Unit 1: Foundation Chemistry Alkanes [1.6]

Joel Biffin

Unit 1 | Alkanes [1.6] Fractional Distillation of Crude Oil

- Fractional Distillation is used to separate the hydrocarbons of different chain lengths.

- Due to van der Waals forces between hydrocarbons:

> Longest chain lengths are tapped off at bottom.

> Highest chain lengths are tapped off at top.

> N.B. branched chain alkanes have lower boiling points than their corresponding straight alkane.

Cracking

- Shorter chained hydrocarbons are more in demand so cracking is used to break down less useful hydrocarbons into more useful, shorter chained hydrocarbons.

- Generally: Long Chained Alkanes -> Shorter Chained Alkanes + Alkenes + H2

- Shorter alkanes have use as fuels and petrochemicals.

- Alkenes are used in the manufacture of polymers.

- Thermal Cracking

> High temperatures (900oC) and pressures (70atm).

> The higher the temperature, the lower Mr alkenes.

> The reaction has a free radical mechanism.

- Catalytic Cracking

> Cracking can occur in the presence of a catalyst at 450oC.

> The catalysts are zeolites.

> Catalytic cracking produces alkanes, cycloalkanes and aromatics.

> The mechanism involves the formation of carbocations.

Unit 1: Foundation Chemistry Alkanes [1.6]

Joel Biffin

Combustion of Alkanes

- Complete combustion occurs in the presence of oxygen, the combustion of alkanes is highly exothermic

- Incomplete combustion occurs when there is a lack of oxygen present.

- Sulfur impurities can be found in petroleum fraction which, when burnt, produce SO2 & SO3.

- The internal combustion engine produces pollutants:

> CO2, H2O, CO, NOx & Unburnt Hydrocarbons.

- Catalytic converters remove these pollutants by converting CO, NOx & Hydrocarbons to CO2, N2 & H2O.

- Converters have a honeycomb coated layer of Pt/Pd/RH - giving a large surface area.

Definitions

Fractional Distillation A method of separating molecules by making use of their different boiling points

Cracking The breaking of C-C bonds in long chain alkanes to give shorter chain, more useful, more valuable hydrocarbons

Thermal Cracking Energy required for bond breaking in hydrocarbon is provided by heat alone. Up to 900oC at 70atm Forms mostly alkanes

Catalytic Cracking

Energy required for bond breaking in hydrocarbon is provided by heat in the presence of a catalyst (zeolite or aluminosilicate) 450oC Forms branched alkanes, cycloalkanes and aromatic hydrocarbons

Fuel Substance which on combustion produces energy

Fossil Fuel Fuel which has been formed over millions of years and which is a finite resource

Greenhouse Gas Any gas which contributes to global warming by absorption of infra-red radiation by their bonds (polar)

CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)

CH4(g) + 3/2 O2(g) -> CO(g) + 2H2O(l)

CH4(g) + O2(g) -> C(s) + 2H2O(l)