Hydrocarbons

Question 1

Properties of Hydrocarbons according to chain length

Answer 1

As the number of carbon atoms in the molecules increase, properties change in a regular pattern. Many of these changes are to do with the increased intermolecular attractions.

Question 2

Bigger molecules have:

Answer 2

• Higher boiling points because they are more attracted to one another (heavier)
• Less volatile liquids- evaporate slower at room temperature because of the higher intermolecular forces
• More viscous (flow less easily) because big molecules are stickier
• Don’t burn as easily, which limits their use as fuels

Question 3

Fractioning column

Answer 3

40C     Refinery gases
           Gasoline (petrol)
           Kerosene
           Diesel oil 
           Fuel oil
400C    Bitumen

Question 4

Homologous series

Answer 4

A family of compounds with similar properties due to similar bonding. They have the same molecular formula (CnHn+2)

Question 5

Hydrocarbon

Answer 5

Organic molecules made up of ONLY carbon and hydrogen

Question 6

Saturated hydrocarbons

Answer 6

Containing only single bonds- maximum possible number of hydrogen atoms to given number of carbons (alkanes)

Question 7

Unsaturated hydrocarbons

Answer 7

Containing a carbon=carbon double bond so that they don’t contain as many hydrogens as the corresponding alkane (alkenes)

Question 8

Isomers

Answer 8

Compounds that have the same molecular formula but different displayed formulae (different ways of fitting together)

CH3-CH2-CH2-CH3 or CH3
|
CH3-CH-CH3

Question 9

Dehydration of alcohols

Answer 9

The dehydration of ethanol produces ethene. Ethanol vapour from mineral wool soaked in ethanol is passed over heated aluminium oxide (catalyst) which then goes through a tube leading into cold water as the tube points upwards into a test tube where ethene gas collects.
CH3CH2OH(g) –> CH2=CH2(g) + H2O(l)

Question 10

Manufacture of ethanol- Fermentation

Answer 10

FERMENTATION- Yeast added to a sugar or starch solution and left in the warm (30C) for several days without air. Enzymes in the yeast convert sugar into ethanol and carbon dioxide.

Question 11

Manufacture of ethanol- Hydration

Answer 11

HYDRATION- Pass ethene and steam over a phosphic acid catalyst at 300C and a pressure of 60-70 atmospheres

Question 12

Pros and cons: Fermentation

Answer 12

ADVANTAGES
-Renewable resources like sugar beet or cane
-Gentle temperatures and ordinary pressure
DISADVANTAGES
-Batch process
-Slow, several days per batch
-Produces impure ethanol that needs further processing

Question 13

Pros and cons: Hydration

Answer 13

ADVANTAGES
-Continuous flow, its efficient
-Very fast
-Produces much purer ethanol
DISADVANTAGES
-Finite resources- oil
-High temperatures and pressure, needing high energy

Question 14

Simple distillation

Answer 14

A side arm flask is filled with sea water and heated from below. The vapour goes into a condenser from the side arm and pure water is produced, collecting in a beaker at the end of the condenser.

Question 15

Crude oil

Answer 15

Crude oil is a mixture of hydrocarbons, which are organic molecules. It is a finite, non renewable resource. They can occur as chains, branched chains, rings or as a combination.

Question 16

Alkanes

Answer 16

ALKANES AINT GOT NO DOUBLE BONDS

CnH2n+2

    H
     |
H -C- H
     |
    H

Question 17

Products of combustion

Answer 17

Complete combustion gives Carbon dioxide and water.
CH4(g) + 2O2(g) –> CO2(g) + 2H2O(l)
Incomplete combustion (when theres not enough oxygen) gives Carbon monoxide or carbon (soot) instead of CO2.

Question 18

Alkanes with bromine

Answer 18

Alkanes react with bromine in the presence of UV light, replacing a hydrogen atom with a bromine atom, a substitution reaction. Methane and bromine gas is brown until exposed to sunlight, making bromomethane and hydrogen bromide.

Question 19

Alkenes

Answer 19

CnH2n

DOUBLE BONDS

H   H
 |    |
C=C
 |   |
H  H

Question 20

Alkenes with bromine

Answer 20

Alkenes undergo addition reactions where part of the double bonds breaks and the electrons are used to join other atoms onto the two carbon atoms. Bromine does this without heat of a catalyst, using bromine solution ('bromine water').
--> H  H
      |   |  
  H-C-C-H
      |   |
     Br Br

Question 21

Test for alkenes

Answer 21

Shake with bromine water and the orange will be decolourised. If it is in gas form, bubble through.

Question 22

Reasons for cracking

Answer 22

Fractional distillation of crude oil produces more long chain hydrocarbons than can be used directly and fewer short chain hydrocarbons than are required, so you need to use cracking.

Question 23

Cracking

Answer 23

Cracking is thermal decomposition- a big molecule splitting into smaller ones on heating, in a fairly random way. Alkanes can be converted to alkenes. The gas oil fraction is heated to give a gas and then passed over a catalyst if MIXED SILICON DIOXIDE and ALUMINIUM OXIDE at about 600-700C. It could be carried out at a higher temperature without a catalyst.
C13H28(l) –> C2H4(g) + C3H6(g) + C8H18(l)

Question 24

Combustion

Answer 24

All hydrocarbons burn in oxygen to form carbon dioxide and water and release a lot of heat in the process which means they can be used as fuels.
Burning methane:
CH4(g) + 2O2(g) –> CO2(g) + 2H2O(l)

Question 25

Incomplete combustion

Answer 25

If theres not enough air (oxygen), you get incomplete combustion, leading to the production of carbon (soot) or carbon monoxide instead of carbon dioxide.
2CH4(g) + 3O2(g) –> 2CO(g) + 4H2O(l)

Question 26

Carbon monoxide

Answer 26

CO is poisonous, odourless and colourless. It combines with haemoglobin in the blood and prevents it from carrying enough oxygen.

Question 27

Fractional distillation (small scale)

Answer 27

Used to separate liquids such as alcohol (ethanol) and water. You can separate them using their different boiling points: water at 100C, alcohol at 78C.