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Flashcards in Chapter 12 Deck (60):
1

How reactive are alkanes? Why?

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

2

Why are alkanes a good fuel?

The combustion of alkanes is very exothermic

3

What is produced in incomplete combustion of alkanes?

CO(g) or C(s)

4

Give 3 reasons why incomplete combustion is bad?

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

5

When do Alkanes react with halogens?

Under UV light

6

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

The hydrogen atoms on the alkane is replaced with Halogen atoms

7

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

A substitution reaction

8

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

A haloalkane and a hydrogen halide

9

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

Radical substitution

10

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

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

11

What occurs during the mechanism Initiation step?

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

12

Is Initiation endo or exothermic?

Endothermic as only bonds are breaking

13

What occurs during the mechanism Propagation step?

Radicals react with molecules forming new radicals and molecules

14

What type of reaction is the Propagation phase?

A chain reaction

15

What occurs during the mechanism Termination step?

Radicals combine, forming a stable molecule

16

Is the termination step exo or endothermic?

Exothermic as only bond making occurs

17

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

Cancel the same molecules on either side of the equation

18

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

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

19

What are alkanes?

Saturated Hydrocarbons

20

What is the general formula for alkanes?

CnH2n+2

21

When do alkanes show chain isomerism?

In a chain

22

When do alkane chains show isomerism?

When its a chain with 4 or more Carbon atoms

23

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

A single SIGMA bond. (Covalent bond)

24

What is a SIGMA bond?

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

25

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

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

What are cycloalkanes?

Alicyclic hydrocarbons

27

What is the general formula for a cycloalkane?

CnH2n

28

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

60° bond angle, very reactive, strained ring

29

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

90°bond angle, very reactive, strained ring

30

What is the bond angle and stability of Cyclopentane?

108° bond angle, less strained

31

What is the bond angle and stability of Cyclohexane?

109.5° bond angle, stable

32

How are alkanes found?

They are extracted from crude oil by fractional distilation

33

How does chain length of alkanes effect boiling point?

The longer the chain the higher the boiling point

34

What state are alkanes with 1-4 carbon atoms?

Colourless gases

35

What state are alkanes with 5-17 carbon atoms?

Colourless liquids

36

What state are alkanes with 18+ carbon atoms?

White waxy solids

37

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

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

If branching on alkanes increases what happens to boiling point?

decreases

39

Why does branching of alkanes decrease boiling point?

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

How reactive are alkanes? Why?

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

41

What are the 2 reactions alkanes can carry out?

1. Combustion
2. Halogen substitution

42

What makes alkanes very good fuels?

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

43

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

Gas

44

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

Liquid

45

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

Solid

46

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

CO(g) or C (s)

47

Why is the incomplete combustion of alkanes an issue?

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

48

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

The presence of UV light

49

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

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

50

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

A haloalkane and a hydrogen halide are formed

51

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

The reaction mechanism involves a radical substitution

52

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

1. Initiation
2. Propagation
3. Termination

53

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

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

54

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

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

55

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

Radicals combine forming a stable molecule

56

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

ENDOTHERMIC

57

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

Chain reaction

58

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

EXOTHERMIC

59

What does further substitution lead to ?

Results in unpredicted compounds being made

60

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

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