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A level Chemistry > Chapter 12 - Alkanes > Flashcards

Chapter 12 - Alkanes Flashcards

1
Q

Alkanes can be?

A

straight chained, branched, or cyclic

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2
Q

each C atom has a ?

A

tetrahedral shape - 109.5 bond angle

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3
Q

intermolecular forces?

A
  • C (2.1) & H (2.5) have very similar electronegativity values
  • meaning molecules are not polar, weak induced dipole forces form
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4
Q

Reactivity of alkanes?

A
  • have a relatively low reactivity
  • the C-H bond has low polarity - the sigma bonds av a low polarity bc the electroneg of C &H is very similar
  • the bonds within have very large bond enthalpies - lots of energy needed to break
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5
Q

combustion?

A
  • O2 from air is combined with alkane
  • rapid process - (alkenes burn very fast though)
  • so alkanes are freq used as, or as a component of a fuel source
  • thru combustion, the chemical energy is transferred to thermal energy
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6
Q

Complete combustion of alkanes?

A
  • in excess air (O2)
  • max amount of energy transferred
  • burns with a clean blue flame
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7
Q

incomplete combustion of alkanes?

A
  • in limited O2 supply

- burns with a dirty yellow flame

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8
Q

products of complete combustion?

A
  • water: simply adds to existing water supplies

- CO2 - greenhouse gas- increasing levels of CO2 are thought to be responsible for increasing global temps

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9
Q

products of incomplete combustion?

A
  • carbon particulates - solid C: observed as soot
  • CO - colourless, odourless gas ‘the silent killer’ prevent the effective transport of O2 around the body -> binds to Hb instead of O2
  • unburned hydrocarbons - some exist in the products
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10
Q

impurities in the alkanes - sulfur?

A
  • Oxides of S may be produced if S atoms contained within hydrocarbon
  • processing such as fractional distillation and cracking may not have removed these impurities
  • Sulfur dioxide and Sulfur trioxide may also be formed
  • these gases r acidic oxides, when they dissolve in H2O , they form sulfuric and sulfurous acid -> these contriute to acid rain
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11
Q

impurities in the alkanes - N?

A
  • combustion of N containing alkanes will lead to formation of N oxides ➡N monoxide and N dioxide - can dissolve in H20 to form acid rain
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12
Q

catalytic converters?

A
  • are fitted to the exhausts of Road Vehicles
  • uses precious metals such as platinum, rhodium, and palladium which are thinly spread over a mesh
  • gas pass over the catalyst (e.g. CO, N oxides) ➡ converted into less harmful gases
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13
Q

examples of some of the reactions that occur in the catalytic converter?

A

-

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14
Q

examples of some of the reactions that occur in the catalytic converter?

A
  • 2CO + O2 ➡ 2CO2

- 2NO + 2CO ➡ N2 + 2CO2

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15
Q

Radical substitution ?

A
  • alkanes react with the halogens, spec. Cl and Br in UV light
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16
Q

what happens in initiation of Radical Sub?

A
  • the radicals are formed
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17
Q

what happens in termination of Radical Sub?

A
  • 2 radicals collide to form a stable product

- (3 equations, this is step 3)

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18
Q

further substituted reactions also occur forming?

A

more substituted alkanes + longer alkanes

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19
Q

methane molecular formula?

A

CH4

empirical: CH4

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20
Q

ethane molecular formula?

A

C2H6

empirical: CH3

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21
Q

Propane molecular formula?

A

C3H8

empirical: C3H8

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22
Q

butane molecular formula?

A

C4H10

empirical: C2H5

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23
Q

Pentane molecular formula?

A

C5H12

empirical: C5H12

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24
Q

Hexane molecular formula?

A

C6H14

empirical: C3H7

25
Q

Heptane molecular formula?

A

C7H16

empirical: C7H16

26
Q

Octane molecular formula?

A

C8H18

empirical: C4H9

27
Q

Nonane molecular formula?

