3. Alkanes Flashcards
(23 cards)
Define “hydrocarbon”.
A compound consisting of carbon and hydrogen only.
Define “saturated hydrocarbon”.
Contains no double or triple carbon bonds.
What is the general formula for alkanes?
CnH2n+2
Define “fractional distillation”.
The separation of components in a mixture based on their boiling points. The technique involves boiling and condensing and collecting fractions over particular boiling point ranges.
What is thermal cracking?
The breakdown of the less useful longer hydrocarbon fractions to more volatile shorter hydrocarbon fractions, using heat in the absence of air.
What is the general equation for thermal cracking?
CabHxy -> CaHx + CbHy
Saturated -> Saturated + Unsaturated
What gases are produced by burning fossil fuel?
Greenhouse gases (carbon dioxide) and acidic gets (sulfur dioxide and oxides of nitrogen). The former absorb IR radiation from Earth’s surface and reflect it back, heating the planet. Causes climate change. The latter dissolve into water and cause acid rain.
What is being done to limit greenhouse gas production?
Removing sulfur and nitrogen from fuels before combustion
Using catalytic converters in cars
Scrubbing emissions with alkali to neutralise acidity levels
Using renewable energy sources
What is the role of catalytic converters?
They convert polluting/harmful exhaust emissions from cars to less polluting products, by oxidising or reducing reactions on the surface of the catalyst.
Honeycomb ceramic structure coated with metal catalyst. LSA, low cost … Lead ruins it.
Physical properties of alkenes?
State changes from gas (methane to butane) to liquid (pentane onwards) to solid (heptadecane).
Viscosity and melting point increase due to van dear Waals forces.
Alkanes are non-polar.
Complete combustion of alkanes?
Needs a source of ignition and a plentiful air supply
Produces water and carbon dioxide
Generally needs fractions for equation
Incomplete combustion of alkanes?
Needs a source of ignition and a limited supply of air
Forms water and carbon monoxide, and maybe soot
Again, half numbers
Monohalogenation of alkanes?
Requires UV light and produces a mixture of haloalkanes.
Methane + chlorine produces HCl and various C-Cl isomers.
Similar with bromine.
Define “substitution reaction”.
Where one group or atom is replaced by another group or atom.
Define “free radical”.
A reactive species with an unpaired electron.
Define “photochemical”.
Requires UV light to initiate.
What are the three steps in the monohalogenation of alkanes?
Initiation
Propagation
Termination
What happens during the initiation step?
A molecule is split into radicals using UV light.
The halogen, I’m assuming.
So you get halogen free radicals.
Example of homolytic fission.
Define “homolytic fission”.
The breaking of a covalent bond in a molecule to form identical species as one electron from the bond goes to each atom.
What happens in the propagation step?
Pattern: molecule + radical -> radical + molecule
So adding the new chlorine radical to a methane molecule gives a chlorine molecule and a methyl radical.
What happens in the termination step?
Pattern: two radicals -> molecule
Chlorine radicals -> chlorine molecule
Methyl radical and chlorine radical -> chloromethane
Two methyl radicals -> ethane
What is the overall equation for monohalogenation of alkanes?
Methane + Chlorine -> Chloromethane + Hydrogen chloride
But also dichloromethane, trichloromethane, tetrachloromethane, and ethane
Define “mechanism”.
The way the reactants change into the products, possibly showing movement of electrons, breaking and formation of covalent bonds, and any intermediate species.
