[3.3.2] Alkanes Flashcards
Fractional Distillation of Crude Oil, Modification of Alkanes by Cracking, Combustion of Alkanes & Chlorination of Alkanes.
What are alkanes?
Saturated hydrocarbons.
What is petroleum?
A mixture consisting mainly of alkane hydrocarbons.
What is a petroleum fraction?
Mixture of hydrocarbons with a similar chain length and boiling point range.
Describe the process of fractional distillation.
FRACTIONAL DISTILLATION PROCESS (KEY POINTS)
- This is a physical process involving the splitting of weak van der Waals forces between molecules.
- Oil is pre-heated and passed into a fractional distillation column.
- The fractions condense at different heights.
- The temperature of the column decreases upwards.
- The separation depends on boiling point and boiling point depends on the size of molecules.
- The larger the molecule, the larger the van der Waals forces.
- Similar molecules (size, boiling point, mass) condense together.
- Small molecules condense at the top at lower temperatures and big molecules condense at the bottom at higher temperatures.
What is vacuum distillation?
- 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.
Describe the process for fractional distillation in the laboratory.
PROCESS
- Fractional distillation is used to separate liquids with similar boiling points.
- Heat the flask with a Bunsen burner or electric mantle.
- This causes vapours of all the components to be produced.
- Vapours pass up the fractionating column.
- The vapour of the substance with the lower boiling point reaches the top of the fractionating column first.
- The thermometer should be at or below the boiling point of the most volatile substance.
- The vapours with higher boiling points condense back into the flask.
- Only the most volatile vapour passes into the condenser.
- The condenser cools the vapours and condenses to a liquid and is collected.
What is cracking?
Conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds.
What are the economic reasons for cracking?
- The petroleum fractions with shorter C chains are in more demand then larger fractions.
- To make use of excess larger hydrocarbons and to supply demand for short ones, longer hydrocarbons are cracked.
- The products of cracking are more valuable than the starting materials
Describe the conditions and products of thermal cracking.
CONDITIONS
- High pressure - 7000kPa.
- High temperature - 400°C to 900°C.
PRODUCTS
- Produces mostly alkenes.
- e.g. ethene used for making polymers and ethanol.
- Sometimes produces hydrogen used in the Haber process and in margarine manufacture.
Describe the conditions and products of catalytic cracking.
CONDITIONS
- Slight or moderate pressure.
-
High temperature - 450°C.
- Cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used.
- Zeolite catalyst.
PRODUCTS
- 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.
What is fuel?
Releases heat energy when burnt.
Why are alkanes used for fuel?
- Alkanes readily burn in the presence of oxygen.
- This combustion of alkanes is highly exothermic, explaining their use in fuels.
What are the conditions and products of complete combustion?
- In excess oxygen alkanes will burn with complete combustion.
- The products of complete combustion are CO₂ and H₂O.
What are the conditions and products of incomplete combustion?
- If there is a limited amount of oxygen, then incomplete combustion occurs.
- The products of incomplete combustion are CO and/or C and H₂O.
- CO is very toxic.
- C (soot) produces a sooty flame and can cause global dimming (reflection of the sun’s light).
- Incomplete combustion produces less energy per mole than complete combustion.
How can the combustion of hydrocarbons containing sulfur cause air pollution?
- Sulfur-containing impurities are found in petroleum fractions which produce SO₂ (sulfur dioxide) when they are burned.
- Coal is also high in sulfur and large amounts of sulfur dioxide are emitted from power stations.
- SO₂ will dissolve in atmospheric water and can produce acid rain.
Explain why sulfur dioxide can be removed from flue gases using calcium oxide.
What equation represents this process?
- SO₂ can be removed from the waste gases from furnaces by flue gas desulfurisation.
- The gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction.
- SO₂ + CaO -> CaSO₃
- The calcum sulfite which is produced can be used to make calcium sulfate for plasterboard.
Internal combustion engines produce a number of pollutants.
What are these pollutants and what are their environmental consequences?
NITROGEN OXIDES (NOₓ)
- Nitrogen oxides form from the reaction between N₂ and O₂ inside car engines.
- The high temperature and spark in the engine provides sufficient energy to break the strong N₂ bond.
- N₂ + O₂ -> 2NO
- NO is toxic and can form acidic gas NO₂ which is also toxic and can form acid rain.
CARBON MONOXIDE
- Toxic.
CARBON DIOXIDE
- Contributes to global warming.
UNBURNT HYDROCARBONS
- Not all fuel burns in the engine.
- This can contribute towards the formation of smog.
SOOT
- Global dimming & respiratory problems.
How can gaseous pollutants formed from internal combustion engines be removed using catalytic converters?
What equations represent this process?
- Catalytic converters remove CO, NOₓ and unburned hydrocarbons (e.g. C₈H₁₈) from exhaust gases, turning them into CO₂, N₂ and H₂O.
- Catalytic converters have a ceramic honeycomb coated with a thin layer of catalyst metals - platinum, palladium, rhodium - to give a large surface area.
- The processes within converters can be represented by:
- 2 CO + 2 NO - > 2 CO₂ + N₂
- C₈H₁₈ + 25 NO -> 8 CO₂ + 12.5 N₂ + 9 H₂O
What are the greenhouse gases?
Describe the mechanism of the greenhouse effect.
What are the problems associated with increasing levels of greenhouse gases?
- Carbon dioxide (CO₂), methane (CH₄) and water vapour (H₂O) are all greenhouse gases.
MECHANISM OF GREENHOUSE EFFECT
- UV wavelength radiation passes through the atmosphere to the Earth’s surface and heats up the Earth’s surface.
- The earth radiates out long wavelength radiation.
- The C=O bonds in CO₂ absorb infrared radiation so the IR radiation does not escape from the atmosphere.
- The energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed.
PROBLEMS ASSOCIATED WITH INCREASING LEVELS OF GREENHOUSE GASES
- Carbon dioxide levels have risen significantly during recent years due to increasing burning of fossil fuels.
- CO₂ is a particularly effective greenhouse gas and it’s thought to be largely responsible for global warming.
Why don’t alkanes react with many reagents?
This is because the C-C bond and the C-H bond are relatively strong.
What are the steps in free radical substitution?
The mechanism proceeds via a series of steps:
- Step 1. Initiation.
- Step 2. Propagation.
- Step 3. Termination.
Explain the reaction of methane with chlorine as a free-radical substitution mechanism.
STEP ONE: INITIATION
- All propagation steps have a free radical in the reactants and in the products.
- Cl₂ -> 2 Cl·
- UV light supplies the energy to break the Cl-Cl bond.
- The bond is broken in a process called homolytic fission which forms two chlorine free radicals.
- A free radical is a reactive species which possesses an unpaired electron.
STEP TWO: PROPAGATION
-
CH₄ + Cl· -> HCl + ·CH₃
- The chlorine free radicals are very reactive and remove an H from the methane leaving a methyl free radical.
-
·CH₃ + Cl₂ -> CH₃Cl + Cl·
- 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.
STEP THREE: TERMINATION
- Collision of two free radicals does not generate further free radicals: the chain is terminated.
- ·CH₃ + Cl· -> CH₃Cl
- ·CH₃ + ·CH₃ -> CH₃CH₃
Write the free radical substitution mechanism of Br₂ and propane.
Write the overall reaction of methane with chlorine.
CH₄ + Cl₂ -> CH₃Cl + HCl
