3.3.2 Alkanes Flashcards
Where do we get alkanes from industrially?
Refining crude oil.
Petroleum is a mixture consisting mainly of alkane hydrocarbons.
Petroleum fraction: mixture of hydrocarbons with a similar chain length and boiling point range.
What are the steps of fractional distillation?
- Oil is pre-heated
- then passed into column.
- The fractions condense at different heights
- The temperature of column decreases upwards
- The separation depends on boiling point
- Boiling point depends on size of molecules
- The larger the molecule the larger the van der waals forces
- Similar molecules (size, bp, mass) condense together
- Small molecules condense at the top at lower temperatures
- and big molecules condense at the bottom at higher temperatures
This is a physical process involving the splitting of weak van der waals forces between molecules
What is a vacuum distillation unit?
• Heavy residues from the fractionating column are distilled again under a vacuum.
• Lowering the pressure over a liquid will lower its boiling point.
Vacuum distillation allows heavier fractions to be further separated without high temperatures which could break them down.
What is cracking?
Conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds
High Mr alkanes → smaller Mr alkanes + alkenes + (hydrogen)
What are the 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 hydrocarbons are cracked.
• The products of cracking are more valuable than the starting materials (e.g. ethene used to make poly(ethene), branched alkanes for motor fuels, etc.)
This is a chemical process involving the splitting of strong covalent bonds so requires high temperatures
What are the 2 types of cracking?
Thermal Cracking
Conditions:
High pressure (7000 kPa)
High temperature (400°C to 900°C)
- Produces mostly alkenes e.g. ethene used for making polymers and ethanol - Sometimes produces hydrogen used in the Haber Process and in margarine manufacture
Example Equations
C8H18 → С6Н14 + С2Н4
C12H26 → C10H22 + С2Н4
Bonds can be broken anywhere in the molecule by C-C bond fission and C-H bond fission.
Catalytic Cracking
Conditions:
Slight or moderate pressure
High temperature (450°C)
Zeolite catalyst
- Produces branched and cyclic alkanes and aromatic hydrocarbons
- Used for making motor fuels
Branched and cyclic hydrocarbons burn more cleanly and are used to give fuels a higher octane number.
(Cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used)
What are fuels?
Release heat energy when burnt.
Why are alkanes used as fuels?
Alkanes readily burn in the presence of oxygen. This combustion of alkanes is highly exothermic.
What is complete combustion?
In excess oxygen alkanes will burn with complete combustion.
The products of complete combustion are CO2 and H2O.
C8H18(g) + 12.5O2(g) → 8CO2(g) + 9H2O(l)
What is incomplete combustion?
If there is a limited amount of oxygen then incomplete combustion occurs.
Producing CO (carbon monoxide) and/or C (soot) and H2O.
CH4(g) + 3/2O2(g) → CO(g) + 2H2O(l)
CH4(g) + O2(g) → C(s) + 2H2O(l)
Incomplete combustion produces less energy per mole than complete combustion.
What are the issues caused by sulfur containing impurities being found in petroleum fractions?
They produce SO2 (sulfur dioxide) when they are burned.
SO2 will dissolve in atmospheric water and can produce acid rain.
Explain how SO2 can 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 filter containing calcium oxide or calcium carbonate which reacts with the acidic sulfur dioxide in a neutralisation reaction.
SO2 + CaO → CaSO3 (calcium sulfite)
What are other pollutants caused by combustion of alkanes as fuels and their environmental impact?
- Nitrogen oxides - form from the reaction between N2 and O2 (from air) inside car engines. The high temperature and spark in the engine provides sufficient energy to break strong N2 bond.
(N2 + O2 → 2NO) (N2 + 2O2 → 2NO2)
= NO is toxic and can form acidic gas NO2
= NO2 is toxic and acidic and forms acid rain - Carbon monoxide (CO) - formed from incomplete combustion of fuels containing C.
= Toxic - Carbon dioxide (CO2) - formed from complete combustion of fuels containing C.
= Contributes towards global warming - Unburnt hydrocarbons - not all fuel burns in the engine.
= Contributes towards formation of smog - Soot (C) - formed from incomplete combustion of fuels containing C.
= Global dimming and respiratory problems
What are catalytic converters?
In the internal combustion engine (e.g. cars):
These remove CO, NOx, 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 + 25/2N2 + 9H2O
What are the greenhouse gases?
Carbon dioxide (CO2), methane (CH4) and water vapour (H2O) are all greenhouse gases.
What is the mechanism of the greenhouse effect?
UV wavelength radiation passes through the atmosphere to the Earth’s surface and heats up Earth’s surface.
The Earth radiates out infrared long wavelength radiation.
The C=O Bonds in CO2 absorb infrared radiation so the IR radiation does not escape from the atmosphere.
This energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed.
Carbon dioxide levels have risen significantly in recent years due to increasing burning of fossil fuels. Carbon dioxide is a particularly effective greenhouse gas and its increase is thought to be largely responsible for global warming.
How does the synthesis of halogenoalkanes occur?
Reaction of alkanes with bromine / chlorine in UV light
Outline the mechanism for the production of halogenoalkanes from alkanes (free radical substitution) (Cl2 and methane)
Step one: Initiation
Essential condition: UV light (The UV light supplies the energy to break the Cl-Cl bond)
Cl2 → 2CI•
Step two: Propagation
The chlorine free radicals are very reactive and remove an H from the methane leaving a methyl free radical.
The methyl free radical reacts with a Cl2 molecule to produce the main product and another Cl free radical.
(As the Cl free radical is regenerated, it can react with several more alkane molecules in a chain reaction.)
CH4 + CI• → HCl + •CH3
•CH3 + Cl2 → CH3Cl + CI•
Step three: Termination
Collision of two free radicals does not generate further free radicals: the chain is terminated.
•CH3 + CI• → CH3CI
•CH3 + •CH3 → CH3CH3 (Minor step leading to impurities of ethane in product. Write this step using structural formulae and do not use molecular formulae.)
CI• + CI• → Cl2
Write mechanism of Br and propane.
Step one: Initiation
Essential condition: UV light
Br2 → 2Br•
Step two: Propagation
CH3CH2CH3 + Br• → HBr + CH3CH2CH2•
CH3CH2CH2• + Br2 → CH3CH2CH2Br + Br•
Step three: Termination
CH3CH2CH2• + Br• → CH3CH2CH2Br
CH3CH2CH2• + CH3CH2CH2• → CH3CH2CH2CH2CH2CH3
Br• + Br• → Br2
If the question asks for the halogen to be substituted onto a middle carbon in the chain, it is important to put the free radical ‘dot’ on the correct carbon in the propagation stages.
Further substitution reactions can occur where, for example, you’ve formed CH3Cl from CH4 and Cl2 and then excess Cl2 present will promote further substitution and could produce CH2Cl2, CHCl3 and CCI4.
Write the overall reaction equation for the formation of CCI4 from CH4 + Cl2.
Write the overall reaction equation for the formation of CFCl3 from CH3F + Cl2.
CH4 + 4Cl2 → CCI4 + 4HCI
CH3F + 3Cl2 → CFCl3 + 3HCI
