Chapter 12 - Alkanes

Question 1

What are alkanes?

Answer 1

Saturated hydrocarbons

Question 2

General formula of alkanes

Answer 2

CnH2n+2

Question 3

What will every carbon atom in an alkane have?

Answer 3

Four single bonds with other carbon atoms

Question 4

What are saturated hydrocarbons?

Answer 4

All carbon-carbon bonds are single

Question 5

What is the name of the bond between the carbon atoms in an alkane?

Answer 5

A sigma bond

Question 6

What is a sigma bond?

Answer 6

The result of an overlap of two orbitals, one from each bonding atom

Question 7

How many electrons are in a sigma bond?

Answer 7

2

Question 8

Where is a sigma bond positioned?

Answer 8

On a line directly between two bonding atoms

Question 9

What are the two bonds where sigma bonds can occur?

Answer 9

C-H or C-C

Question 10

What is the bond angle in an alkane and why?

Answer 10

109.5, the four pairs of bonding electrons repel eachother equally

Question 11

What structure does an alkane form?

Answer 11

A tetrahedral structure

Question 12

Why are alkanes not rigid?

Answer 12

The sigma bonds act as an axes which the shape can rotate around

Question 13

How is crude oil separated into fractions?

Answer 13

Fractional distillation

Question 14

As chain length increases, what happens to the boiling point of alkanes?

Answer 14

It increases

Question 15

Why does fractional distillation work?

Answer 15

Alkanes have different boiling points, dependent on chain length

Question 16

What type of bonds are responsible for the boiling point of alkanes?

Answer 16

London forces between the molecules

Question 17

Where are covalent bonds found?

Answer 17

Inside molecules

Question 18

Where are London forces found?

Answer 18

Between molecules

Question 19

Why do alkanes with a longer chain length have higher boiling points?

Answer 19

The longer the carbon chain, the larger the surface area available over which London forces can act. There are also more electrons to interact. This means more energy is needed to overcome the London forces

Question 20

Will branched isomers have higher or lower boiling points?

Answer 20

Lower

Question 21

Why do branched chain alkanes have lower points?

Answer 21

There are fewer surface points of contact between molecules of the branched out, giving fewer London forces. The shape of the molecule prevents the branched molecules getting as close together as straight-chain molecules hence decreasing the intermolecular force further

Question 22

Why are alkanes very unreactive?

Answer 22

• C-H and C-C Sigma bond are strong
• C-C Bonds are non-polar
• The electronegativity of carbon and hydrogen is so similar that the C-H bond is non-polar

Question 23

What are alkanes uses?

Answer 23

•fuel as readily available/easy to transport/burn in plentiful supplies of oxygen

Question 24

What is produced with the combustion of alkanes and oxygen?

Answer 24

Carbon dioxide and water

Question 25

What is produced in an incomplete combustion of alkanes and oxygen?

Answer 25

Carbon monoxide/soot and water

Question 26

What is carbon monoxide?

Answer 26

Colourless, odourless and highly toxic gas which combines with haemoglobin preventing the transport oxygen around the body

Question 27

In radical substitution what are the three stages take place?

Answer 27

• Initiation
• propagation
• termination

Question 28

What is initiation phase from radical substitution?

Answer 28

The first stage in a radical reaction in which a radical starts when a covalent bond is broken by homolytic fission of a covalent bond

Question 29

What is propagation phase from radical substitution?

Answer 29

The 2nd phase that continue a free radical reaction, in which a radical reacts with a reactive molecules to form a new molecule and another radical , causing a chain reaction

Question 30

What is termination phase from radical substitution?

Answer 30

The step at the end of the radical substitution which two radicals combine to form a molecule

Question 31

What are the limitations of a radical substitution?

Answer 31

When CH3Br is formed in the second propagation step, further substitution can occur. Another bromine radical can collide with the molecule and replace a hydrogen atom, forming CH2Br2. This can happen until all of the hydrogen atoms are used up (CBr4)

Question 32

What is the mechanism for the bromination of methane an example of?

Answer 32

Radical substitution

Question 33

What is another problem of free radical substitution which is a result of chain length?

Answer 33

It can take place at any point along the carbon chain, so a mixture of isomers can be formed

Question 34

What is radical substitution used for?

Answer 34

Making haloalkanes

Question 35

When will termination occur?

Answer 35

When two radicals collide to form a stable molecule

Question 36

What happens during initiation?

Answer 36

Sunlight provides the energy to break the Br-Br or Cl-Cl bond
Homolytic fission occurs - the bond splits equally and each atom gains one electron from the bond
The atoms are now free radicals because of this unpaired electron
Br - Br -> Br. + .Br

Question 37

What happens during termination?

Answer 37

Two radicals collide to form a stable molecule. There are lots of ways this could happen, e.g.:

• Br. + .Br -> Br2
• .CH3 + .CH3 -> C2H6

Question 38

What happens during the second propagation step?

Answer 38

Each methyl radical reacts with another bromine molecule to form the organic product of bromomethane and another bromine radical. This radical then reacts with a CH4 molecule and the first step repeats

Question 39

What are the two reactions that occur during propagation?

Answer 39

1) CH4 + Br. -> .CH3 + HBr

2) .CH3 + Br2 -> Ch3Br + Br.

Question 40

What is photodissociation?

Answer 40

Sunlight providing the energy to break a bond

Question 41

What happens during the first propagation step?

Answer 41

A bromine (or chlorine) radical reacts with a C-H bond in the methane to form a methyl radical (.CH3) and a molecule of hydrogen(halide)