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Chemistry > 12.Alkanes > Flashcards

12.Alkanes Flashcards

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1
Q

What are alkanes

A

Saturated hydrocarbons

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2
Q

What are saturated hydrocarbons

A

A compound containing only H and C with only C-C and C-H bonds, no C=C

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3
Q

Describe the physical properties of alkanes: polarity

A
  • non polar

- as electronegativity between C and H are so similar

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4
Q

What are the intermolecular forces that act between alkanes

A

only van der Waals

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5
Q

Describe the physical properties of alkanes: boiling points

A
  • as chain length increases
  • so does van der Waal forces strength
  • as more electrons present
  • so boiling point increases
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6
Q

Why do alkanes with branched chains have lower melting points than straight chain alkanes with the same number of carbon atoms

A
  • cannot pack together as closely
  • less interaction of van der Waal forces
  • weaker van der Wall forces
  • lower boiling point
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7
Q

Describe the physical properties of alkanes: solubility

A
  • insoluble in water
  • water is held by much stronger hydrogen bonds than the van der Waals
  • lipid soluble
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8
Q

How do alkanes react

A
  • strong H-C bonds so relatively unreactive
  • react with halogens
  • burn in oxygen
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9
Q

What is crude oil made of

A

Branched and unbranched alkanes and impurities

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10
Q

How are alkanes in crude oil separated

A

Fractional distillation in a fractionating tower

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11
Q

Describe the process of fractional distillation

A
  • crude oil heated in furnace
  • mixture of liquid and vapour passes into a tower that is cooler at the top than at the bottom
  • vapour pass up tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool and condense
  • shorter chains condense in towers near the top (lower boiling points)
  • thick residue collects at base called tar
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12
Q

What is cracking

A

The breaking of C-C bonds in a long hydrocarbon chain to make shorter chains

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13
Q

What is useful about cracking

A
  • produces shorter, more useful chains (especially petrol)

- some of the products are alkenes which are more reactive than alkanes

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14
Q

What does cracking produces

A
  • shorter alkanes

- high % of alkenes

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15
Q

What are the two types of cracking

A
  • thermal

- catalytic

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16
Q

Describe thermal cracking conditions

A
  • high temperature (700-1200 K)

- high pressure (7000 kPa).

17
Q

What is produced during thermal cracking

A

A mixture of alkanes, alkenes and sometimes hydrogen

18
Q

Describe catalytic cracking

A
  • lower temperature (720 K) (still high)
  • lower pressure
  • zeolite catalyst
19
Q

What is produced during catalytic cracking

A
  • branched alkanes
  • cycloalkanes
  • aromatic compounds
20
Q

What commercial product is produced by catalytic cracking

A

Motor fuel

21
Q

Name alkane fuels

A
  • methane
  • propane
  • butane
  • petrol
  • paraffin
22
Q

What is incomplete combustion

A

A combustion reaction in which there is insufficient oxygen for all the carbon in the fuel to burn to carbon dioxide. Carbon monoxide and/or carbon are formed

23
Q

List pollutants hydrocarbon based fuels produce

A
  • CO (poisonous gas)
  • nitrogen oxides (produced when there is enough energy for O2 and N2 to react, happens in engines)
  • sulfur dioxide (produced from sulfur containing impurities in oil)
  • CO2 (climate change)
  • unburned hydrocarbons
24
Q

What are contributors to acid rain

A

NOx and SO2

25
Q

Name two methods of flue gas desulfurisation

A
  • reacting it with calcium oxide to form gypsum

- reacting it with calcium carbonate

26
Q

What is the equation for the reaction of flue gas and calcium oxide

A

CaO (s) + 2H2O (l) + SO2 (g) + 1/2O2 (g) —-> CaSO4.2H2O (s)

27
Q

What is the equation for the reaction of flue gas and calcium carbonate

A

CaCO3 (s) + 1/2O2 (g) +SO2 (g) —-> CaSO4 (s) + CO2 (g)

28
Q

What do catalytic converters

A

Reduce the output of CO, NOx and unburnt hydrocarbons in the exhaust gas mixture

29
Q

Describe structure of a catalytic converter

A

Honeycomb made of a ceramic material coated with platinum and rhodium metals. Honeycomb shape provides an enormous surface area

30
Q

Write the two reactions that occur in catalytic converters

A
  • 2CO (g) + 2NO (g) —> N2 (g) + 2CO2 (g)

- hydrocarbons + nitrogen oxide —> nitrogen + carbon dioxide + water

31
Q

Name the stages of free radical substitution

A
  • initiation
  • propagation
  • termination
32
Q

How do Cl-Cl bonds break under UV light

A
  • UV light energy is greater than Cl-Cl bond so it breaks
  • both atoms are the same so it breaks homiletically
  • forms two free radicals
33
Q

Write the initiation step of free radical sub

A

Cl-Cl —-> 2Cl•

…………uv

34
Q

Write the propagation steps

A
  1. Cl• + CH4 —> HCl + •CH3

2. •CH3 + Cl2 —> CH3Cl + Cl•

35
Q

Write possible termination steps

A
  1. Cl•+ Cl• —-> Cl2
  2. •CH3 + •CH3 —->C2H6
  3. Cl• + •CH3 —-> CH3Cl
36
Q

What is the overall reaction of free radical substitution

A

CH4 (g) + Cl2 —> CH3Cl (g) + HCl

37
Q

Write how dichloromethane can be made in the propagation stage

A
  1. CH3CL + Cl• —-> •CH2Cl +HCl

2. •CH2Cl +Cl2 —>CH2Cl2 + Cl•

38
Q

Why are chain reactions important

A

Chlorine free radicals formed from CFCs are destroying the ozone layer which protects the earth from UV

39
Q

What is the reaction for the breaking down of the ozone layer

A

Cl• + O3 —->ClO• + O2
ClO + O3 —-> 2O2 + Cl•
2O3 —> 3O2

Chemistry (27 decks)

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