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Chemistry🧪 > O1: Alkanes + Halogenoalkanes > Flashcards

O1: Alkanes + Halogenoalkanes Flashcards

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1
Q

Are alkanes un-saturated or saturated?

A

Saturated hydrocarbons

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2
Q

What does saturated hydrocarbon mean?

A

Contain only single bonds

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3
Q

What are some uses of alkanes?

A

-Fuels
-Lubricants
-Crude oil

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4
Q

What is the general formula for Alkanes?

A

CnH2n+2

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5
Q

Describe the boiling point trends of alkanes

A

The greater the chain length, the greater the number of electrons, and VDW forces.

Therefore as chain length increases, boiling point increases.

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6
Q

Why do branched chained alkanes have lower boiling points than straight chain?

A

They cannot pack together as closely (as straight chain) so the VDW forces are less effective.

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7
Q

Are alkanes soluble in water, why/why not?

A

They are insoluble- water molecules are held together by H bonds which are much stronger than VDW forces between alkane molecules, however alkanes do mix with other relatively non-polar liquids.

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8
Q

How do alkanes react?

A

-Unreactive
-Have strong C-C and C-H bonds
-However can burn and will react with halogens under suitable conditions
-Require lots of oxygen to burn, or else carbon monoxide is produced.

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9
Q

What is fractional distillation?

A

Separating a liquid (crude oil) by their boiling points into fractions

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10
Q

What is cracking?

A

Breaking longer chains into smaller, more useful ones.

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11
Q

As Mr increases, what happens to the boiling points (fractional distillation)?

A

The greater the Mr, the greater the boiling point.

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12
Q

Crude oil can contain other compounds such as sulphur, why is this a problem?

A

Because when burnt it will make sulphur dioxide, then mixes with oxygen high in the atmosphere to make sulphur trioxide, which reacts with water to make sulphuric acid (acid rain).

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13
Q

Briefly describe the process of fractional distillation

A
  • Crude oil heated in a furnace, vapourises
  • Goes into fractionating column where it is cooler at the top than the bottom.
  • Vaporises and travels up, then condense at their boiling point into liquid.
  • Shorter chain are more at the top as they have lower boiling points.
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14
Q

List the fractions of crude oil that condense in order.

A

Top:

Petroleum gases
Naphtha (petrol)
Kerosene
Diesel oil
Lubricating oil
Fuel oil
Bitumen

Bottom:

British Football Leagues Don’t Kick Penalties Good

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15
Q

Why do we crack longer chain hydrocarbons?

A

Shorter chain hydrocarbons are more useful in industry (e.g naphtha) and yet we have a lot more longer chains.

We crack to break the longer ones in to shorter ones.

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16
Q

What is thermal cracking?

A

Heating alkanes to a high temperature: 700-1200K
Under a high pressure: 1000kPa

Produces a high % of alkenes due to it producing free radicals originally.

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17
Q

What is catalytic cracking?

A

A high temperature of 720K and a zeolite catalyst, and low pressures

Produces motor fuels and aromatic hydrocarbons, branched and cycloalkanes

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18
Q

When in Bromine water, how do alkanes and alkenes react?

A

Alkanes: orange > orange

Alkenes: orange > colourless

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19
Q

Why are shorter chain alkanes good fuels?

A

They are unreactive, but if burned in plenty of Oxygen they will produce CO2 and H2O. This gives out a lot of heat energy.

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20
Q

What is incomplete combustion?

A

When an alkane is burned in a limited supply of oxygen, that produces a poisonous gas called carbon monoxide, and water

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21
Q

When hydrocarbon based fuels are burnt, the produce polluting products, list some

A

-Carbon monoxide > poisonous gas
-Nitrogen oxides > acid rain (nitric acid)
-Sulphur dioxide > acid rain
-Carbon particulates > asthma and cause cancer, smog
-Unburnt hydrocarbons > greenhouse gas, add to photochemical smog
-Carbon dioxide > greenhouse gas
-Water vapour > greenhouse gas

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22
Q

What is ‘Flue gas desulfurisation’?

