Chapter 12- Properties of alkanes and their reactions

Question 1

What are alkanes?

Answer 1

a saturated hydrocarbon that contains only hydrogen and carbon atoms that are joined by single covalent bonds (sigma bonds)

Question 2

What is a sigma bond?

Answer 2

an overlap of two orbitals, one from each bonding atom
–> the electron density of a sigma bond is positioned on the line between two atoms

Question 3

Alkanes:
General formula?
Bond angle?
shape?

Answer 3

CnH2n+2 (non cyclic alkanes)
109.5 degrees
Tetrahedral

Question 4

What is special about sigma bonds

Answer 4

they can act as axes for atoms to rotate freely

Question 5

How is crude oil separated?

Answer 5

Through fractional distillation

Question 6

Effect of chain length on boiling point

Answer 6

As chain length increases, the strength of london forces increases
–> due to greater surface are of contact
–> BP increases so more energy is required to overcome the forces

Question 7

Effect of branching on boiling point

Answer 7

Greater branching means there is less surface area of contact
–> results in weaker london forces
–> BP decreases so less energy is needed to overcome the forces

Branches can also get in the way and prevent the branched molecules from getting as close together as straight chains
–> decreases intermolecular forces further

Question 8

Complete combustion of alkanes

Answer 8

Forms CO2 and Water

Question 9

Incomplete combustion

Answer 9

produces either CO (carbon monoxide) and water
OR
C (Carbon particulates) and water

Question 10

How to work out the amount of gas made during combusiton

Answer 10

1.calculate number of mols
2. balance equation
3. Rearrange volume formula (N=V/Vm)

Question 11

Reaction of alkanes with halogens

Answer 11

UV radiation from sun provides initial energy for the reaction between an alkane and halogen

Question 12

What type of reaction is bromination of alkanes?

Answer 12

Radical substitution is a type of chemical reaction in which a functional group in a molecule is replaced by another group through the intermediacy of a radical.

Question 13

Bromination of alkanes: Initiation

Answer 13

covalent bond between a bromine molecule is broken via homolytic fission
–> forms two highly reactive bromine radicals

Question 14

What is a radical?

Answer 14

a very reactive species with an unpaired electron

Question 15

Bromination of alkanes: Propagation

Answer 15

Reaction propagates through two propagation steps (chain reaction)
1. Bromine radical reacts with CH4 to form a methyl radical *CH3 and a hydrogen bromide molecule HBr

2. Each methyl radical reacts with a bromine molecule, forming bromomethane CH3Br, together with a new bromine radical

This reaction continues until all the reactants are used up (in theory)

Question 16

Bromination of alkanes: Termination

Answer 16

Two radicals collide and form a molecule with all electrons paired
In this case, there are 3 possible reactions

1. Br* + *Br –> Br2
2. *CH3 + *CH3 –> C2H6
3. *CH3 + *Br –> CH3Br

These slow down the overall reaction

Question 17

Limitations of radical substitution

Answer 17

Further substitution
- unwanted products can form if during the propagation step, a product like CH3Br reacts with another radical to form CH2Br2

Substitution at different positions
- for longer chains, there can be a mixture of monosubstituted isomers by substitution at different positions