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3.3.4 Alkanes Flashcards

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1
Q

_____, _____ and _____ have no isomers

A

Methane, ethane and propane

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2
Q

Alkanes with branched chains have _____ melting points than straight chain alkanes (with same no. of C)

A

LOWER melting points than straight chain alkanes (with same no. of C)

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3
Q

Why do alkanes with branched chains have lower melting points than straight chain alkanes (with same no. of C)?

A

∵ less surface area & weaker van der Waals forces

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4
Q

Solid alkanes have a ____ feel

A

waxy

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5
Q

Shorter chains of alkanes are ___ at room temp.

A

gases

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6
Q

Why are alkanes insoluble in water?

A

Hydrogen bonds holding water molecules together = much stronger than van der Waals forces between alkane molecules

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7
Q

What do alkanes react with?

A

halogens (& burn under suitable conditions)

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8
Q

What is petroleum (crude oil)?

A

Mixture of hydrocarbons - mostly alkanes

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9
Q

How did crude oil form?

A

Formed millions of years ago by breakdown of plant and animal remains at high pressure and temperature

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10
Q

Why does crude oil contains other elements?

A

Contains some elements that were in original plants and animals

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11
Q

Describe the stages of fractional distillation

A
  1. Crude oil = vaporised at 350°C
  2. Vaporised crude oil goes into fractionating column & rises up through trays
  3. Largest hydrocarbons don’t vaporise = run to the bottom & form a gooey residue
  4. Crude oil vapour goes up = gets cooler
  5. Each fraction condenses at different temperature
    • Drawn off at different levels in column
  6. Hydrocarbon with lowest boiling points don’t condense
    • Drawn off as gases at top of column
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12
Q

Give 2 conditions of thermal cracking

A
  • 900°C - high temperature
  • Up to 70 atm - high pressure
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13
Q

How do can we avoid the decomposition (of C and H) in thermal cracking?

A

Alkanes kept in these conditions for very short time (e.g. a second)

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14
Q

Name 3 conditions of catalytic cracking

A
  • Uses zeolite catalyst (hydrated aluminosilicate)
  • 1-2 atm - slight pressure
  • 450°C - high temperature
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15
Q

What is the structure of a zeolites & why?

A

Have honeycomb structure = enormous surface area

& are acidic

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16
Q

What is the benefit of using a catalyst?

A
  • Cuts costs ∵ reaction can be done at a low pressure & low temperature
  • & catalyst speeds up reaction - saving time (& time is money)
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17
Q

What do aromatic compounds contain?

A

Benzene rings

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18
Q

What is the structure of benzene rings?

A

Have 6 carbon atoms with 3 double bonds

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19
Q

Why are benzene rings stable?

A

Because the electrons are delocalised around the carbon ring

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20
Q

Why doesn’t cracking produce 2 alkanes?

A

Since there isn’t enough hydrogen atoms to produce 2 alkanes = one of new chains has C=C

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21
Q

How are the products obtained from cracking separated?

A

By fractional distillation

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22
Q

Why are alkanes are great fuels?

A

Burning small amount = huge amount of energy

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23
Q

Give the word equation for complete combustion

A

Alkanes + Oxygen → Carbon Dioxide + Water

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24
Q

Give the word equations for incomplete combustion

A

Alkanes + Oxygen → Carbon Monoxide + Water (+ sometimes Carbon Dioxide)

Alkanes + Oxygen → Carbon + Water

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25
Q

Why is carbon monoxide gas poisonous?

A

Binds to same sites on haemoglobin molecules in red blood cells as oxygen molecules ∴ oxygen cannot be carried around the body

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26
Q

Name 2 issues with soot (particulates)

A

Soot causes breathing problems & builds up in engines = can’t work properly

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27
Q

Describe the greenhouse effect

A
  • Greenhouse gases in atmosphere = good at absorbing infrared energy (heat)
  • Emit some energy they absorb back towards Earth, keeping it warm = greenhouse effect
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28
Q

How does rising temperature affect water vapour?

A
  • Higher temps = more water vapour in air = more greenhouse effect
  • Greater cloud formation and clouds reflect solar radiation
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29
Q

How can carbon monoxide be removed from exhaust gases?

A

By catalytic converters on cars

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30
Q

Where do unburnt hydrocarbons come from?

A

Engines (they don’t burn all fuel molecules = come out as unburnt hydrocarbons)

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31
Q

How are oxides of nitrogen (NOx) produced?

A

Produced when high pressure and temperature in car engine causes nitrogen and oxygen atoms in air to react together

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32
Q

Describe how smog (O3) is created?

