3.2 Alkanes Flashcards
Give the definition of a petroleum fraction.
A petroleum fraction is a mixture of hydrocarbons with a similar chain length and boiling point range.
Describe industrial fractional distillation in steps.
(see page 1 in the chemrevise revision guide)
What is the vacuum distillation unit in fractional distillation? What is the benefit of this?
(see page 1 in the chemrevise revision guide)
Describe how you would execute fractional distillation in the laboratory.
(see page 1 in the chemrevise revision guide)
Give the definition of cracking.
Cracking is the conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
Give a general formula of cracking.
High Mr alkanes -> smaller Mr alkanes + alkenes + (hydrogen)
Cracking is a ______ process involving the splitting of strong _______ bonds so requires _____ temperatures.
Cracking is a chemical process involving the splitting of strong covalent bonds so requires high temperatures.
Fractional distillation is a ______ process involving the splitting of weak ________ forces between molecules.
Fractional distillation is a physical process involving the splitting of weak van der waals forces between molecules.
Draw a labelled diagram of fraction distillation in industry including the different fractions with the boiling points of each.
(8 fractions)
(see page 1 in the chemrevise revision guide)
What are the conditions of thermal cracking?
- High pressure (7000kPa)
- High temperature (400ºC to 900ºC)
What are the conditions of catalytic cracking?
- Slight or moderate pressure
- High temperature (450ºC)
- Zeolite catalyst
What products are produced from thermal cracking?
- Mostly alkenes
(e. g. ethene used for making polymers and ethanol) - Sometimes produces hydrogen
(used in the Haber process and in the margarine manufacture)
What products are produced from catalytic cracking?
- Branched and cyclic alkanes
(burn more cleanly and are used to give fuels a higher octane number) - Aromatic hydrocarbons
(used for making motor fuels)
Is thermal or catalytic cracking cheaper? Explain why.
Catalytic cracking is cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used.
Why are alkanes good as fuels?
Alkanes readily burn in the presence of oxygen. This combustion of alkanes is highly exothermic, explaining their use as fuels.
Give two example equations of thermal cracking.
(see page 2 in the chemrevise revision guide)
What is the definition of fuel?
Fuel releases heat energy when burnt.
What are the products of complete combustion in alkanes?
In excess oxygen alkanes will burn with complete combustion.
The products of complete combustion are CO2 and H2O
Write an equation of the complete combustion of C8H18 in an excess of oxygen.
(see page 2 in the chemrevise revision guide)
What are the products of incomplete combustion in alkanes?
If there is a limited amount of oxygen then incomplete combustion occurs.
Producing CO (very toxic) and/or C (sooty flame)
Write two of the equations for the incomplete combustion of CH4 in a limited amount of oxygen.
(see page 2 in the chemrevise revision guide)
Give two negatives of the incomplete combustion.
- Incomplete combustion produces less energy per mole than complete combustion.
- Carbon (soot) can cause global dimming - reflection of the suns’s light.
What is petroleum?
Petroleum is a mixture consisting mainly of alkane hydrocarbons.
What is fractional distillation used for?
Fractional distillation is used to separate liquids with different boiling points.
Draw a labelled diagram of the apparatus setup for fraction distillation in the laboratory.
(see page 1 in the chemrevise revision guide)
Give 3 economic reasons for cracking.
- The petroleum fractions with shorter C chains (e.g, petrol and naphtha) are in more demand than larger fractions.
- To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hardocrabons are cracked.
- The products of cracking are more valuable than the starting materials (e.g. ethene used to make poly(ethene), branches alkanes for motor fuels, etc.)
What do Sulfur containing impurities produce when burned?
SO2
S + O -> SO2
CH3SH + O2 -> SO2 + CO2 + 2H2O
What is the problem with SO2 in the atmosphere?
SO2 will dissolve in atmosphere water and can produce acid rain.
Why do power stations produce lots of sulfur oxides?
Coal is high in sulfur content, and large amounts of sulfur oxides are emitted from power stations.
How can SO2 be removed from the waste gases from furnaces (e.g. coal fired power stations)?
SO2 can be removed from the waste gases from furnaces (e.g. coal fired power stations) by flue gas desulfurisation. The gases pass through a scrubber containing basic calcium oxide which reacts with the acidic suffer dioxide in a neutralisation reaction.
SO2 + CaO -> CaSO3
The calcium sulphate formed can be used to make calcium sulfate for plasterboard.
How are Nitrogen Oxides commonly formed?
Nitrogen oxides form from the reaction between N2 and O2 inside the car engine.
The temperature and spark in the engine provides sufficient engird to break strong N2 bond.
N2 + O2 -> 2NO
N2 + 2O2 -> 2NO2
What are the environmental consequences of these pollutants:
- Nitrogen oxides
- Carbon monoxide
- Carbon dioxide
- Unburnt hydrocarbons
- Soot
(see page 3 in the chemrevise revision guide)
What do catalytic converters do?
Catalytic converters remove CO, nitrogen oxides, and unburned hydrocarbons (e.g. octane, C8H18) from the exhaust gases, turning them into ‘harmless’ CO2, N2 and H2O.
2CO + 2NO -> 2CO2 + N2
C8H18 + 25NO -> 8CO2 + 12.5N2 + 9H2O
Converters have a ceramic honeycomb coated with a thin layer of catalyst metals platinum, palladium, rhodium - to give a large surface area.
Name 3 greenhouse gases, how do they affect the environment?
Greenhouse gases trap the Earth’s radiated infra red energy in the atmosphere.
- Methane (CH4)
- Water vapour (H2O)
The main greenhouse gas (but its natural) - Carbon dioxide (CO2)
The levels have risen significantly in recent years due to increasing burning of fossil fuels.
Considered to be largely responsible for global warming.
What are the 3 steps in free radical substitution?
- Initiation
- Propagation
- Termination
What is the essential condition for free radical substitution?
UV light.
It supplies the energy to break the bond (e.g. Cl-Cl)
What process of fission occurs in the initiation step of free radical substitution?
Homolytic fission.
Draw all 3 free radical substitution steps for the reaction of methane with chlorine to produce chloromethane and HCl.
(see page 4 in the chemrevise revision guide)
In general why do alkanes not react with many reagents?
In general, alkanes do not react with any reagents. This is because the C-C bond and the C-H bond are relatively strong.
Write the mechanism of Br2 and propane. What is the essential condition of this reaction?
(see page 5 in the chemrevise revision guide)
Write the overall equation for the formation of CCl4 from CH4 + Cl2
(see page 5 in the chemrevise revision guide)
Write the overall equation for the formation of CFCl3 from CH3F + Cl2
(see page 5 in the chemrevise revision guide)
