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A-level Chemistry > Alkanes > Flashcards

Alkanes Flashcards

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1
Q

alkanes, type of molecules, general formula

A

Homologous series of saturated hydrocarbons with the general formula CnH2n+2 (ring alkanes: CnH2n)

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2
Q

saturated definition

A

contains only single C-C bonds

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3
Q

hydrocarbon definition

A

a compound that only contains hydrogen and carbon atoms

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4
Q

reactivity of alkanes

A

• Unreactive, burn, react with halogens

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5
Q

types of forces between alkanes molecules (why)

A

• Only van der waals forces

o Electronegativity between C-H is very similar so no dipole-dipole attractions or H-bonds

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6
Q

how does increase in chain length affect boiling point

A

• boiling point increases with increase in chain length due to more points of contact between molecules so more van der waals. Higher Mr so stronger van der waals forces between molecules require lots of energy to break

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7
Q

how does increase in branching affect boiling point

A

• boiling point decreases with increase in branching as it results in fewer points of contact (unable to pack as closely together) so there are fewer and weaker van der waals forces between forces require less energy to break

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8
Q

fractional distillation definition

A

a method of separating mixtures of liquids according to their different boiling points

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9
Q

fraction definition

A

a mixture of hydrocarbons with similar boiling points

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10
Q

fractional distillation steps (5)

A
  1. crude oil is vaporised
  2. vapour enters the bottom of the column which is hot and the bottom and cold at the top
  3. as vapour rises it cools and condenses
  4. different molecules condense at different heights due to different boiling points
  5. the larger the molecule, the higher the boiling point and so the lower down the column it condenses
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11
Q

as carbon chain increases in length: (4)

A
  • more viscous (longer chains become tangled)
  • harder to ignite
  • less volatile
  • higher boiling points
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12
Q

number of carbons in chain to be a gas

A

1-4

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13
Q

number of carbons in chain to be a liquid

A

5-17

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14
Q

number of carbons in chain to be a solid

A

> 17

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15
Q

cracking definition

A

a process for breaking long-chained saturated hydrocarbons into smaller ones, producing a mixture of saturated and unsaturated products

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16
Q

why do we need to crack (2)

A

High demand for useful, short hydrocarbon chains but low supply. Excess long hydrocarbons can be cracked into shorter ones

Alkanes are more useful than alkanes as starting materials for further chemical reactions

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17
Q

type of reaction for cracking

A

thermal decomposition of alkanes breaks C-C bonds

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18
Q

thermal cracking conditions (2)

A

900°C, 70atm

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19
Q

what does thermal cracking mostly produce

A

alkenes

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20
Q

catalytic cracking conditions (3)

A

450°C, 1atm, zeolites catalyst (e.g. Al2O3 with honeycomb structure)

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21
Q

what does catalytic cracking mostly produce (4)

A

motor fuels, aromatics, cyclic alkanes, branched alkanes

22
Q

why can alkanes be used as fuels

A

Can burn readily in oxygen and is highly exothermic

23
Q

when does complete combustion occur and in what does it produce

A

a sufficient supply of O2 and produces CO2 and H2O

24
Q

when does incomplete combustion occur and in what does it produce

A

insufficient supply of O2 and produces CO/C and H2O

25
Q

why are some molecules greenhouse gases (3 examples)

A

Molecules with covalent bonds absorb IR radiation e.g. CO2, CH4, H2O

26
Q

catalytic converter structure and metal

A

honeycomb structured with thin layer of platinum, palladium and rhodium

27
Q

equation for catalytic converter with CO

A

2CO + 2NO -> 2CO2 + N2

28
Q

equation for catalytic converter for hydrocarbons (e.g. C8H18)

A

C8H18 + 25NO -> 8CO2 12.5N2 + 9H2O

29
Q

CO2 formation, problems and ways to reduce problem

A

Complete combustion of fuels containing C
Greenhouse gas- global warming
Burn less fossil fuels

30
Q

CO formation, problems and ways to reduce problem

A

Incomplete combustion of fuels containing C
Toxic – binds to haemoglobin causes suffocation
Catalytic converter – 2CO + 2NO -> 2CO2 + N2

31
Q

C (soot) formation, problems and ways to reduce problem

A

Incomplete combustion of fuels containing C
Global dimming, triggers asthma
Ensure sufficient supply of oxygen when burning

32
Q

H2O formation, problems

A

Combustion of fuels containing H Greenhouse gas – global warming but falls as rain

33
Q

SO2 formation, problems and ways to reduce problem

A

S impurities in fuels burn with O2

Acid rain Desulfurisation – remove SO2 in flue gas by reacting with CaO base so neutralisation reaction forms CaSO4

34
Q

NOx formation, problems and ways to reduce problem

A

N2 and O2 in air react under high temperatures of engines and furnaces
Acid rain and breathing problems
Catalytic converter – 2CO + 2NO -> 2CO2 + N2

35
Q

unburnt hydrocarbons formation, problems and ways to reduce problem

A

Incomplete combustion of fuel
Global dimming – reflect back rays of light back into space
Ensure sufficient supply of oxygen C8H18 + 25NO -> 8CO2 12.5N2 + 9H2O

36
Q

mechanism definition

A

a sequence of steps showing the path taken by electrons in a reaction

37
Q

homolytic fission definition

A

Breaking of a covalent bond with each atom getting one electron from the covalent bond to form two radicals

38
Q

heterolytic fission definition

A

one atom gets the bonding pair of electrons from the covalent bond

39
Q

free radical definition

A

a species with an unpaired electron (represented as •) which is highly reactive

40
Q

radical substitution definition

A

a type of substitution in which a radical replaces a different atom or group of atoms

41
Q

overall synthesis of chloroalkanes

A

CH4 + Cl2  CH3Cl + HCl

42
Q

type of mechanism for synthesis of chloroalkanes

A

free radical substitution

43
Q

mechanism steps for synthesis of chloroalkanes

A

Initiation: Cl2  2Cl• (UV light and >350⁰C)
Propagation: CH4 + Cl•  •CH3 + HCl
•CH3 + Cl2  CH3Cl + Cl•
Termination: •CH3 + Cl•  CH3Cl

44
Q

how can further substitution be minimised for synthesis of chloroalkanes

A

add methane to excess

45
Q

why is the ozone important

A

protective layer from UV radiation which can cause mutation and skin cancer

46
Q

ozone formation overall

A

O2 + O ↔ O3

47
Q

ozone formation steps (4)

A
  1. O2  2O (UV with high frequency)
  2. O + O2  O3 (+ heat)
  3. O3  O2 + O (UV with lower frequency)
  4. O2 + O  O3 (+ heat)
48
Q

removal of ozone overall

A

O3 + O  2O2

49
Q

what can act as catalysts to remove ozone and where do they come from

A

CFCs are used for refrigerants, aerosols, insecticides etc. and are released through these uses
NO (from lightning or aircraft engines)

50
Q

removal of ozone with Cl• catalyst steps

A 
Initiation: CFCl3  •CFCl2 + Cl•
Propagation: Cl• + O3  ClO• + O2 
                         ClO• + O3  Cl• + 2O2
Termination: 2O3  3O2 
`
51
Q

removal of ozone with NO catalyst steps

A

N2 + O2  2NO
NO + O3  NO2 + O2
NO2 + O3  NO + 2O2
2O3  3O2

52
Q

How is distillation different to fractional distillation

A

Distillation separates all volatile components of a mixture from non-volatile ones

A-level Chemistry (17 decks)

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