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Chemistry > Alkanes > Flashcards

Alkanes Flashcards

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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
Q

under what conditions is catalytic cracking carried out

A

high temp, slight pressure and in presence of a zeolite

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26
Q

what does catalytic cracking produce

A

cycloalkanes, branched alkanes and aromatic hydrocarbons

27
Q

what are the products of catalytic cracking used for

A

motor fuels

28
Q

what is obtained from alkane fuels when they’re combusted

A

energy

29
Q

what do the products formed in the combustion of an alkane depend on

A

the supply of oxygen

30
Q

what products are formed in complete combustion

A

carbon dioxide and water

31
Q

what products form during incomplete combustion

A

carbon monoxide and water

32
Q

when does incomplete combustion occur

A

in a limited supply of oxygen

33
Q

what products form during further incomplete combustion

A

solid carbon (soot) and water

34
Q

when does further incomplete combustion occur

A

in an even more limited supply of oxygen

35
Q

what does the combustion of hydrocarbon fuels produce

A

pollutants

36
Q

what is the effect of unburned hydrocarbons

A

low level ozone (respiratory problems)

37
Q

how do unburned hydrocarbons produce low level ozone

A

reactions with NO gases

38
Q

what is the effect of carbon dioxide

A

global warming

39
Q

what is carbon dioxide produced from

A

combustion of fuels

40
Q

what effect does carbon monoxide have

A

toxic gas

41
Q

how is carbon monoxide produced

A

incomplete combustion of fuels in limited supply of oxygen

42
Q

effect of carbon

A

particles exacerbate asthma

43
Q

how is carbon produced

A

incomplete combustion of fuels in a very limited supply of oxygen

44
Q

effect of nitrogen oxides

A

acid rain and photochemical smog

45
Q

how are nitrogen oxides produced

A

nitrogen and oxygen from the air react at high temperatures in engines

46
Q

effect of sulphur dioxide

A

acid rain

47
Q

how is sulphur dioxide produced

A

sulphur from fuel impurities reacts with oxygen in the air

48
Q

catalytic converters

A

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
Q

how are NO and CO removed

A

react to produce less polluting products eg carbon dioxide and nitrogen

50
Q

flue gas desulfarisation

A

chimneys/flues coated with CaO or CaCO3 which absorb and react with SO2 produced

51
Q

what is halogenation

A

reacting an alkane with a halogen

52
Q

why are alkanes unreactive generally

A

non-polar bonds

lots of strong covalent bonds need to be broken

53
Q

what conditions are needed for halogenation

A

uv light

54
Q

what type of reaction is halogenation

A

substitution- one atom replaced by other

55
Q

what is formed when alkanes with 3 or more carbons react with a halogen

A

a mixture of position isomers

56
Q

what happens if an alkane is reacted with an excess halogen

A

each hydrogen atom can be replaced by halogen atom

57
Q

What is the mechanism for halogenation of alkanes

A

Free radical substitution

58
Q

What is a radical

A

A species with an unpaired electron

59
Q

What are the 3 stages of free radical substitution

A

Initiation

Propagation

Termination

60
Q

What is initiation

A

Formation of radicals

61
Q

What is propagation

A

Formation of products

62
Q

what is termination

A

removal of radicals

63
Q

how is the unpaired electron of a radical shown

A

by a dot

Chemistry (42 decks)

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