A

Hydrocarbons and nitrogen oxides react in presence of sunlight to form ground-level ozone (O3) = major component in smog

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33
Q

Why is ground-level ozones bad? (3x)

A

Irritates people’s eyes and increases respiratory problems + lung damage

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34
Q

How can we remove unburnt hydrocarbons and NOx from exhausts?

A

By catalytic converters

35
Q

What is a catalytic converter?

A

Honeycomb made of ceramic material coated with platinum and rhodium metals

36
Q

Describe how acid rain occurs

A
  1. Some fossil fuels contain sulfur: when burnt, sulfur reacts to form sulfur dioxide gas (SO2)
  2. Sulfur dioxide in the atmosphere reacts with water vapour and oxygen to form sulfuric acid = acid rain
37
Q

Name 3 things acid rains destroys

A
  • Trees and vegetation
  • Corroding buildings/statues
  • Kills fish in lakes
38
Q

What are flue gases?

A

Gases given out by power stations

39
Q

Describe flue gas desulfurisation

(how flue gases can be removed from power stations before it gets into atmosphere)

A
  • Powdered calcium carbonate (limestone) or calcium oxide is mixed with water to make an alkaline slurry
  • When flue gases mix with alkaline slurry = acidic sulfur dioxide gas reacts with calcium compounds to form harmless salt (calcium sulfate)
    • Oxides 1st to produce calcium sulphite & then oxidises again to form calcium sulfate (aka gypsum)
40
Q

What is calcium sulfate (AKA gypsum) used for?

A

Used to make builders’ plaster and plasterboard

41
Q

Give the symbol equation for forming NO

A

N2 (g) + O2 (g) → 2NO(g)

42
Q

Give the symbol equation for forming sulfuric acid

A

SO2(g) + 1/2 O2(g) + H2O(l) → H2SO4(l)

43
Q

Give 2 word equations of a catalytic converter converting polluting gases into less harmful products

A

Carbon Monoxide + Nitrogen Oxide → Nitrogen + Carbon Dioxide

Hydrocarbons + Nitrogen Oxide → Nitrogen + Carbon Dioxide + Water

44
Q

What are free radicals?

A

Particles with unpaired electrons

45
Q

When do free radicals form?

A

When a covalent bond spilts equally, giving one electron to each atom

46
Q

Why are free radicals very reactive?

A

∵ electrons are unpaired

47
Q

Example: Reacting Chlorine with Methane

What happens after the termination reactions?

A

Depends on whether there’s more chlorine or methane around:

  1. Chlorine’s in excess: Cl• free radicals = start attacking chloromethane, producing dichloromethane CH2Cl, trichloromethane CHCl3 & tetrachloromethane CCl4
  2. Methane’s in excess: product mostly be chloromethane
48
Q

What are chlorofluorocarbons (CFCs)?

A

Halogenoalkane molecules where all of hydrogen atoms have been replaced by chlorine and fluorine atoms

49
Q

What does Ozone (O3) in upper atmosphere do?

A

Acts as a chemical sunscreen → absorbs UV radiation from Sun

50
Q

How is ozone formed naturally?

A
  1. When oxygen molecule (O2) is broken down into 2 free radicals by UV radiation
  2. Free radicals attack other oxygen molecules forming ozone
51
Q

Explain how CFCs are creating holes in ozone layer (include symbol equations)

A
  • Chlorine free radicals (Cl•) = formed in upper atmosphere when C-Cl bonds in CFCs are broken down by UV
    e. g. CCl3F → CCl2F• + Cl•
  • These free radicals = catalysts → react with ozone to form an intermediate (ClO•) & oxygen molecule

Cl• + O3 → O2 + ClO•

ClO• + O3 → 2O2 + Cl•

  • Overall reaction: 2O3 → 3O2 & Cl• = catalyst
52
Q

Name 3 properties of CFCs

A

Unreactive, non-flammable & non-toxic

53
Q

CFCs are _____

A

BANNED

54
Q

What can we now use instead of CFCs?

