Alkanes

Question 1

What are alkanes?

Answer 1

A homologous series of saturated hydrocarbons with the general formula (CnH2n+2).

Question 2

What does saturated mean?

Answer 2

Contains single bonds only.

Molecule containing no double bonds.

Question 3

What does unsaturated mean?

Answer 3

Molecule containing double bonds.

Question 4

What is a hydrocarbon?

Answer 4

Molecule containing hydrogen and carbon only.

Question 5

What is a homologous series?

Answer 5

A family of compounds with the same general formula and similar chemical properties.
In a series each number differs by the addition of a CH2 group and there is a gradual change in physical properties.

Question 6

Longer carbon chain means what for the boiling point?

Answer 6

Higher boiling point.
Stronger Van der Waal’s forces between molecules.
More electrons in the molecules.

Question 7

More branched chain means what for the boiling point?

Answer 7

Lower boiling point.
Weaker Van der Waal’s forces between molecules.
Not being able to pack as close together.

Question 8

What is the point of fractional distillation of petroleum?

Answer 8

Crude oil has no use in its raw form.

But you can separate it out into more useful bite (fractions) by fractional distillation.

Question 9

As the C chain gets longer, the hydrocarbons are:

Answer 9

More viscous.
Harder to ignite.
Less volatile.
Higher boiling point.

Question 10

What is the basic idea for the separation process of fractional distillation?

Answer 10

1. The crude oil is vaporised at about 350^oc
2. The vaporised crude oil goes into the bottom of the fractioning column and rises through the trays.
3. As this happens, it gets cooler creating a temperature gradient .
4. Molecules condense at different heights (different boiling points)
   Larger the molecule = lower down it condenses.

Question 11

What happens to hydrocarbons with a boiling point that is too high or low during fractional distillation?

Answer 11

Too high = doesn’t vaporise at all, runs to the bottom and forms a gooey residue (this can then be separated at a lower pressure)
Too low = doesn’t condense, drawn off as gases at the top of the column.

Question 12

Why is cracking used?

Answer 12

To make use of excess larger hydrocarbons and to supply demand for shorter ones.

Question 13

What takes place during thermal cracking?

Answer 13

High temp = pressure (1000^oc, 70atm)

Produces lots of alkenes e.g. valuable product= polyethene (made from ethene)

Question 14

What takes place during catalytic cracking?

Answer 14

Uses a zeolite catalyst
Slight pressure and high temp (500^oc, 1-2atm)
Produces aromatic hydrocarbons and the alkanes needed to produce motor fuels (cycloalkanes)

Question 15

Why does using a catalyst cut costs?

Answer 15

Reaction can be done at a lower temp + pressure

Also speeds up the rate of reaction = saving time (time is money)

Question 16

How does SO2 form?

Answer 16

Sulfur-containing alkanes can produce SO2 during combustion.

Question 17

Why is SO2 bad?

Answer 17

It dissolves in rain water to form acid rain.

Question 18

Why is flue gas desulfurisation used?

Answer 18

So SO2 can be removed from the waste gases from furnaces.

Question 19

How does flue gas desulfurisation work?

Answer 19

The gases pass through a scrubber containing calcium oxide or calcium carbonate (which reacts with the SO2) = (acid base reaction)
CaO + SO2 -> CaSO3
CaO3 + SO2 -> CaSO3 + CO2
CaSO3 + gypsum -> plasterboards

Question 20

What does a catalytic converter do?

Answer 20

Removes toxic gases ( CO, NOx &unburned hydrocarbons) from the exhaust gases, turning them into CO2, N2, H2O

Question 21

What is a catalytic converter?

Answer 21

It has a ceramic honeycomb coated with a thin layer of catalyst metals (Pt, Pd, Rh) to give a large surface area.

Question 22

What happens during initiation in the chlorination of alkanes?

Answer 22

The Cl-Cl bond is broken by UV radiation by homolytic fission
(Covalent bond split down the middle)
(Free radicals formed)
Cl2 -> Cl* + Cl*

Question 23

What happens during propagation in the chlorination of alkanes?

Answer 23

Continuing the reaction
Free radicals are used up and created in a chain reaction
Cl* + CH4 -> CH3* + HCl
CH3* + Cl2 -> CH3Cl + Cl*

Question 24

What happens during termination in the chlorination of alkanes?

Answer 24

Free radicals are removed
Cl* + Cl* -> Cl2
CH3* + CH3* -> C2H6
CH3* + Cl* -> CH3Cl
^All include half arrows from star to middle to show the movement of electrons.

Question 25

What is a condition for the free radical substitution? and why?

Answer 25

Must be excess methane so no further substitutions take place
As favoured product= chloromethane (not di or tri etc)

Question 26

What does the ozone layer do?

Answer 26

Naturally protects us from the harmful UV radiation produced by the sun.

Question 27

What are the equations for the decomposition of the ozone layer? and what does this do?

Answer 27

Cl* + 03 -> ClO* + O2
ClO* + O2 -> 202 + Cl*
The chlorine radicals are regenerated and act as a catalyst in the breakdown of ozone molecules into oxygen.

Question 28

What are now used instead of CFCs?

Answer 28

HFCs as an alternative.