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
Name two methods of flue gas desulfurisation
- reacting it with calcium oxide to form gypsum
- reacting it with calcium carbonate
What is the equation for the reaction of flue gas and calcium oxide
CaO (s) + 2H2O (l) + SO2 (g) + 1/2O2 (g) —-> CaSO4.2H2O (s)
What is the equation for the reaction of flue gas and calcium carbonate
CaCO3 (s) + 1/2O2 (g) +SO2 (g) —-> CaSO4 (s) + CO2 (g)
What do catalytic converters
Reduce the output of CO, NOx and unburnt hydrocarbons in the exhaust gas mixture
Describe structure of a catalytic converter
Honeycomb made of a ceramic material coated with platinum and rhodium metals. Honeycomb shape provides an enormous surface area
Write the two reactions that occur in catalytic converters
- 2CO (g) + 2NO (g) —> N2 (g) + 2CO2 (g)
- hydrocarbons + nitrogen oxide —> nitrogen + carbon dioxide + water
Name the stages of free radical substitution
- initiation
- propagation
- termination
How do Cl-Cl bonds break under UV light
- UV light energy is greater than Cl-Cl bond so it breaks
- both atoms are the same so it breaks homiletically
- forms two free radicals
Write the initiation step of free radical sub
Cl-Cl —-> 2Cl•
…………uv
Write the propagation steps
- Cl• + CH4 —> HCl + •CH3
2. •CH3 + Cl2 —> CH3Cl + Cl•
Write possible termination steps
- Cl•+ Cl• —-> Cl2
- •CH3 + •CH3 —->C2H6
- Cl• + •CH3 —-> CH3Cl
What is the overall reaction of free radical substitution
CH4 (g) + Cl2 —> CH3Cl (g) + HCl
Write how dichloromethane can be made in the propagation stage
- CH3CL + Cl• —-> •CH2Cl +HCl
2. •CH2Cl +Cl2 —>CH2Cl2 + Cl•
Why are chain reactions important
Chlorine free radicals formed from CFCs are destroying the ozone layer which protects the earth from UV
What is the reaction for the breaking down of the ozone layer
Cl• + O3 —->ClO• + O2
ClO + O3 —-> 2O2 + Cl•
2O3 —> 3O2
