Alkanes Flashcards
What are alkanes?
A homologous series of saturated hydrocarbons with the general formula (CnH2n+2).
What does saturated mean?
Contains single bonds only.
Molecule containing no double bonds.
What does unsaturated mean?
Molecule containing double bonds.
What is a hydrocarbon?
Molecule containing hydrogen and carbon only.
What is a homologous series?
A family of compounds with the same general formula and similar chemical properties.
In a series each number differs by the addition of a CH2 group and there is a gradual change in physical properties.
Longer carbon chain means what for the boiling point?
Higher boiling point.
Stronger Van der Waal’s forces between molecules.
More electrons in the molecules.
More branched chain means what for the boiling point?
Lower boiling point.
Weaker Van der Waal’s forces between molecules.
Not being able to pack as close together.
What is the point of fractional distillation of petroleum?
Crude oil has no use in its raw form.
But you can separate it out into more useful bite (fractions) by fractional distillation.
As the C chain gets longer, the hydrocarbons are:
More viscous.
Harder to ignite.
Less volatile.
Higher boiling point.
What is the basic idea for the separation process of fractional distillation?
- The crude oil is vaporised at about 350^oc
- The vaporised crude oil goes into the bottom of the fractioning column and rises through the trays.
- As this happens, it gets cooler creating a temperature gradient .
- Molecules condense at different heights (different boiling points)
Larger the molecule = lower down it condenses.
What happens to hydrocarbons with a boiling point that is too high or low during fractional distillation?
Too high = doesn’t vaporise at all, runs to the bottom and forms a gooey residue (this can then be separated at a lower pressure)
Too low = doesn’t condense, drawn off as gases at the top of the column.
Why is cracking used?
To make use of excess larger hydrocarbons and to supply demand for shorter ones.
What takes place during thermal cracking?
High temp = pressure (1000^oc, 70atm)
Produces lots of alkenes e.g. valuable product= polyethene (made from ethene)
What takes place during catalytic cracking?
Uses a zeolite catalyst
Slight pressure and high temp (500^oc, 1-2atm)
Produces aromatic hydrocarbons and the alkanes needed to produce motor fuels (cycloalkanes)
Why does using a catalyst cut costs?
Reaction can be done at a lower temp + pressure
Also speeds up the rate of reaction = saving time (time is money)
How does SO2 form?
Sulfur-containing alkanes can produce SO2 during combustion.
Why is SO2 bad?
It dissolves in rain water to form acid rain.
Why is flue gas desulfurisation used?
So SO2 can be removed from the waste gases from furnaces.
How does flue gas desulfurisation work?
The gases pass through a scrubber containing calcium oxide or calcium carbonate (which reacts with the SO2) = (acid base reaction)
CaO + SO2 -> CaSO3
CaO3 + SO2 -> CaSO3 + CO2
CaSO3 + gypsum -> plasterboards
What does a catalytic converter do?
Removes toxic gases ( CO, NOx &unburned hydrocarbons) from the exhaust gases, turning them into CO2, N2, H2O
What is a catalytic converter?
It has a ceramic honeycomb coated with a thin layer of catalyst metals (Pt, Pd, Rh) to give a large surface area.
What happens during initiation in the chlorination of alkanes?
The Cl-Cl bond is broken by UV radiation by homolytic fission
(Covalent bond split down the middle)
(Free radicals formed)
Cl2 -> Cl* + Cl*
What happens during propagation in the chlorination of alkanes?
Continuing the reaction
Free radicals are used up and created in a chain reaction
Cl* + CH4 -> CH3* + HCl
CH3* + Cl2 -> CH3Cl + Cl*
What happens during termination in the chlorination of alkanes?
Free radicals are removed Cl* + Cl* -> Cl2 CH3* + CH3* -> C2H6 CH3* + Cl* -> CH3Cl ^All include half arrows from star to middle to show the movement of electrons.
What is a condition for the free radical substitution? and why?
Must be excess methane so no further substitutions take place
As favoured product= chloromethane (not di or tri etc)
What does the ozone layer do?
Naturally protects us from the harmful UV radiation produced by the sun.
What are the equations for the decomposition of the ozone layer? and what does this do?
Cl* + 03 -> ClO* + O2
ClO* + O2 -> 202 + Cl*
The chlorine radicals are regenerated and act as a catalyst in the breakdown of ozone molecules into oxygen.
What are now used instead of CFCs?
HFCs as an alternative.