A

C9H20

empirical: C9H20

28
Q

Decane molecular formula?

A

C10H22

C5H11

29
Q

explain what happens when an alkane boils?

A

induced dipole forces between mols are broke. NO COVALENT BONDS ARE BROKEN

30
Q

Explain differences in BP?

A
  • Highest BP = the molecule that has the greatest SA as there is the most induced dipole forces
  • lowest= lowest SA so least no. of induced dipole forces between molecules so less energy required to boil
31
Q

What are aliphatic alkanes and what is their general formula?

A
  • straight or branched chains of C atoms

- GF: CnH2n+2

32
Q

what r alicyclic/ cycloalkanes + their GF?

A

Homologous series of alkanes that form rings

CnH2n (same as alkenes)

33
Q

for a cycloalkane add what to the alkane ?

A
  • just cyclo to start

e. g. cyclohexane

34
Q

which cycloalkane does not exist?

A

cycloethane

35
Q

why are alkanes so unreactive?

A
  • there is only a small difference in electronegativity between C&H meaning molecules not polar, permanent dipole forces cannot arise
  • large bond enthalpies, so lots of energy required for a reaction to occur
36
Q

initiation step definition?

A

the step that starts the reaction forming radicals by homolytic fission

37
Q

mechanism definition?

A

a series of steps that show how the reaction takes place

38
Q

subsitution reaction definition?

A

a reaction where an atom of one molecule is swapped for an atom of an other

39
Q

photochemical reaction definition?

A

absorption of light energy causing a bond to break

40
Q

radical or free radical definition?

A

an atom or group of atoms with an unpaired electron

41
Q

termination step definition?

A

radicals react with each other so the reaction stops

42
Q

homolytic fission definition?

A

when a bond breaks to give 2 free radicals

43
Q

series of steps in FRS?

A

Initiation -> propagation -> termination

- always label

44
Q

how many initiation steps r there?

A

1

45
Q

how many propagation steps r there?

A

2

46
Q

how many termination steps r there?

A

3

47
Q

conditions for FRS?

A

UV light

48
Q

what happens in the initiation step?

A
  • this starts the reaction, forming radicals
  • UV light breaks the bond between the 2 Cl atoms homolytically
  • this means that each Cl takes one electron back from the bond
  • the • represents an unpaired electron
49
Q

what happens in the propagation steps?

A

these 2 steps use up the r and make up the p. Hydrogen radicals are very unstable so never form.

  • radical + reactant -> product and radical (1)
  • radical (1) + reactant -> product + radical
50
Q

what happens in the termination steps?

A

radicals react with each other and this ends the reaction. (think of the combinations)

51
Q

further substitution can occur

A

(when more of the halogen is added)

52
Q

what are alkanes?

A

main components of natural gas and crude oil, are amongst the most stable organic compounds, mainly used as fuels

53
Q

A sigma bond is?

A
  • the result of the direct overlap of 2 orbitals, one from each bonding atom
  • a sigma bond is positioned on a line directly between the bonding atoms
54
Q

effect of chain length on BP?

A
  • As the chain length increases, the molecules have a larger SA so more surface contact is possible between molecules
  • induced dipole forces will be grater and so more energy is required to overcome the forces
55
Q

branched isomers have a ? BP than straight chain isomers?

A

lower

56
Q

why do branched isomers have a lower BP?

A
  • There are less surface points of contact between molecules of the branched alkanes, giving fewer induced dipole forces
  • also: shape - the branches get in the way and prevent the branched molecules from getting as close together as straight chain mols, dec. the IMF further
57
Q

why are alkanes used as fuels?

A

they r readily available, easy to transport, burn in plentiful supply of O2 w/o releasing toxic products

58
Q

termination: what happens when 2 radicals collide and react?

A

the radicals are removed from the reaction mixture, stopping the reaction

59
Q

further sub can continue until?

A

all H atoms have been substituted

A level Chemistry (39 decks)

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