Alkanes (Organic) (complete)

Question 1

Name the similarities between alkanes.

Answer 1

• Similar chemical properties
• Trend in physical properties.

Question 2

How is boiling point affected in alkanes?

Answer 2

• Higher Boiling point:
  > the longer the carbon chain; due to stronger van der waals forces between molecules because of more electrons in the molecules.
  > the less branched the carbon chain; due to weaker can der waals forces between molecules as they can’t pack as close together.

Question 3

Describe how the compounds in crude oil can be separated?
(Why are they able to be separated using this technique)

Answer 3

Fractional Distillation:
- Because the compounds in crude oil have different boiling points, due to C-H bonds being virtually non-polar, they only contain van der waals forces between molecules. Therefore as the alkanes chain gets longer, the melting and boiling points increase due to greater van der waals forces.
- Method:
> the crude oil is vaporised.
> the vapour is passed into a distillation tower which is hot at the bottom and cold at the top.
> as the vapour rises, it cools.
> molecules will condense at different heights as they have different boiling points.
> the larger the molecules, the lower down the column it condenses.

Question 4

Define fraction.

What happens to the hydrocarbons in fractions as their carbon chain increases?

Answer 4

• Fraction - a mixture of hydrocarbons with similar boiling points.

As their carbon chain increases, they become:
- more viscous
- harder to ignite
- less volatile
- have higher boiling points

Question 5

What happens to the residue left after the first distillation of crude oil?

Answer 5

• the residue contains useful substances (e.g: lubricating oil, fuel oil, and bitumen) that boil above 350 degrees at atmospheric pressure, however they would decompose at such high temperatures, so they must be seperated further by distillation at lower pressure (vacuum distillation)

Question 6

What is cracking?

Give the differences in:
- Temperature
- Pressure
- Catalyst used?
- Products
: of Thermal cracking versus Catalytic cracking.

Answer 6

Cracking is a process which splits up larger hydrocarbons with longer chain into smaller hydrocarbons with shorter chains, which are in higher demand. (Thermal decomposition of alkanes.)

Thermal Cracking - Temp = 900 degrees, Pressure = 70 atm, No Catalyst used, Produces alkenes.
Catalytic Cracking - Temp = 450 degrees, Pressure = 1-2 atm, Zeolite catalyst used, Produces motor fuels (aromatics, cyclic alkanes, branched alkanes.)

Question 7

What is reforming?

Answer 7

• The process where straight chain hydrocarbons are converted into branched chain alkanes and cyclic alkanes.
• Both these products burn more efficiently and are used in petrol for cars.

Question 8

Name the seven pollutants from burning fossil fuels, how they are formed, the problems they cause, and ways to reduce the problem for each.

Answer 8

CO2 - complete combustion of fuels containing carbon; greenhouse gas (global warming); burn less fossil fuels.
CO - incomplete combustion of fuels containing carbon; toxic; ensure a good supply of oxygen when burning fuels.
C - incomplete combustion of fuels containing carbon; blackens buildings, can cause respiratory problems, global dimming; ensure a good supply of oxygen when burning fuels.
H2O - combustion of fuels containing hydrogen; no problems caused…
SO2 - combustion of fuels containing sulphur; acid rain (kills fish, damages plants); remove sulphur from fuel before burning, flue gas desulfurisation.
NOx - reaction of N2 in the air with O2 in the air at very high temperatures (in engines); acid rain (kills fish, damages plants); use catalytic converters in car engines.
Unburned hydrocarbons - some of the fuel may not actually burn; wastes fuel, greenhouse gas; ensure engines are well-tuned and there is a good supply of oxygen when burned.

Question 9

What is flue gas desulfurisation?
How is it carried out?
Give the equations.

Answer 9

• The process of removing SO2 from the waster gases from furnaces.
• The gases pass through a scrubber containing calcium oxide or calcium carbonate, which reacts with SO2.
• CaO + SO2 —> CaSO3 or CaCO3 + SO2 —> CaSO3 + CO2.
• The reaction forms gypsum (calcium sulphate) which is used too make plasterboard.

Question 10

How do Catalytic Converters work?
Give the equations.

Answer 10

• They remove CO2 and NO2 and unburned hydrocarbons from the exhaust gases, turning them into CO2 and H2O.
• They have ceramic honeycomb coated with a thin layer of catalyst metals to give a large surface area.
• 2CO + 2NO —> 2CO2 + N2 or C8H18 + 25NO —> 8C02 + 12.5N2 + 9H2O

Question 11

How does Global Warming occur?

Answer 11

• The incoming short wave radiation from the Sun enter the Earth’s atmosphere.
• Long wave radiation mostly escapes the Earth’s atmosphere as it is sent back, however, whatever doesn’t get through the atmosphere, is reflected back into the Earth, to be re-absorbed, warming up the Earth.