alkanes and alkenes

Question 1

4:03 know what is meant by the terms homologous series, functional group and isomerism

Answer 1

functional group:
- an atom or a group of atoms that determine the chemical properties of a compound

Homologous series is a group of substances with:
- the same general formula
- similar chemical properties because they have the same functional group
- a trend (graduation) in physical properties

Isomers:
- molecules with the same molecular formula but with a different structure

Question 2

4:02 understand how to represent organic molecules using empirical formulae, molecular formulae, general formulae, structural formulae and displayed formulae

Answer 2

molecular formula:
- actual number of atoms of each element in a molecule

empirical formula
- the simplest whole number ratio of atoms present in a compound

general formula:
- shows the relationship between the number of atoms of one element to another within a molecule
- Members of a homologous series share the same general formula.

structural formula:
- shows how the atoms in a molecule are joined together

displayed formula:
- a full structural formula which shows all the bonds in a molecule as individual lines

Question 3

4:19 know the general formula for alkanes

Answer 3

CnH2n+2

Question 4

4:20 explain why alkanes are classified as saturated hydrocarbons

Answer 4

Saturated:
- A molecule containing only single bonds between carbon atoms

Unsaturated:
- A molecule containing a carbon-carbon double or triple bond. For example, alkenes as described as unsaturated molecules

Question 5

4:22 describe the reactions of alkanes with halogens in the presence of ultraviolet radiation, limited to mono-substitution knowledge of reaction mechanisms is not required

Answer 5

• alkanes react with bromine in the presence of UV light, e.g. sunlight
• a hydrogen atom in the alkane is replaced by a bromine atom
• this is substitution

Question 6

4:24 know the general formula for alkenes

Answer 6

CnH2n

Question 7

4:27 describe the reactions of alkenes with bromine, to produce dibromoalkanes

Answer 7

• alkenes react with bromine water. UV light is not required for this reaction
• the double bond is broken and the bromine atoms are added. This is an addition reaction
• during this reaction there is a colour change from orange to colourless

Question 8

4:28 describe how bromine water can be used to distinguish between an alkane and an alkene

Answer 8

alkane:
- will not react when bromine water is added without UV light
- bromine water stays orange

alkene:
- react with bromine water even without Uv light
- bromine water changes form orange to colourless

Question 9

4:17 describe how long-chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking

Answer 9

• cracking involves the thermal decomposition of long-chain alkanes into shorter-chain alkanes and alkenes
• temperature: 600
• catalyst: aluminium oxide, Al2O3