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Chemistry AS > Chapter 12 > Flashcards

Chapter 12 Flashcards

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1
Q

How reactive are alkanes? Why?

A

Alkanes are very unreactive due to the strong and non-polar C-C and C-H bonds

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2
Q

Why are alkanes a good fuel?

A

The combustion of alkanes is very exothermic

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3
Q

What is produced in incomplete combustion of alkanes?

A

CO(g) or C(s)

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4
Q

Give 3 reasons why incomplete combustion is bad?

A
  1. Less energy per mole is released
  2. Carbon monoxide is toxic
  3. Carbon particles are respiratory irritants/ carcinogenic and cause global dimming
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5
Q

When do Alkanes react with halogens?

A

Under UV light

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6
Q

What occurs during a reaction between a alkane and a halogen?

A

The hydrogen atoms on the alkane is replaced with Halogen atoms

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7
Q

What type of reaction is the reaction between an alkane and a halogen?

A

A substitution reaction

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8
Q

What is formed during a reaction between an alkane and a halogen?

A

A haloalkane and a hydrogen halide

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9
Q

What does the reaction mechanism involve for the halogenation of alkane?

A

Radical substitution

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10
Q

What are the tree stages of the mechanism for the halogenation of alkanes?

A

The mechanism takes place in three stages: initiation, propagation and termination

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11
Q

What occurs during the mechanism Initiation step?

A

A molecules absorbs UV light, homolytic fission occurs and radicals form

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12
Q

Is Initiation endo or exothermic?

A

Endothermic as only bonds are breaking

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13
Q

What occurs during the mechanism Propagation step?

A

Radicals react with molecules forming new radicals and molecules

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14
Q

What type of reaction is the Propagation phase?

A

A chain reaction

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15
Q

What occurs during the mechanism Termination step?

A

Radicals combine, forming a stable molecule

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16
Q

Is the termination step exo or endothermic?

A

Exothermic as only bond making occurs

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17
Q

How do you create the final equation after a halogenation of alkanes reaction?

A

Cancel the same molecules on either side of the equation

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18
Q

What are the 3 limitations of radical substitution in organic synthesis?

A
  1. Further substitution
  2. Substitution at different positions in a carbon chain
  3. Reactions produce a mixture of products
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19
Q

What are alkanes?

A

Saturated Hydrocarbons

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20
Q

What is the general formula for alkanes?

A

CnH2n+2

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21
Q

When do alkanes show chain isomerism?

A

In a chain

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22
Q

When do alkane chains show isomerism?

A

When its a chain with 4 or more Carbon atoms

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23
Q

What bonds are present between the Carbon atoms in an alkane?

A

A single SIGMA bond. (Covalent bond)

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24
Q

What is a SIGMA bond?

A

A sigma bond is a head on overlap of the p-orbitals, one from each carbon atom.

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25
Q

What is the shape of an alkane and why does it adopt this shape?

A

A tetrahedral shape with a bond angle of 109.5°because around each central C-atom are 4 regions of electron density, which repel each other as far apart as possible in order to minimise repulsion.

26
Q

What are cycloalkanes?

A

Alicyclic hydrocarbons

27
Q

What is the general formula for a cycloalkane?

A

CnH2n

28
Q

What is the bond angle, reactivity and stability of Cyclopropane?

A

60° bond angle, very reactive, strained ring

29
Q

What is the bond angle, reactivity and stability of Cyclobutane?

A

90°bond angle, very reactive, strained ring

30
Q

What is the bond angle and stability of Cyclopentane?

A

108° bond angle, less strained

31
Q

What is the bond angle and stability of Cyclohexane?

A

109.5° bond angle, stable

32
Q

How are alkanes found?

A

They are extracted from crude oil by fractional distilation

33
Q

How does chain length of alkanes effect boiling point?

A

The longer the chain the higher the boiling point

34
Q

What state are alkanes with 1-4 carbon atoms?

A

Colourless gases

35
Q

What state are alkanes with 5-17 carbon atoms?

A

Colourless liquids

36
Q

What state are alkanes with 18+ carbon atoms?

A

White waxy solids

37
Q

Why does the boiling point of alkanes increase as chain length does?

A

More electrons means stronger induced dipole-dipole interactions (London Forces) so more energy is needed to overcome them resulting in a higher boiling point.

38
Q

If branching on alkanes increases what happens to boiling point?

A

decreases

39
Q

Why does branching of alkanes decrease boiling point?

A

Branched alkanes have less surface area in contact so weaker/ fewer intermolecular forces hence less energy is needed to overcome them so lower boiling point.

40
Q

How reactive are alkanes? Why?

A

Alkanes are very unreactive due to the strong and non-polar C-C and C-H bonds

41
Q

What are the 2 reactions alkanes can carry out?

A
  1. Combustion

2. Halogen substitution

42
Q

What makes alkanes very good fuels?

A

Combustion of alkanes is very exothermic hence making them good fuels.

43
Q

What state is an alkane with a C1-C4 chain length?

A

Gas

44
Q

What state is an alkane with a C5-C17 chain length?

A

Liquid

45
Q

What state is an alkane with a C18+ chain length?

A

Solid

46
Q

What is produced in the incomplete combustion of an alkane in a limited air supply of oxygen?

A

CO(g) or C (s)

47
Q

Why is the incomplete combustion of alkanes an issue?

A
  1. Less energy released per mole
  2. Carbon monoxide is poisonous
  3. Carbon particles are respiratory irritants/ carcinogenic and add to global warming.
48
Q

What causes alkanes to react with halogens like Bromine and Chlorine?

A

The presence of UV light

49
Q

What reaction occurs between a alkane and a halogen under UV light?

A

A substitution reaction where the halogen replaces the hydrogen atoms in the alkane molecule

50
Q

What is the product of a substitution reaction between a halogen and a alkane?

A

A haloalkane and a hydrogen halide are formed

51
Q

What is the reaction mechanism for halogen substitution of an alkane involve?

A

The reaction mechanism involves a radical substitution

52
Q

What are the three stages of a halogen substitution reaction with an alkane?

A
  1. Initiation
  2. Propagation
  3. Termination
53
Q

What is a definition for the Initiation stage of a halogen substitution reaction?

A

A molecule adsorbs UV light, homolytic fission occurs and radicals form.

54
Q

What is a definition for the Propagation stage of a halogen substitution reaction?

A

Radicals react with a molecule forming new radicals and molecules - chain reaction

55
Q

What is a definition for the Termination stage of a halogen substitution reaction?

A

Radicals combine forming a stable molecule

56
Q

Is the Initiation stage of a halogen substitution reaction endo or exothermic?

A

ENDOTHERMIC

57
Q

What type of reaction is the propagation stage of halogen substitution?

A

Chain reaction

58
Q

Is the Termination stage of a halogen substitution reaction endo or exothermic?

A

EXOTHERMIC

59
Q

What does further substitution lead to ?

A

Results in unpredicted compounds being made

60
Q

What does substitution at different positions in a carbon chain lead to?

A

With longer chained alkanes, halogens can occur in different positions , these then require separation which is expensive.

Chemistry AS (16 decks)

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