Organic Chemistry

Question 1

What are the characteristics of crude oil?

Answer 1

• Crude oil is a non-renewable (finite) resource.
• Crude oil is formed from the fossilised remains of ancient plankton.
• Crude oil is found in rocks and is used to produce fuels and other important chemicals.
• Crude oil is a mixture of lots of different compounds that are not chemically combined.
• Crude oil is largely made up of hydrocarbons.

Question 2

What are the properties of longer chain hydrocarbons?

Answer 2

• Boiling point: hydrocarbons with longer chains have higher boiling points.
• Viscosity: hydrocarbons with longer chains have higher viscosity (i.e. they are thicker and flow less easily).
• Flammability: hydrocarbons with longer chains are less flammable.

Question 3

What are alkanes?

Answer 3

• Alkanes are hydrocarbons where each carbon atom is bonded to four other atoms (hydrogen or carbon).
• We call them saturated hydrocarbons as they only contain single bonds with other atoms.

Question 4

What is the general formula for alkanes?

Answer 4

• The general formula for alkanes is CnH(2n+2).
  
  • E.g. if a hydrocarbon contained 7 carbon
    atoms, the number of hydrogen atoms
    would be (2 x 7) + 2 = 16.
  • Therefore, the formula for this
    hydrocarbon would be C7H16.

Question 5

What are alkanes used for?

Answer 5

Alkanes burn well so make good fuels.

Question 6

What are the four smallest alkanes?

Answer 6

Methane (CH4).
Ethane (C2H6).
Propane (C3H8).
Butane (C4H10).

Question 7

What is fractional distillation?

Answer 7

Fractional distillation is the process of separating crude oil into groups of hydrocarbons with similar numbers of carbon atoms. We call these groups of hydrocarbons “fractions”.

Question 8

Describe the process of fractional distillation.

Answer 8

• Evaporation: crude oil is heated until it evaporates.
• Condensation: Long-chain hydrocarbons have higher boiling points so condense at the bottom and short-chain hydrocarbons have lower boiling points so condense at lower temperatures nearer the top.
• Collection: the products are collected and processed to create end products like fuel.

Question 9

What are homologous series?

Answer 9

Homologous series are families of similar compounds containing carbon atoms.

Question 10

What is cracking?

Answer 10

breaking down a hydrocarbon with a long chain into smaller molecules

Question 11

What is a fraction?

Answer 11

a group of hydrocarbons with similar chain lengths

Question 12

Name five useful materials produced from crude oil fractions.

Answer 12

petrol, diesel oil, kerosene, heavy fuel oil, and liquefied petroleum gases

Question 13

Name two methods of cracking.

Answer 13

steam and catalytic

Question 14

What are alkenes used for?

Answer 14

formation of polymers

Question 15

What happens when hydrocarbons are burned with plenty of oxygen?

Answer 15

The combustion (burning) of hydrocarbons when there is a good air supply involve:
Energy release.
The oxidation of carbon and hydrogen to create carbon dioxide and water.

Question 16

What happens when hydrocarbons are burned with a lack of oxygen?

Answer 16

• If alkanes are burned without enough oxygen, then carbon monoxide can be created:
  2C + O2 → 2CO.
• Carbon monoxide is a colourless, odourless, toxic gas.

Question 17

Why don’t heavy fractions of crude oil make good fuels?

Answer 17

• Heavy fractions of crude oil don’t ignite easily.
• Heavy fractions of crude oil have high boiling points.
• Heavy fractions of crude oil have low volatility.

Question 18

What happens in catalytic cracking?

Answer 18

Vapourised heavy hydrocarbons are passed over a hot catalyst.
The products include alkanes and alkenes.
E.g. hexane → butane + ethene

Question 19

What happens in steam cracking?

Answer 19

Vapourised heavy hydrocarbons are mixed with steam in a high temperature environment.
The products include alkanes and alkenes.
E.g. decane → octane + ethene

Question 20

What are alkenes?

Answer 20

Alkenes are hydrocarbons with the functional group C=C. This means that alkenes have a carbon-carbon double bond and are unsaturated.

Question 21

What is the general formula for alkenes?

Answer 21

Alkenes are a homologous series with the general formula CnH2n.

Question 22

What are the four smallest alkenes?

Answer 22

Ethene with the formula C2H4.
Propene with the formula C3H6.
Butene with the formula C4H8.
Pentene with the formula C5H10.

Question 23

Describe the combustion reactions of alkenes.

Answer 23

Alkenes react with oxygen in combustion reactions. They react in a similar way to other hydrocarbons.
E.g. ethene (C2H4) + oxygen (O2) → carbon monoxide (CO) + carbon (C) + water (H2O)
Because their combustion is incomplete, they burn with smoky flames.

Question 24

Describe the reactions of alkenes with hydrogen.

Answer 24

In the presence of a nickel catalyst, hydrogen can be added to an alkene to give an alkane.
E.g. ethene (C2H4) + hydrogen (H2) → ethane (C2H6).
Adding hydrogen atoms across a carbon-carbon double bond is called hydrogenation.

Question 25

Describe the reactions of alkenes with water.

Answer 25

In the presence of a phosphoric acid catalyst, water, in the form of steam, can be added to an alkene to give an alcohol.
E.g. ethene (C2H4) + steam (H2O) → ethanol (C2H5OH).

Question 26

Describe the reactions of alkenes with halogens.

Answer 26

Alkenes, unlike alkanes, will react when shaken with bromine water. This causes the solution to change colour from orange-brown to colourless.
E.g. ethene (C2H4) (colourless) + bromine (Br2) (orange-brown solution) → dibromoethane (CH2BrCH2Br) (colourless).
This can be used as a test to distinguish between alkenes and alkanes.