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Chemistry > alkanes > Flashcards

alkanes Flashcards

1
Q

why do branched molecules have lower boiling points

A

there are weaker forces of attraction between branched molecules - prevents closer approach to neighbouring molecules.

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2
Q

what happens when you add an alkane to water

A

forms 2 layers (insoluble)

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3
Q

what conditions are needed for cracking

A

Over 400°c
moderate pressure
zeolite catalyst

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4
Q

% error =

A

error on equipment x times used
———————————————- x 100
measurement taken

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5
Q

which equipment has an error x 2

A

balance
burette

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6
Q

define hydrocarbon

A

compounds containing hydrogen and carbon only

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7
Q

what does an empirical formula show

A

simplest whole number ratio of atoms present in the molecule

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8
Q

what does a molecular formula show

A

number of each type of atom present in the molecule

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9
Q

what does structural formula show

A

functional groups and how the atoms are bonded together

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10
Q

what does displayed formula show

A

shows all the bonds and all the atoms and how they are bonded

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11
Q

what does skeletal formula show

A

does not show C atoms or C-H bonds

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12
Q

define structural isomerism

A

occurs when two or more compounds have the same molecular formula but different structural formulae

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13
Q

define chain isomers

A

a type of structural isomer
they have a different arrangement of the carbon chain but the same molecular formula

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14
Q

why does boiling point increase with increasing chain length

A

more carbon atoms more electrons so stronger london forces
require more energy to overcome

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15
Q

how does branching affect boiling point

A

branching reduces the boiling point

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16
Q

how does fractional distillation work

A

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

17
Q

define cracking

A

the breakdown of molecules into shorter ones by heating with a catalyst

18
Q

what happens during cracking

A

fresh feedstock and catalyst powder flow into the vertical reactor
cracked products pass to a fractionating column
catalyst flows to a regenarator

19
Q

what is reforming

A

the conversion of straight chained hydrocarbons intto branched and cyclic hydrocarbons by heating with a platinum catalyst

20
Q

what is crude oil used for

A

fuel and lubricants
manufacture of paints and polymers

21
Q

what is the purpose of cracking

A

increases our supply of gasoline
produces short chain alkanes that are used to manufacture polymers

22
Q

what do alkanes produce when burned in excess 02

A

CO2 and H2

23
Q

what pollutants are formed from combustion of fuels

A

carbon (soot)
CO
sulfur dioxide
nitrogen dioxide
unburnt hydrocarbons

24
Q

method to find solubility of alkanes in water

A

add 10 drops of each to a half a test tube of water and shake

25
Q

what is required for alkanes to react with chlorine in a substitution reaction to form chloroalkanes

A

UV light

26
Q

what kind of mechanism is an alkane reaction with halogens

A

free radical substitution

27
Q

what is the initiation step

A

in the presence of UV light Cl2 breaks into 2 chlorine free radicals via homolytic fission

28
Q

what do the propagation steps do

A

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

29
Q

what do the termination steps do

A

produce a stable product that wont continue to react

30
Q

method for reaction of hexane with bromine

A
  • 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
31
Q

observations of Br and hexane in UV light
(and with ammonia vapour)

A

red/brown colour-> decolourised
steamy fumes

with ammonia vapour -> white smoke

32
Q

observations of Br and hexane in the dark

A

remains red/brown colour
no reaction

Chemistry (24 decks)

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