6.2 - Hydrocarbons/ Alkanes, Alkenes Flashcards
What are Alkanes?
SATURATED hydrocarbons
what re Alkenes?
UNSATURATED hydrocarbons
Alkanes general formula?
CnH2n+2
Alkenes general formula?
CnH2n
Octane numbers?
=A measure of the performance of the fuel
=higher octane means a more efficient fuel
=straight chains are prone to knocking, exploding due to activation energy
=knocking is increased by straight chained hydrocarbons, therefore branched molecules burn better ‘higher octane number’
Reforming?
a catalyst is used together with high temperature
Cracking?
useful, break down hydrocarbons to make more useful fuels
thermal cracking?
temperature and pressure
Catalytic cracking?
catalyst, temperature, pressure
Burning alkanes?
Fractional distillation
Fractional distillation?
Inject crude oil vapourised rises cools condenses collected as fractions
What is the point in burning alkanes?
To make useful substances such as petrol and diesel
Free radical substitution steps?
Initiation
Propagation
Termination
What are the conditions needed for homolytic fission?
U.V light
300 degrees
Initiation?
the molecule is split by homolytic fission, forming 2 radicals
Propagation?
during the halogenation of alkanes, the radical will then go onto react with eg. methane
Rapid process
Termination?
the end of radical substitution where 2 radicals combine to form a molecule
Problems with free radical substitution?
other radicals can be formed
if there’s too much mixture, other products will be formed and therefore have to be separated
Catalytic converters?
Redox reaction
one reduction catalyst and one oxidation
Molecules bond with the catalyst and elements separate off as a harmless gas
Testing for alkenes?
place sample in test tube with drops of bromine water, the more drops needed to go colourless, the more unsaturated
Why are alkenes insoluble in water?
They cant form H bonds because there are no oxygens
Name the 2 bonds in alkenes?
Sigma and pi
When do bonds form?
bonds form between atoms when atomic orbitals overlap and form molecular orbitals
When is a sigma bond formed?
when 2 s orbitals overlap (in a straight line) which is a single covalent bond
When is a pi bond formed?
when 2 p orbitals overlap sideways, it forms an area above and below the molecular axis.
which bond is weaker?
pi bond is MUCH weaker
Why do trans isomers have lower boiling points?
Because the U shape of the cis isomer doesnt pack as well as the straighter shape. Intermolecular forces arent as strong as they should be
Making alkanes from alkenes?
Hydrogenation
150 degrees
nickel catalyst
Heterolytic fission?
is the breaking of a covalent bond so that both bonding electrons are taken by one atom
Making alcohols?
Hydration reaction
eg. Ethene+water = Ethanol
Stability of carbocations?
stable- least stable Quarternary, tertiary, secondary, primary! the more methyl groups, the more stable it is!!
Making diols?
Addition and oxidation takes place in this reaction
The requirements are dilute acidified potassium manganate
monomers and polymers?
know how to draw them
Propagation?
reactions continue until all hydrogen atoms have been replaced with halogen atoms
Everything about natural gas?
naturally occuring
hydrogen based]
used for heating, cooking and electrics
produced as a result of decaying material
Everything about bio diesel?
most economical
renewable fuel
many uses, generators
Nuclear power?
no CO2
hard to dispose of waste
need enriched uranium 235- awkward
