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A - Level Chemistry > Chapter 12 > Flashcards

Chapter 12 Flashcards

1
Q

What is the bonding like in an alkane?

A

2 p-orbitals, 1 from each bonding atom

overlap and contain 1 electron each

sigma bond has 2 electrons that are shared between bonding atoms

each carbon in an alkane has 4 sigma bonds

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2
Q

What is a sigma bond?

A

The head on overlap of 2 atomic orbitals 1 from each bonding atom, with electron density centred around a line directly between the bonding atoms nuclei.

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3
Q

How many bonds can a Carbon form in an alkane?

A

4 covalent bonds

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4
Q

Are alkanes saturated or unsaturated?

A

saturated hydrocarbons (single bonds only)

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5
Q

What is the general formula of an alkane?

A

CnH2n+2

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6
Q

What’s is the shape of an alkane?

A

tetrahedral

bond angle = 109.5*

caused by = each c atom surrounded by 4 e pairs in 4 sigma bonds, repulsion between these e pairs cause this shape around each C atom

Lone pairs repel more than bonding electrons
4 electron pairs in 4 sigma bonds (4 bonding pairs)

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7
Q

Why does BP of alkanes increase as chain length increases?

A

because of weak LFs

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8
Q

What is the effect of chain length on boiling point?

A
  • chain length increases
  • increases surface area
  • Greater (stronger) London forces
  • more energy required to overcome the forces
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9
Q

What’s is the effect of branching on boiling point?

A

(Example may include isomers of alkanes- branched isomers has lower boiling point than straight chain isomers)

  • more branching
  • fewer surface points of contact (decreased surface area)
  • weaker (fewer) London forces

Also another reason is to do with the shape.
- branches get in the way and prevent branched molecules getting as close together as straight chain molecules, decreasing IMFs further

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10
Q

What is an alkane?

A

A saturated hydrocarbons containing C-H bonds only.

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11
Q

Are alkane bonds polar? Why/why not?

A

Non-polar because carbon and hydrogen have similar electronegativities.

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12
Q

Describe the sigma bond in alkane?

A

The sigma bond is a covalent bond which has a direct overlap of the electron clouds of the bonding atoms.

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13
Q

What type of intermolecular forces do alkanes have? Why?

A

London forces -> induced dipole-dipole interaction, because the bonds are non-polar

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14
Q

What happens to the boiling point as alkane chain length increases? Why?

A

BP increases

Increased surface area

Stronger LFs

more energy required to overcome LFs

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15
Q

Does branched have a lower/higher BP compared straight chain?

A

lower BP

have fewer surface area

less LFs

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16
Q

Are alkanes insoluble/soluble in water? Why?

A

Insoluble

bonds in H2O stronger than LFs in alkanes

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17
Q

How reactive are alkanes?

A

Very unreactive

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18
Q

What reactions will alkanes undergo?

A

Combustion and reaction with halogens

19
Q

What is complete combustion?

A

Combustion in plentiful supply of oxygen

20
Q

What are the products of complete combustion when alkanes are used?

A

Carbon dioxide and water

21
Q

What is the colour of the Bunsen burner flame during complete combustion?

A

Blue flame

22
Q

What is incomplete combustion and what products are formed in the case of alkanes?

A

Combustion in limited supply of O2

Products = H2O, CO2 and CO

23
Q

What type of hydrocarbon are most likely to undergo incomplete combustion?

A

Long chain Hydrocarbons

24
Q

What is the environmental impact of carbon monoxide?

A

It is toxic/poisonous

25
Q

What is the environmental impact of soot (carbon)?

A

Asthma, cancer, global dimming

26
Q

What is the name of the mechanism for the reaction of alkanes with halogens

A

Free radical substitution

27
Q

In the presence of what does alkane react with halogens?

A

UV light
(Sunlight)

28
Q

What are the 3 stages of free radical substitution?

A

Initiation - breaking halogen bond to form free radicals

Propagation - chain part of the reaction where products are formed but free radicals remains.

Termination - free radicals removed, stable products formed

29
Q

Write equations for the reaction of CH4 with Cl2 to form CH3Cl

A

Initiation: Cl2 —> 2Cl* (in presence of UV light)

Propagation: Cl* + CH4 —>HCl + *CH3
CH3 + Cl2 —> CH3Cl + Cl

Termination:
CH3 + Cl —> CH3Cl
2Cl* —> Cl2
*CH3 + *CH3 —> CH3CH3

30
Q

What are limitations of free radical substitution

A

Further substitution

Substitution at different positions in carbon chain

31
Q

What are the 3 steps In free radical substitution

A

Initiation

Propagation

Termination

32
Q

What happens in initiation UV radiation

A

Homolytic fission

Form radicals

Energy provided by

33
Q

What happens in propagation

A

Chain reaction

Step terminated when 2 radicals collide

34
Q

What happens in termination

A

2 radicals collide, form molecule with all electrons paired, both radicals removed from reaction mixture, stopping reaction.

35
Q

How can you separate crude oil

A

Separate into fractions by fractional distillation in distillation tower

36
Q

What happens to boiling point of alkanes as chain length increases

A

BP increases as chain length increases

37
Q

The effect of chain length on BP

A

Chain length increases

Larger SA

More surface contact possible between molecules

LFs between molecules will be greater

More energy required to overcome stronger LFs

38
Q

Why do branched isomers have lower BP compared to straight chain isomers

A

Fewer surface points of contact

Fewer LFs

Branches get in the way + prevent branched molecules getting as close together as straight chain molecules

Decreasing IMFs further

39
Q

Simple explanation of effect of branching on BP

A

More branching

Less surfed points of contact

Weaker/fewer LFs

40
Q

Why do alkanes lack reactivity

A

C-C and C-H sigma bonds are strong

C-C are non-polar

EN of C + H, so similar C-H considered non-polar

41
Q

Why are alkanes used as fuels

A

Readily available

Easily transport

Burn in plentiful supply of oxygen without releasing toxic products

42
Q

What does combustion give out

A

Heat

43
Q

What does the incomplete combustion of alkanes produce

A

Limited supply of oxygen, form toxic gas CO or C(s)

E.g. C7H16 + 7 1/2 O2 —> 7CO + 8H2O

      C7H16 + 4O2 —> 7C + 8H2O

A - Level Chemistry (11 decks)

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