Chapter 12

Question 1

How reactive are alkanes? Why?

Answer 1

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

Question 2

Why are alkanes a good fuel?

Answer 2

The combustion of alkanes is very exothermic

Question 3

What is produced in incomplete combustion of alkanes?

Answer 3

CO(g) or C(s)

Question 4

Give 3 reasons why incomplete combustion is bad?

Answer 4

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

Question 5

When do Alkanes react with halogens?

Answer 5

Under UV light

Question 6

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

Answer 6

The hydrogen atoms on the alkane is replaced with Halogen atoms

Question 7

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

Answer 7

A substitution reaction

Question 8

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

Answer 8

A haloalkane and a hydrogen halide

Question 9

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

Answer 9

Radical substitution

Question 10

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

Answer 10

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

Question 11

What occurs during the mechanism Initiation step?

Answer 11

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

Question 12

Is Initiation endo or exothermic?

Answer 12

Endothermic as only bonds are breaking

Question 13

What occurs during the mechanism Propagation step?

Answer 13

Radicals react with molecules forming new radicals and molecules

Question 14

What type of reaction is the Propagation phase?

Answer 14

A chain reaction

Question 15

What occurs during the mechanism Termination step?

Answer 15

Radicals combine, forming a stable molecule

Question 16

Is the termination step exo or endothermic?

Answer 16

Exothermic as only bond making occurs

Question 17

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

Answer 17

Cancel the same molecules on either side of the equation

Question 18

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

Answer 18

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

Question 19

What are alkanes?

Answer 19

Saturated Hydrocarbons

Question 20

What is the general formula for alkanes?

Answer 20

CnH2n+2

Question 21

When do alkanes show chain isomerism?

Answer 21

In a chain

Question 22

When do alkane chains show isomerism?

Answer 22

When its a chain with 4 or more Carbon atoms

Question 23

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

Answer 23

A single SIGMA bond. (Covalent bond)

Question 24

What is a SIGMA bond?

Answer 24

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

Question 25

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

Answer 25

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.

Question 26

What are cycloalkanes?

Answer 26

Alicyclic hydrocarbons

Question 27

What is the general formula for a cycloalkane?

Answer 27

CnH2n

Question 28

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

Answer 28

60° bond angle, very reactive, strained ring

Question 29

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

Answer 29

90°bond angle, very reactive, strained ring

Question 30

What is the bond angle and stability of Cyclopentane?

Answer 30

108° bond angle, less strained

Question 31

What is the bond angle and stability of Cyclohexane?

Answer 31

109.5° bond angle, stable

Question 32

How are alkanes found?

Answer 32

They are extracted from crude oil by fractional distilation

Question 33

How does chain length of alkanes effect boiling point?

Answer 33

The longer the chain the higher the boiling point

Question 34

What state are alkanes with 1-4 carbon atoms?

Answer 34

Colourless gases

Question 35

What state are alkanes with 5-17 carbon atoms?

Answer 35

Colourless liquids

Question 36

What state are alkanes with 18+ carbon atoms?

Answer 36

White waxy solids

Question 37

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

Answer 37

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

Question 38

If branching on alkanes increases what happens to boiling point?

Answer 38

decreases

Question 39

Why does branching of alkanes decrease boiling point?

Answer 39

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.

Question 40

How reactive are alkanes? Why?

Answer 40

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

Question 41

What are the 2 reactions alkanes can carry out?

Answer 41

1. Combustion | 2. Halogen substitution

Question 42

What makes alkanes very good fuels?

Answer 42

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

Question 43

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

Answer 43

Gas

Question 44

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

Answer 44

Liquid

Question 45

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

Answer 45

Solid

Question 46

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

Answer 46

CO(g) or C (s)

Question 47

Why is the incomplete combustion of alkanes an issue?

Answer 47

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

Question 48

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

Answer 48

The presence of UV light

Question 49

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

Answer 49

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

Question 50

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

Answer 50

A haloalkane and a hydrogen halide are formed

Question 51

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

Answer 51

The reaction mechanism involves a radical substitution

Question 52

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

Answer 52

1. Initiation 2. Propagation 3. Termination

Question 53

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

Answer 53

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

Question 54

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

Answer 54

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

Question 55

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

Answer 55

Radicals combine forming a stable molecule

Question 56

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

Answer 56

ENDOTHERMIC

Question 57

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

Answer 57

Chain reaction

Question 58

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

Answer 58

EXOTHERMIC

Question 59

What does further substitution lead to ?

Answer 59

Results in unpredicted compounds being made

Question 60

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

Answer 60

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