Topic 3

Question 1

Explain the terms homologous series, hydrocarbon, saturated, unsaturated, general formula and isomerism.

Answer 1

Homologous Series:
A series of organic compounds that have the same general formula, similar chemical reactions and where each member differ from the next by a given formula.

Hydrocarbon:
A compound that only consists of carbon and hydrogen.

Saturated:
An organic compound in which all the bonds are singe bonds

Unsaturated:
An organic compound that contains a carbon-carbon double bond.

General Formula:
A formula that states the ratio of atoms of each elements in the formula of every compound in a particular homologous series.

Isomerism:
Compounds that have the same molecular formula but different formulae are said to exhibit isomerism, the different compounds are called isomers.

Question 2

Recall that alkanes have the general formula CnH2n+2

Answer 2

Alkane are a homologous series of compounds that have the general formula of CnH2n+2

Question 3

Draw displayed formulae for alkanes with up to five carbon atoms in a molecule, and name the straight-chain isomers

Answer 3

All you need to remember when drawing displayed formulas for alkanes is that every carbon is bonded to two hydrogens (usually draw them one on top and one below) and there are two hydrogens on the end of the whole thing.

C1H4 - Methane
C2H6 - Ethane
C3H8 - Propane
C4H10 - Butane
C5H12 - Pentane

Question 4

Recall the products of the complete and incomplete combustion of alkanes

Answer 4

Complete Combustion:
If there is lots of oxygen, complete combustion will occur. This releases lots of energy whilst producing water and carbon dioxide. These are the equations for the combustion of methane, for example…

Methane + Oxygen -> Carbon Dioxide + Water
CH4 + 2(O2) -> CO2 + 2(H20)

NOTE: When there is lots of oxygen and the combustion is complete, the gas burns with a clean blue flame.

Incomplete Combustion:
Incomplete combustion occurs when there is not enough oxygen. Less energy is produced than in complete combustion, along with this energy, water and carbon dioxide, carbon monoxide is also produced (which is poisonous). These are the equations for an example of incomplete combustion…

4(CH4) + 6(O2) -> C + 2(CO) + CO2 + 8(H2O)

NOTE: Incomplete combustion will burn with a smoky yellow flame. Also, the product will depend on how much oxygen is present (e.g. using the example above, if 7O2 were present, no C would be produced, instead it would be another CO2).

Question 5

Describe the substitution reaction of methane with bromine to form bromomethane in the presence of UV light.

Answer 5

In the presence of UV light, halogens will react with alkanes (producing haloalkanes). When this reaction occurs, a hydrocarbon atom from the alkane is replaced by the halogen (bromine). THis is known as a substitution reaction.

Methane + Bromine -> Bromomethane + Hydrogen Bromine

NOTE: The reaction must have UV light to work, just like photosynthesis can’t work without light, substitution reactions will not work without UV light.

Question 6

Recall that alkenes have the general formula CnH2n

Answer 6

Alkenes are another homologous series of compounds that have the general formula of CnH2N

Question 7

Draw displayed formulae for alkenes with up to four carbon atoms in a molecule, and name the straight-chain isomers (knowledge of cis- and transisomers
is not required)

Answer 7

Alkenes have one carbon-carbon double bond in their carbon chains (meaning they are saturated).

C2H4 - Ethene
C3H6 - Propene
C4H8 - Butene

There are two possibilities for butene as the carbon-carbon double bond can go in two places.

NOTE: Ethene is the first alkene as ‘methene’ can not exist as alkenes have double bonds.

Question 8

Describe the addition reaction of alkenes with bromine, including the decolourising of bromine water as a test for alkenes.

Answer 8

Alkenes undergo addition reactions with halogens. For example, a bromine molecule will add across the double bond of ethene to form 1,2- dibromoethane.

Since all alkenes contain a carbon-carbon double bond, they will all decolourise bromine.

This reaction, therefore can be used to test for the presence of a carbon-carbon double bond.

Saturated compounds will not immediately decolourise bromine.