Alkanes and Nomenclature

Question 1

What is the alkanes general formula?

Answer 1

Cn H2n+2.

They’ve only got carbon and hydrogen atoms- they’re hydrocarbons.

Question 2

How many bonds does each carbon atom in an alkane have?

Answer 2

Four single bonds with other atoms.

It’s impossible for carbon to make more than four bonds, so alkanes are saturated.

Question 3

Give examples of alkanes.

Answer 3

Methane 
Ethane 
Propane
Butane
Pentane

Question 4

What are cyclohexanes?

Answer 4

They have a ring of carbon atoms with two hydrogens attached to each carbon.
Cyclohexane’s have two fewer hydrogens than other alkanes so they have a general formula of Cn H2n but they’re still saturated.

Question 5

1- Stem and Alkane?

Answer 5

Meth

Methane

Question 6

2-Stem and Alkane?

Answer 6

Eth

Ethane

Question 7

3- Stem and Alkane?

Answer 7

Prop

Propane

Question 8

4- Stem and Alkane?

Answer 8

But

Butane

Question 9

5- Stem and Alkane?

Answer 9

Pent

Pentane

Question 10

6- Stem and Alkane?

Answer 10

Hex

Hexane

Question 11

7- Stem and Alkane?

Answer 11

Hept

Heptane

Question 12

8- Stem and Alkane?

Answer 12

Oct

Octane

Question 13

9- Stem and Alkane?

Answer 13

Non

Nonane

Question 14

10- Stem and Alkane?

Answer 14

Dec

Decane

Question 15

What is the side chain prefix for a side chain with 1 carbon atom?

Answer 15

Methyl

Question 16

What is the side chain prefix for a side chain with 2 carbon atoms?

Answer 16

Ethyl

Question 17

What is the side chain prefix for a side chain with 3 carbon atoms?

Answer 17

Propyl

Question 18

What is the side chain prefix for a side chain with 4 carbon atoms?

Answer 18

Butyl

Question 19

What is the side chain prefix for a side chain with 5 carbon atoms?

Answer 19

Pentyl

Question 20

What is the side chain prefix for a side chain with 6 carbon atoms?

Answer 20

Hexyl

Question 21

What shape do carbon molecules in an alkane form?

Answer 21

A tetrahedral shape around each carbon.

Each carbon atom has four pairs of bonding electrons around it that all repel each other equally.

Question 22

How is the boiling point of an alkane dependent on its size and shape?

Answer 22

The smallest alkanes, like methane, are gases at room temperature and pressure- they’ve got very low boiling points.
Larger alkanes are liquids- they have higher boiling points.
This is due to differences in there intermolecular forces.

Question 23

Why is the boiling point of an alkane dependent on its size and shape?

Answer 23

Alkanes have covalent bond inside the molecules.
Between the molecules, there are van der Waals force which hold them all together.
The longer the carbon chain, the more van der Waals forces there are.
This is because they have a larger molecular surface area so there is more surface contact between molecules.
As a result, more electrons interact and it takes more energy to overcome the forces and separate them.

Question 24

What is the complete combustion reaction?

Answer 24

If you burn (oxidise) alkanes with enough oxygen, you get carbon dioxide and water.

Question 25

Why do larger alkanes release more energy per mole?

Answer 25

When alkanes burn, some energy is used to break the covalent bonds but more energy is released by forming new product bonds in CO2 and H2O- larger alkanes form more CO2 and H2O.

Question 26

What is propane used for?

Answer 26

Central heating and cooking fuel.

Question 27

What is butane used for?

Answer 27

Bottled and sold as camping gas.

Question 28

What is petrol and diesel used for?

Answer 28

Both made up of a mixture of alkanes and are used as fuels for transport.

Question 29

What is the incomplete combustion reaction?

Answer 29

If there isn't much oxygen around an alkane will still burn, but it will produce carbon monoxide and water.

Question 30

Why is incomplete combustion a problem?

Answer 30

Carbon monoxide is poisonous- the oxygen in your body is carried around by haemoglobin. Carbon monoxide is better at binding to heamoglobin than oxygen is. This means less oxygen will reach your cells.

Question 31

Describe the molecular shape around each carbon atom in an alkane.

Answer 31

Molecules form tetrahedral shapes around each carbon.

Question 32

Name two factors that affect the boiling point of an alkane.

Answer 32

The length of the carbon-carbon chain and the amount of branching in the molecule.

Question 33

What are the products of the complete and incomplete combustion of alkanes?

Answer 33

The products of complete combustion are carbon dioxide and water. The products of incomplete combustion are carbon monoxide and water (and sometimes particulate carbon and carbon dioxide).

Question 34

The structure of an Alkane, as with all Hydrocarbons, can be represented with what four things?

Answer 34

- A 3D model, with all the atoms and bonds shown, modelled in 3 dimensions. - Full Structural Formula, with all the atoms and bonds shown. - Skeletal Formula, with only the bonds between the Carbon atoms shown. - Shortened Structural Formula, a linear form where all the atoms are shown but no bonds.

Question 35

What are the ten first alkanes in the homologous series?

Answer 35

``` Methane Ethane Propane Butane Pentane Hexane Heptane Octane Nonane Decane ```

Question 36

Why are alkanes inert?

Answer 36

They have a lack of polarity due to the fact carbon and hydrogen have similar electronegativities- causing them to be non polar.

Question 37

What can substitute the hydrogen atoms in an alkane with halogen atoms?

Answer 37

UV light

Question 38

What is the reaction for chlorine mixing with methane?

Answer 38

CH4 (g) + Cl2 (g) -> CH3Cl(g) + HCl (g)

Question 39

If a reaction requires ultraviolet light what is it therefore known as?

Answer 39

A photochemical reaction.

Question 40

What is the absorption of light energy causing a bond to break known as?

Answer 40

Photodissociation.

Question 41

What happens when UV light breaks a Cl-Cl bond in the initiation step?

Answer 41

Homolytic fission and two chlorine atoms are formed, each having seven electrons in their router shell. The chlorine atoms each have one unpaired electron and are thus free radicals.

Question 42

Write the equation for the bond breaking of Cl-Cl in the initiation step.

Answer 42

Cl-Cl (g) -> Cl. (g) + Cl. (g)

Question 43

What does the reaction of a chlorine free radical with a methane molecule produce?

Answer 43

Hydrogen chloride and a CH3. methyl free radical

Question 44

What can a methyl free radical and a chlorine molecule react to produce?

Answer 44

Chloromethane and a new chlorine free radical (a chain reaction).

Question 45

How long will the propagation reaction go on for?

Answer 45

As long as there is a supply of methane molecules and undissociated chlorine.

Question 46

What is the termination stage?

Answer 46

Take place when two free radicals meet which removes the free radicals and disrupts the propagation steps.

Question 47

Summarise alkane substitution.

Answer 47

The Cl-Cl or Br-Br bond undergoes homolytic fission in ultraviolet light producing reactive Cl or Br free radicals. This initation step of free-radical substitution is followed by propagation steps involving a chain reaction which regenerates the halogen free radicals. Termination of the reaction may occur, for example, when two radicals combine.

Question 48

What does the boiling point of an alkane depend on?

Answer 48

Its size and shape. Alkanes have covalent bonds inside the molecules and van der Waals forces between the molecules. The longer the carbon chain, the stronger the forces because there is a greater molecular surface area and more electrons to interact. It therefore takes more energy to overcome.

Question 49

Why does a branched alkane chain have a lower boiling point than straight chained alkanes?

Answer 49

Branched chain alkanes can't pack closely together and they have smaller molecular surface areas so van der Waals forces are reduced.