ch 12 alkanes

Question 1

alkane structure explain σ-bonds and its implications

Answer 1

a type of covalent bond when atomic orbitals overlap head on
C-C bond : pp overlap
c-h: ps overlap
HH: ss overlap
σ-bonds act as an axes, and therefore allow free rotation. tetra hedral 109.5

Question 2

impact of branches on boiling point in alkanes

Answer 2

Each isomer has the same number of electrons but the strength of these forces
are not the same.

The shape of the molecule affects the boiling points. The more branches the
lower the boiling point.

Straight chains allow close packing and maximum surface contact - more London
forces. More branches means the molecules can’t pack as closely.

More london forces means more energy to break.

as branching increases the boiling point decreases

branched fewer points of contact so fewer London forces

Less energy required to break London forces

Question 3

impact of chain length on alakens boiling points, viscosity and flamability

Answer 3

• As the length of an alkane chain increases, the boiling point increases. This is because there are more points of contact with each adjacent molecule. This means there are stronger induced dipole to dipole forces between adjacent molecules.
• In addition to this, the viscosity, or resistance to flow, of alkanes also increases with chain length. Longer chains tend to tangle and resist flow more than shorter ones, making them more viscous.
• The flammability of alkanes is another property that is affected by chain length. Short-chain alkanes are generally more flammable than long-chain alkanes. This is because short-chain alkanes vaporise more easily, and it is the vapour of a substance, not the liquid, that burns. Therefore, substances with shorter chains, which can vaporise at lower temperatures, are more likely to catch fire.

Question 4

branched/longer or shorter alkanes burn cleaner?

Answer 4

Straight chain alkanes have a tendency to pre ignite in a combustion engine whereas branched chains do not.

Branched chain alkanes burn cleaner than straight chain alkanes so they are usualaly converted

The shorter the alkane the cleaner it burns.

Question 5

larger alkanes release …. energy because ….

Answer 5

more because they have heaps more energy per mole beacuease more bonds

Question 6

Complete combustion of alkanes general formula

Answer 6

CxHy (g) + (x+y/4)O2 -> XCO2 (g) + Y/2 H2O

Question 7

Incomplete combustion of alkanes

Answer 7

Incomplete combustion is where there is an insufficient supply of oxygen. It produces
carbon monoxide,

CH4(g) + 11/2O2(g) → CO(g) + 2H2O(g)

The orange/yellow colour of the flames is due to incomplete combustion. This
colour is due to particles of carbon glowing in the heat. More oxygen (air)
increases oxidation of the carbon.

The products of burning contain some carbon monoxide, CO and carbon, C.
Carbon monoxide is a poisonous gas. It has no colour or odour so is not noticed. It
combines with haemoglobin in the blood in preference to oxygen –
carboxyhaemoglobin. Deaths occur from faulty gas fires or boilers in poor ventilated
rooms. Landlords are now required to have gas appliances serviced annually.

Question 8

reactions of alkanes

Answer 8

• halogenation (free radical sun)
• straight->branched
• cracking
• cyclic

Question 9

Cracking:

Answer 9

Cracking breaks longer, less useful alkanes into shorter, more useful alkanes and
alkenes.

This is done at heating the alkanes with a catalyst.

Both C – C and C – H bonds are broken in the process. Random bonds are broken which
means a variety of products are produced including hydrogen. Some of the
intermediates can react to produce branched chain alkanes.

Alkenes are used in the polymer industry.

Question 10

Producing branched alkanes

Answer 10

Straight chain alkanes have a tendency to pre ignite in a combustion engine whereas
branched chains do not.

Branched chain alkanes burn cleaner than straight chain alkanes so they are usually
converted using isomerisation reactions.

Isomerisation reactions:
pentane -> 2,2-dimethylpropane

Question 11

Producing cyclic hydrocarbons

Answer 11

As straight chain alkanes have a tendency to pre ignite they are also converted to cyclic
and aromatic hydrocarbons.

Any hydrogen produced is used in other processes - ammonia production, margarines.

These also burn cleaner than straight chain alkanes so they are usually converted using
reforming reactions.

Reforming reactions

pentane -> cyclopentane + H2

Question 12

Halogenation of the alkanes - FREE RADICAL SUBSTITUTION (FRS):

Answer 12

Alkanes are unreactive due to the lack of polarity of the C-H bonds. This is due
to the fact that carbon and hydrogen are similar in electronegativities.

In the presence of ultraviolet light (or 300oC) the halogen will substitute a
hydrogen in an alkane, a haloalkane and hydrogen halide are produced.

Reactions that occur in light are called photochemical reactions.

Question 13

steps of FRS

Answer 13

1. initiation
2. propaogation
3. termination

Question 14

limitations of FRS

Answer 14

• further subsitusions end up with range of products that need seperating
• subsitution in variety of positions end up with mixture of isomers

Question 15

conditions for free radical subsitution

Answer 15

• Uv light (300’c)
• halogen Br2 (always form di bromide) or HBr hydrogen halide

Question 16

name the process by which straight chain alkanes are changed into there cyclic counterparts of same chain length

Answer 16

reformation

Question 17

branched chain alkanes are prefered in petroleum industry becuase …

Answer 17

• ignite easier