A

e.g. HFCs (hydrofluorocarbons) & hydrocarbons

55
Q

For the conversion of methane to tetrachloromethane:

Give the overall equation

A

CH4 + 4 Cl2 → CCl4 + 4 HCl

56
Q

Free Radical Substitution

Condition

A

UV light neeed to break halogen bond

57
Q

For the conversion of ethane to 1,1-dibromoethane

Give the overall equation

A

C2H6 + 2Br2 → C2H4Br2 + 2HBr

58
Q

For the conversion of ethane to 1,1-dibromoethane

Give the pair of propagation steps to form bromoethane from ethane

A

C2H6 + Br• → C2H5• + HBr

C2H5• + Br2 → C2H5Br + Br•

59
Q

For the conversion of ethane to 1,1-dibromoethane:

Give the pair of propagation steps to form 1,1-dibromoethane from bromoethane

A

C2H5Br + Br• → C2H4Br• + HBr

C2H4Br• + Br2 → C2H4Br2 + Br•

60
Q

For the conversion of ethane to 1,1-dibromoethane:

Give the termination step that forms butane

A

2C2H5• → C4H10

61
Q

Write the symbol equation for when carbon monoxide reacts with nitrogen oxide

A

2CO(g) + 2NO(g) → N2(g) + 2CO2(g)

62
Q

Represent how calcium oxide is used to remove sulfur dioxide from flue gases by using an equation

A
63
Q

Represent how calcium carbonate is used to remove sulfur dioxide from flue gases by using an equation

A
64
Q

Deduce an equation to show how nitrogen dioxide reacts with oxygen and water to form nitric acid

A

4NO2 + 2H2O + O2 → 4HNO3

65
Q

Explain the main economic reason why alkanes are cracked (1)

A

To produce substances which are more in demand / products with high value

66
Q

Suggest why calcium oxide reacts with sulfur dioxide (1)

A

It is basic/SO2 is acidic

(neutralises the gas)

67
Q

Write an equation to show how gypsum (calcium sulfate) is made

A

CaSO3(s) + 1/2O2(g) → CaSO4

Calcium sulfite is oxidised to calcium sulfate (gypsum)

68
Q

what are the steps of the process?

A

initiation (formation of free radicals)
propagation (chain reactions)
termination (free radicals combine)

69
Q

homolytic fission

A

splitting an atom into 2 free radicals

71
Q

heterolytic fission

A

splitting of an atom into a cation and an anion

72
Q

cons of reaction:

A

needs UV light for reaction to start

substitution is random soon control over which H is substituted in longer alkanes- not a precise process

if left, multiple substitutions can occur-not a precise process

74
Q

why is propagation a chain reaction?

A

further reactions produce more free radicals

75
Q

(5) properties of alkanes and why

A

saturated: every carbon atom has 4 single covalent bonds around it

non-polar (and non-soluble in water): no distinct dipole movement in the molecules therefore will not dissolve in polar water

: electronegatives of C and H = similar therfore IMF = weak van der waals

unreactive: strong, non-polar covalent bonds make them resistant to attack by other reactive species

varied mp/bp: increases with increased carbon chain length (Mr)

: stronger induced-dipole IMF in larger molecules as more e- are involved therefore more E required to overcome IMF i.e IMF increases

: more branching genenerally lowers mp/bp as molecules have a lower SA therefore ID forces are weaker between molecules

76
Q

why is the boiling point important to know in fractional distillation

A

more Mr = increased bp

stronger ID

therefore energy needed to separate them

= higher condensing point (g => l)

77
Q

process of fractional distillation

A

1) crude oil is vapourised
2) vapour passed into a tower with a temperature gradient
3) vapour rises and it cools
4) molecules condense at different heights cause different bp
5) larger the molecule and the lower it condenses

78
Q

what is cracking

A

the process of breaking up larger, less useful, hydrocarbons into smaller, more useful ones

79
Q

what does thermal cracking form?

A

homolytic fission occurs forms 2 free radicals

goes onto form : mostly alkenes (makes polymers)

: smaller alkanes (makes fuels)

: H2 (useful in industry/fuel)

80
Q

what does catalytic cracking form?

A

(greener than other method)

heterolytic fission occurs which forms carbocations

goes on to form : mostly smaller alkanes (motor fuels)

: aromatic hydrocarbons

: branched alkanes and cycloslkanes

81
Q

what is reforming

A

where straight chain alkanes

which forms either: branched chain alkanes (fuels) at random length therfore further distillation is required

: benzene

82
Q

combustion of alkanes

A

conditions: reaction with O2 and exothermic

83
Q

what are pollutant made in combustion

A

CO2, H2O, SO2, NO, CO, unburned alkanes and carbon particulates

84
Q

what does catalytic convertor remove

A

removes some polluting gases: CO, unburned alkanes and various nitrogen oxides made by internal combustion engines

85
Q

ways in which alkanes mp/bp changes?

A

increases with carbon chain (bp) because stronger induced-dipole foces IMF in larger molecules as more e- involved to overcome IMF

more branching decreases mp as molecules have higher SA therfore ID forces are weaker between molecules

chem (13 decks)

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