[3.3.2] Alkanes

Question 1

What are alkanes?

Answer 1

Saturated hydrocarbons.

Question 2

What is petroleum?

Answer 2

A mixture consisting mainly of alkane hydrocarbons.

Question 3

What is a petroleum fraction?

Answer 3

Mixture of hydrocarbons with a similar chain length and boiling point range.

Question 4

Describe the process of fractional distillation.

Answer 4

FRACTIONAL DISTILLATION PROCESS (KEY POINTS)

• This is a physical process involving the splitting of weak van der Waals forces between molecules.

1. Oil is pre-heated and passed into a fractional distillation column.
2. The fractions condense at different heights.
3. The temperature of the column decreases upwards.
4. The separation depends on boiling point and boiling point depends on the size of molecules.
5. The larger the molecule, the larger the van der Waals forces.
6. Similar molecules (size, boiling point, mass) condense together.
7. Small molecules condense at the top at lower temperatures and big molecules condense at the bottom at higher temperatures.

Question 5

What is vacuum distillation?

Answer 5

• 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.

Question 6

Describe the process for fractional distillation in the laboratory.

Answer 6

PROCESS

• Fractional distillation is used to separate liquids with similar boiling points.

1. Heat the flask with a Bunsen burner or electric mantle.
2. This causes vapours of all the components to be produced.
3. Vapours pass up the fractionating column.
4. The vapour of the substance with the lower boiling point reaches the top of the fractionating column first.
5. The thermometer should be at or below the boiling point of the most volatile substance.
6. The vapours with higher boiling points condense back into the flask.
7. Only the most volatile vapour passes into the condenser.
8. The condenser cools the vapours and condenses to a liquid and is collected.

Question 7

What is cracking?

Answer 7

Conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds.

Question 8

What are the economic reasons for cracking?

Answer 8

1. The petroleum fractions with shorter C chains are in more demand then larger fractions.
2. To make use of excess larger hydrocarbons and to supply demand for short ones, longer hydrocarbons are cracked.
3. The products of cracking are more valuable than the starting materials

Question 9

Describe the conditions and products of thermal cracking.

Answer 9

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.

Question 10

Describe the conditions and products of catalytic cracking.

Answer 10

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.

Question 11

What is fuel?

Answer 11

Releases heat energy when burnt.

Question 12

Why are alkanes used for fuel?

Answer 12

• Alkanes readily burn in the presence of oxygen.
• This combustion of alkanes is highly exothermic, explaining their use in fuels.

Question 13

What are the conditions and products of complete combustion?

Answer 13

• In excess oxygen alkanes will burn with complete combustion.
• The products of complete combustion are CO₂ and H₂O.

Question 14

What are the conditions and products of incomplete combustion?

Answer 14

• 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.

Question 15

How can the combustion of hydrocarbons containing sulfur cause air pollution?

Answer 15

• 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.

Question 16

Explain why sulfur dioxide can be removed from flue gases using calcium oxide.

What equation represents this process?

Answer 16

• 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.

Question 17

Internal combustion engines produce a number of pollutants.

What are these pollutants and what are their environmental consequences?

Answer 17

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.

Question 18

How can gaseous pollutants formed from internal combustion engines be removed using catalytic converters?

What equations represent this process?

Answer 18

• 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

Question 19

What are the greenhouse gases?

Describe the mechanism of the greenhouse effect.

What are the problems associated with increasing levels of greenhouse gases?

Answer 19

• Carbon dioxide (CO₂), methane (CH₄) and water vapour (H₂O) are all greenhouse gases.

MECHANISM OF GREENHOUSE EFFECT

1. UV wavelength radiation passes through the atmosphere to the Earth’s surface and heats up the Earth’s surface.
2. The earth radiates out long wavelength radiation.
3. The C=O bonds in CO₂ absorb infrared radiation so the IR radiation does not escape from the atmosphere.
4. 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.

Question 20

Why don’t alkanes react with many reagents?

Answer 20

This is because the C-C bond and the C-H bond are relatively strong.

Question 21

What are the steps in free radical substitution?

Answer 21

The mechanism proceeds via a series of steps:

• Step 1. Initiation.
• Step 2. Propagation.
• Step 3. Termination.

Question 22

Explain the reaction of methane with chlorine as a free-radical substitution mechanism.

Answer 22

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

1. CH₄ + Cl· -> HCl + ·CH₃
   
   • The chlorine free radicals are very reactive and remove an H from the methane leaving a methyl free radical.
2. ·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

Question 23

Write the free radical substitution mechanism of Br₂ and propane.

Answer 23

Question 24

Write the overall reaction of methane with chlorine.

Answer 24

CH₄ + Cl₂ -> CH₃Cl + HCl

Question 25

Answer 25

(Note HCl is always the side product - never H₂)

Question 26

Answer 26

(Note HCl is always the side product - never H₂)