A

Process of removing sulphur dioxide from flue gases (given out by power stations).

-Lime water is sprayed inro flue gas to form calcium sulfite, and oxidised inro calcium sulphate and gypsum.

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23
Q

What can a catalytic converter do?

A

Catalyses unburnt hydrocarbons into H2O and CO2, and nitric oxide into nitrogen and CO2, using a platinum and rhodium catalyst.

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24
Q

How is a halogenoalkane formed?

A

Mixing an alkane and halogen into bright sunlight, or photoflood lamp. A substitution reaction occurs.

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25
Q

What is a substitution reaction?

A

A reaction which occurs when one functional group is swapped for another.
e.g hydrogen is swapped with chlorine.

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26
Q

What is a photochemical reaction?

A

Caused by UV light- the reaction cam repeat itself until the hydrocarbons have been replace and we are left with tetrachloromethane.
Conditions: UV light, room temperature.

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27
Q

What is a free radical?

A

An uncharged molecule (typically highly reactive and short-lived) having an unpaired valency (outer) electron.

e.g Cl-Cl&raquo_space; 2•Cl

The Cl-Cl has 2 arrows outward from the bond with a singular head

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28
Q

What is the first step of Chlorination?

A

Initiation:

-Breaking the Cl-Cl bond to form 2 chlorine atoms
-Absorbs the UV light and breaks
-One electron goes to each atom, leaving an unpaired electron
-This is a free radical

Cl-Cl > 2Cl•

29
Q

What is the second step of Chlorination?

A

Propagation:

-2 stages

  1. Chlorine free radical rips off a hydrogen atom, now the methane becomes a free radical

Cl• + CH4 > HCl + •CH3

  1. Methyl free radical is also very reactive, so reacts with a chlorine molecule, ripping a Cl off.

•CH3 + Cl2 > CH3Cl + Cl•

These step repeat thousands of times.

30
Q

What is the third step of Chlorination?

A

Termination:

-Free radicals are removed by pairing up.
-Using the free radicals from propagation, can do 3 ways:

Cl• + Cl• > Cl2

•CH3 + •CH3 > C2H6

Cl• + •CH3 > CH3Cl

Form stable compounds.

31
Q

Can you have a hydrogen free radical?

A

No

32
Q

What are other names for Chlorination?

A

-Free radical substitution
-Chain reaction

33
Q

What is the role of the ozone layer?

A

-Protects us from UV light, made from ozone (O3)
-Naturally breaks down CFC bonds and forms free radicals, which react with O3 to produce O2
-Part of the atmosphere

34
Q

What are the effects of too much ozone at ground level?

A

-Lung irritation
-Degradation of paints and plastics

35
Q

What do chlorine free radicals do to the ozone?

A

Cl free radicals are produced from halogenoalkanes and break down additional ozone faster than its replaced.

36
Q

What are CFCs and what do they do?

A

Chloroflurocarbons- damage the ozone layer, but can be replaced with HFCs (hydrogenflurocarbons) which do not undergo radicalisation in the atmosphere to produce chlorine atoms.

37
Q

What is a Nucleofile?

A

An electron pair donor

-Attacks the partial charge on the carbon (+) as it doesn’t have enough electrons.

38
Q

What happens in nucleophilic substitution?

A

Replace the halogen in a halogenoalkane.

-The nucleofile will attack the partially positive carbon atom
-A curly arrow is drawn from the lone pair on the nucleofile to the carbon.
-Another curly arrow is dram from the bond C-X to X, electron pair is moving to the halogen, now a halide ion, this is called the leaving group.

39
Q

What are the ions, reagents, and halogens involved in nucleophilic substitution?

A

Halogens: Cl, Br, I, F

Reagents: NaOH, KCN, NH3

Nucleofile: -:OH -:CN :NH3

40
Q

What does reaction of a halogenoalkane and cyanide produce?

A

Nitrile

41
Q

What does the reaction of a halogenoalkane and ammonia produce?

