alkanes Flashcards
why do branched molecules have lower boiling points
there are weaker forces of attraction between branched molecules - prevents closer approach to neighbouring molecules.
what happens when you add an alkane to water
forms 2 layers (insoluble)
what conditions are needed for cracking
Over 400°c
moderate pressure
zeolite catalyst
% error =
error on equipment x times used
———————————————- x 100
measurement taken
which equipment has an error x 2
balance
burette
define hydrocarbon
compounds containing hydrogen and carbon only
what does an empirical formula show
simplest whole number ratio of atoms present in the molecule
what does a molecular formula show
number of each type of atom present in the molecule
what does structural formula show
functional groups and how the atoms are bonded together
what does displayed formula show
shows all the bonds and all the atoms and how they are bonded
what does skeletal formula show
does not show C atoms or C-H bonds
define structural isomerism
occurs when two or more compounds have the same molecular formula but different structural formulae
define chain isomers
a type of structural isomer
they have a different arrangement of the carbon chain but the same molecular formula
why does boiling point increase with increasing chain length
more carbon atoms more electrons so stronger london forces
require more energy to overcome
how does branching affect boiling point
branching reduces the boiling point
how does fractional distillation work
crude oil vapourised at base of column
this temperature < the bp of the longest molecules which liquefy/solidify as tar
others remain as vapours and they rise up to the next layer
pass through bubble traps and condense
shorter molecules may revapourise and continue to rise to the next layer where they condense
define cracking
the breakdown of molecules into shorter ones by heating with a catalyst
what happens during cracking
fresh feedstock and zeolite flow into the vertical reactor
cracked products pass to a fractionating column
catalyst flows to a regenarator
what is reforming
the conversion of straight chained hydrocarbons intto branched and cyclic hydrocarbons by heating with a platinum catalyst
what is crude oil used for
fuel and lubricants
manufacture of paints and polymers
what is the purpose of cracking
increases our supply of gasoline
produces short chain alkanes that are used to manufacture polymers
what do alkanes produce when burned in excess 02
CO2 and H2
what pollutants are formed from combustion of fuels
carbon (soot)
CO
sulfur dioxide
nitrogen dioxide
unburnt hydrocarbons
method to find solubility of alkanes in water
add 10 drops of each to a half a test tube of water and shake
what is required for alkanes to react with chlorine in a substitution reaction to form chloroalkanes
UV light
what kind of mechanism is an alkane reaction with halogens
free radical substitution
what is the initiation step
in the presence of UV light Cl2 breaks into 2 chlorine free radicals via homolytic fission
what do the propagation steps do
set up a chain reaction where the chlorine free radical is regenerated at the end of step 2 ready to attach another molecule of methane
what do the termination steps do
produce a stable product that wont continue to react
method for reaction of hexane with bromine
- 1cm3 hexane in 2 text tubes
- 1 drop of bromine to each
- one in sunlight or UV
- wrap foil around the other
- leave for 5-10 minutes
- hold a glass rod dipped in concentrated ammonia solution so that gas blows across the top of each tube
observations of Br and hexane in UV light
(and with ammonia vapour)
red/brown colour-> decolourised
steamy fumes
with ammonia vapour -> white smoke
observations of Br and hexane in the dark
remains red/brown colour
no reaction
