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Chemistry > Alkanes > Flashcards

Alkanes Flashcards

1
Q

General formula alkanes

A

CnH2n+2

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2
Q

Alkanes

A

Contain single carbon-carbon bonds only

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3
Q

How many bonding pairs of electrons is each carbon atoms in an alkane surrounded by

A

4

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4
Q

What is the shape and bond angle around a carbon atom in an alkane

A

Tetrahedral

109.5

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5
Q

Structural isomers

A

Compounds with the same molecular formula but a different structural formula

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6
Q

Cycloalkanes have…

A

Two less hydrogen atoms than the open chain alkane and are names according to the largest ring

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7
Q

Explain the polarity of alkanes

A

C and H- similar electronegativities- non-polar bonds

Neighbouring molecules attracted to each other by VDWs- weak intermolecular forces

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8
Q

Why are alkanes not soluble in water

A

Water molecules held together by hydrogen bonds which are stronger than VDWs in alkanes

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9
Q

What is the correlation between length of carbon chain and boiling point of the alkabe

A

As length or carbon chain increases, boiling point of alkane increases

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10
Q

Why does boiling point increase with length of carbon chain

A

More electrons- VDWs stronger- require more energy to break

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11
Q

What is the correlation between number of branches and boiling point

A

As number of branches increase boiling point decreases

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12
Q

Why does boiling point decreases as more branches are added

A

Fewer points of contact between molecules- VDWs weaker- require less energy to break

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13
Q

Crude oil

A

A mixture consisting mainly of alkane hydrocarbons- main source of fuels and petrochemicals

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14
Q

Fractional distillation

A

The separation of the components of a liquid into fractions which differ in boiling point

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15
Q

Each fraction is a group of…

A

Compounds that have similar boiling points and are removed at the same level

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16
Q

What happens to boiling points as you go up the column

A

They decrease

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17
Q

Process of fractional distillation

A
  • crude oil vaporised- vapour introduced at bottom of column
  • vapour rises and creates temp gradient
  • alkanes have different boiling points so condense at different levels- fractions collected
  • hydrocarbons with lowest boiling points don’t condense and are drawn off as gases at top of tower
  • largest hydrocarbons don’t vaporise and are collected at base of tower as thick residue
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18
Q

what is the issue with crude oil

A

demand for shorter chain hydrocarbons doesn’t match abundancies in crude oil

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19
Q

how are the issues with crude oil solved

A

longer, less useful alkanes converted into more useful molecules through cracking

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20
Q

two types of cracking

A

thermal and catalytic

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21
Q

what conditions is thermal cracking carried out under

A

very high temp and pressure

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22
Q

what does thermal cracking produce

A

alkanes and a high percentage of alkenes

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23
Q

why can thermal cracking create a mixture of products

A

c-c bonds break at different positions in the chain

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24
Q

what can the products of thermal cracking be used for

A

to make polymers

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25
under what conditions is catalytic cracking carried out
high temp, slight pressure and in presence of a zeolite
26
what does catalytic cracking produce
cycloalkanes, branched alkanes and aromatic hydrocarbons
27
what are the products of catalytic cracking used for
motor fuels
28
what is obtained from alkane fuels when they're combusted
energy
29
what do the products formed in the combustion of an alkane depend on
the supply of oxygen
30
what products are formed in complete combustion
carbon dioxide and water
31
what products form during incomplete combustion
carbon monoxide and water
32
when does incomplete combustion occur
in a limited supply of oxygen
33
what products form during further incomplete combustion
solid carbon (soot) and water
34
when does further incomplete combustion occur
in an even more limited supply of oxygen
35
what does the combustion of hydrocarbon fuels produce
pollutants
36
what is the effect of unburned hydrocarbons
low level ozone (respiratory problems)
37
how do unburned hydrocarbons produce low level ozone
reactions with NO gases
38
what is the effect of carbon dioxide
global warming
39
what is carbon dioxide produced from
combustion of fuels
40
what effect does carbon monoxide have
toxic gas
41
how is carbon monoxide produced
incomplete combustion of fuels in limited supply of oxygen
42
effect of carbon
particles exacerbate asthma
43
how is carbon produced
incomplete combustion of fuels in a very limited supply of oxygen
44
effect of nitrogen oxides
acid rain and photochemical smog
45
how are nitrogen oxides produced
nitrogen and oxygen from the air react at high temperatures in engines
46
effect of sulphur dioxide
acid rain
47
how is sulphur dioxide produced
sulphur from fuel impurities reacts with oxygen in the air
48
catalytic converters
fitted to cars to remove CO, NO and unburned hydrocarbons contain honeycomb structure coated with Pt/Pd/Rh metals honeycomb structure- large surface area
49
how are NO and CO removed
react to produce less polluting products eg carbon dioxide and nitrogen
50
flue gas desulfarisation
chimneys/flues coated with CaO or CaCO3 which absorb and react with SO2 produced
51
what is halogenation
reacting an alkane with a halogen
52
why are alkanes unreactive generally
non-polar bonds lots of strong covalent bonds need to be broken
53
what conditions are needed for halogenation
uv light
54
what type of reaction is halogenation
substitution- one atom replaced by other
55
what is formed when alkanes with 3 or more carbons react with a halogen
a mixture of position isomers
56
what happens if an alkane is reacted with an excess halogen
each hydrogen atom can be replaced by halogen atom
57
What is the mechanism for halogenation of alkanes
Free radical substitution
58
What is a radical
A species with an unpaired electron
59
What are the 3 stages of free radical substitution
Initiation Propagation Termination
60
What is initiation
Formation of radicals
61
What is propagation
Formation of products
62
what is termination
removal of radicals
63
how is the unpaired electron of a radical shown
by a dot

Chemistry (42 decks)

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