A

Amine

42
Q

What does the reaction of a halogenoalkane and a hydroxide produce?

A

Ethanol

43
Q

What are the reagents, nucleophile, conditions and products of a Nucleophilic substitution reaction with a hydroxide?

A

Reagent: NaOH or KOH
Nucleophile: :OH-
Conditions: aqueous (dissolved in water), warm
Products: alcohol

44
Q

What are the reagents, nucleophile, conditions and products of a nucleophilic substitution reaction with cyanide?

A

Reagents: NaCN or KCN
Nucleophile: :CN-
Conditions: ethanolic, heated
Produces: nitrile

45
Q

What are the reagents, nucleophile, conditions, and products of a nucleophilic substitution reaction with ammonia?

A

Reagents:
Nucleophile: :NH3
Conditions: ethanolic, heat, high pressure, excess ammonia
Products: Amine

46
Q

What is the rate of reactions for the halogenoalkanes?

A

Increases down the group, iodine reacts the fastest

-This is due to iodine having the lowest bond enthalpy so requires less energy to break

47
Q

What is the test for halide ions?

A

Aqueous silver nitrate

F-= no ppt
Cl-= white ppt
Br-= cream ppt
I-= pale yellow ppt

48
Q

Why is the ozone layer being broken down a problem?

A

Once broken into O2, it does not protect from UV rays

49
Q

What does the Cl free radical act as in ozone depletion?

A

Catalyst

50
Q

What are CFCs used for?

A

Refrigerants
Solvents
Propellents

51
Q

What has replaced CFCs?

A

Hydroflurocarbons and hydrocarbons

-Can’t produce Cl free radicals
-Have high bond enthalpies so bonds are less likely to break

52
Q

What is Complete combustion?

A

Alkane + oxygen&raquo_space;> CO2 + H2O

53
Q

How is acid rain formed?

A
  1. NO or SO (also cause respiritory issues) react with O2
  2. Reacts with O2 and H2O in clouds
  3. Dorms a dioxide, acid, and falls as acid rain
54
Q

What sort of gradient does a fractionating column have?

A

Negative temp gradient

55
Q

How are free radicals formed?

A

UV breaks the covalent bond, each atom ends up with 1 electron

56
Q

What are the conditions for free radical substitution?

A

-UV light
-Temps above 450 degrees

57
Q

What happens in an elimination reaction of a halogen?

A

2 single bonds break and a double bond forms , eliminating the halogen

58
Q

What is the nucleophile known as in elimination reactions?

A

Base (still a proton acceptor)

59
Q

Explain the elimination reaction mechanism

A
  1. Arrow from bases lone pair to the hydrogen on the adjacent carbon to the halogen
  2. C-H bond breaks and arrow goes from there to C-C bond (double bond form)
  3. C-X bond breaks and arrow goes to X
60
Q

Elimination reactions can produce 2 isomers:

A

Structural
E-Z

61
Q

What are the conditions of an elimination reaction?

A

High temps
Ethanolic hydroxide

62
Q

What is the base in an elimination reaction?

A

:OH-

from KOH or NaOH

63
Q

What is a halogenoalkane?

A

Alkane where the hydrogen has been replaced with halogens

64
Q

General formula of halogenoalkane?

A

CnH2n+1X

65
Q

What can we classify halogenoalkanes as?

A

Primary, secondary, or tertiary

66
Q

What are the properties of halogenoalkanes?

A

-Halogen tends to be more electronegative than carbon, so a polar bond > therefore experiences VDW and dipole-dipole forces
-The longer the carbon chain, the more VDW forces, increasing MP and BP.

67
Q

Are halogenoalkanes soluble?

A

Polar dissolves polar, and water is polar, so yes they do dissolve.

The longer the chain, the less soluble they get as H-C bonds are insoluble.

68
Q

What is a free radical?

A

An uncharged molecule or atom with an unpaired valence electron

69
Q

What is an electrophile?

A

An electron pair acceptor

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