12.Alkanes
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What are alkanes
Saturated hydrocarbons
What are saturated hydrocarbons
A compound containing only H and C with only C-C and C-H bonds, no C=C
Describe the physical properties of alkanes: polarity
• non polar - as electronegativity between C and H are so similar
What are the intermolecular forces that act between alkanes
only van der Waals
Describe the physical properties of alkanes: boiling points
• as chain length increases • so does van der Waal forces strength • as more electrons present • so boiling point increases
Why do alkanes with branched chains have lower melting points than straight chain alkanes with the same number of carbon atoms
• cannot pack together as closely • less interaction of van der Waal forces • weaker van der Wall forces • lower boiling point
Describe the physical properties of alkanes: solubility
• insoluble in water • water is held by much stronger hydrogen bonds than the van der Waals • lipid soluble
How do alkanes react
• strong H-C bonds so relatively unreactive • react with halogens • burn in oxygen
What is crude oil made of
Branched and unbranched alkanes and impurities
How are alkanes in crude oil separated
Fractional distillation in a fractionating tower
Describe the process of fractional distillation
• crude oil heated in furnace • mixture of liquid and vapour passes into a tower that is cooler at the top than at the bottom • vapour pass up tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool and condense • shorter chains condense in towers near the top (lower boiling points) • thick residue collects at base called tar
What is cracking
The breaking of C-C bonds in a long hydrocarbon chain to make shorter chains
What is useful about cracking
• produces shorter, more useful chains (especially petrol) - some of the products are alkenes which are more reactive than alkanes
What does cracking produces
• shorter alkanes - high % of alkenes
What are the two types of cracking
• thermal - catalytic
Describe thermal cracking conditions
• high temperature (700-1200 K) - high pressure (7000 kPa).
What is produced during thermal cracking
A mixture of alkanes, alkenes and sometimes hydrogen
Describe catalytic cracking
• lower temperature (720 K) (still high) • lower pressure • zeolite catalyst
What is produced during catalytic cracking
• branched alkanes • cycloalkanes • aromatic compounds
What commercial product is produced by catalytic cracking
Motor fuel
Name alkane fuels
• methane • propane • butane • petrol • paraffin
What is incomplete combustion
A combustion reaction in which there is insufficient oxygen for all the carbon in the fuel to burn to carbon dioxide. Carbon monoxide and/or carbon are formed
List pollutants hydrocarbon based fuels produce
• CO (poisonous gas) • nitrogen oxides (produced when there is enough energy for O2 and N2 to react, happens in engines) • sulfur dioxide (produced from sulfur containing impurities in oil) • CO2 (climate change) • unburned hydrocarbons
What are contributors to acid rain
NOx and SO2