12.Alkanes Flashcards
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
