed?
- Flue gas desulfurisation
What is used in flue gas desulfurisation?
- CaO
- CaCO3
What is needed for production of halogenoalkanes?
- UV light
What are 3 stages of chlorination of alkanes?
- Initiation
- Propagation
- Termination
What happens in initiation?
- Halogen broken down
Initiation equation
- Cl2 —> 2Cl* (UV needed)
What happens in propagation?
- H is replaced by Cl* radical reformed as catalyst
Propagation equations
- Cl* + CH4 —> CH3* + HCl
- CH3* + Cl2 —> CH3Cl + Cl*
What happens in termination?
- 2 radicals join to end reaction + form stable product
Termination equations
- *CH3 + *CH3 —> C2H6
- *Cl + *Cl —> Cl2
- *Cl + *CH3 —> CH3Cl
Are alkane bonds polar?
- No, similar electronegativities
Alkanes soluble in water?
- No, H bonds in water stronger than alkanes VDWs
How reactive alkane?
- Very unreactive
What reactions happen w alkanes?
- Combustion
- Reaction w/ halogens
Fractions
- Gases
- Petrol
- Kerosene
- Diesel
- Lubricating oil
- Fuel oil
- Bitumen
How does fractional distillation work?
- Crude oil vapourised + entered into fractionating column.
- Hot bottom, cooler top.
- HCs have diff boiling points ∴ condense at diff points of fractionating column.
What is fracking + how is it done?
- Natural gas in shale rock.
- Drill into shale + force water + sand into rock to fracture it.
- Collect gas.
During fracking what is added to break up shale + prevent corrosion?
- HCl + methanol.
What are 2 pros of fracking?
- Gas supply for years.
- Reduced imported gas.
What are 4 cons of fracking?
- Traffic to local area.
- Lots of water used.
- Pollution of water supplies.
- Can cause small earthquakes.
What are the products of cracking alkanes?
- Shorter chain alkane.
- Alkene.
What is the benefit of cracking a long alkane to produce a short alkane?
- More economically valuable as used as fuel.
What is the benefit of cracking a long alkane to produce an alkene?
- More reactive.
- Starting point for many products.
What are the conditions of thermal cracking?
- 1000℃.
- 70 atm.
What are the main products of thermal cracking?
- Alkenes.
Conditions of catalytic cracking?
- 450℃.
- Zeolite catalyst (honeycomb structure).
- Just above 1 atm.
3 products of catalytic cracking?
- Cycloalkanes.
- Aromatic HCs.
- Branched alkanes.
What is a fuel?
- Something which releases heat energy when combusted.
What are the products of incomplete combustion in the case of alkanes?
- Carbon monoxide.
- Carbon.
- Water.
What is the environmental impact of CO?
- Poisonous/ toxic.
What is the environmental impact of C?
- Global dimming.
- Asthma.
- Cancer.
Which type of HCs are most likely to undergo incomplete combustion?
- Longer chain HCs.
What is the environmental impact of nitrogen oxides?
- Forms nitric acid –> acid rain.
- Photochemical smog.
What is the environmental impact of Sulphur dioxide/ sulphur impurities?
- Sulphuric acid –> acid rain.
What is the environmental impact of unburnt HCs?
- Photochemical smog.
What is the environmental impact of CO2?
- Greenhouse gas –> global warming.
What is the environmental impact of water vapour?
- Greenhous gas –> global warming.
What are flue gases?
- Gases given out by power stations.
What are catalytic converters made up of?
- Ceramic honeycomb.
- Coated in Platinum, palladium + rhodium.
What are greenhouse gases?
- Gases which trap infrared radiation + make earth act like greenhouse.
What is greenhouse effect?
- Short UV waves enter, reflected as long infrared waves.
- Greenhouse gases trap infrared, atmosphere heats up.
Carbon neutral activities?
- Activities that produce no net CO2 emissions.
How are halogenoalkanes formed from alkanes?
- Free radical substitution reactions.
What are 3 stages of free radical substitution?
- Initiation, propagation + termination.
What happens in initiation?
- Breaking halogen bond to form free radicals.
What happens in propagation?
- Products are formed but free radical remains.
What happens in termination?
- Free radicals removed, Stable products formed.
What is needed for formation of free radical chlorine atom?
- UV light.
What is the function of the ozone layer?
- Protects earth from harmful exposure to too many UV rays.
How do CFCs break down ozone layer?
- Free radical substitution.
Flue gas desulfurization with CaO equation
- CaO(s) + 2H2O(g) + SO2(g) + 1/2O2(g) —> CaSO4.2H2O(s).
Flue gas desulfurization with CaCO3 equation
- CaCO3 + 1/2O2(g) + SO2(g) —> CaSO4(s) + CO2(g).
