Homologous series

Question 1

How many bonds can carbon form?

Answer 1

Carbon can only ever form 4 bonds (valency 4). It will not form more or less bonds than this as it would not obtain a complete electron energy level.

Question 2

How many bonds can hydrogen form?

Answer 2

Hydrogen can only form 1 bond (valency 1). It will not form more or less bonds than this as it would not obtain a complete electron energy level.

Question 3

What is a hydrocarbon?

Answer 3

A hydrocarbon is a compound made from hydrogen and carbon only.

Question 4

Describe what is meant by a saturated compound.

Answer 4

The alkanes are a homologous series of hydrocarbons that have single covalent bonds only between their carbon atoms.

These are saturated hydrocarbons and the name of all the members of the alkane homologous series ends in -ane. e.g. butane.

Question 5

Describe what is meant by a unsaturated compound.

Answer 5

The alkenes are a homologous series of hydrocarbons that contain at least one carbon-carbon double bond.

Alkenes are unsaturated hydrocarbons and the name of all the members of the alkene homologous series ends in -ene.

Question 6

What does the first part of the name of a hydrocarbon tell us?

Answer 6

The first part of the name of hydrocarbon tells you the number of carbon atoms joined together in a straight chain.
meth:1
eth:2
prop;3
but:4
pent:5
hex:6
hept:7
Oct:8

Question 7

What are the homologous series?

Answer 7

A homologous series is a family of compounds with similar chemical properties who share the same general formula.
3 different homologous series of hydrocarbons include:
Alkanes
Alkenes
Cycloalkanes

Question 8

What are the three ways to write or draw the formula of a hydrocarbon?

Answer 8

1. Full structural formula
2. Shortened structural formula
3. Molecular formula

Question 9

What is shown in a full structural formula?

Answer 9

This shows all the atoms and all the bonds in a molecule.

Question 10

What is shown in a shortened structural formula?

Answer 10

This shows the number of hydrogen atoms attached to each carbon atom (in order). The bonds between the carbon atoms may or may not be shown.

Question 11

What is shown in a molecular formula?

Answer 11

This shows the type and total number of each atom only.

Question 12

What is the general formula for alkanes?

Answer 12

CnH2n+2 (where n = number of carbon atoms)

Question 13

What are the uses of alkanes?

Answer 13

Alkanes are:

• Used as fuels e.g. methane (natural gas) , propane and butane (bottled gas), petrol, kerosine (aircraft fuel) and diesel.
• Used as chemicals such as solvents, lubricants and waxes.
• Converted into alkenes by a process known as catalytic cracking. Alkenes are important chemicals in industry as they are used to manufacture many other chemicals and are referred to as feedstock chemicals.

Question 14

Are alkanes soluble or insoluble in water?

Answer 14

Alkanes are insoluble in water.

Question 15

What is the first member of the alkene family called?

Answer 15

The first member of the alkene family is ethene as there must be a minimum 2 carbon atoms present to have a double bond present.

Question 16

What is the general formula for alkenes?

Answer 16

CnH2n (where n = number of carbon atoms)

Question 17

What are the uses of alkenes?

Answer 17

Alkenes are very useful feedstock chemicals that are used in the making of:

• plastics (polymers)
• ethanol (alcohol) and ethanoic acid (vinegar)
• pesticides
• medicine
• toiletries and cosmetics etc

Question 18

Are alkenes soluble or insoluble in water?

Answer 18

Alkenes are insoluble in water.

Question 19

Describe cycloalkanes.

Answer 19

Cycloalkanes are a homologous series of saturated, cyclic (C atoms joined in rings) hydrocarbons.

The name of all the members of the cycloalkane homologous series starts with cyclo- and ends in -ane.

Question 20

What is the first member of the cycloalkane family?

Answer 20

The first member of the cycloalkane family is cyclopropane as a minimum of 3 carbon atoms are required to form a ring.

Question 21

What is the general formula for cycloalkanes?

Answer 21

CnH2n (where n=number of carbon atoms)

Question 22

What is an isomer?

Answer 22

Compounds with the same molecular formula but different structural formula are called isomers.

Question 23

Which two homologous series are isomers of each other?

Answer 23

Alkenes and cycloalkanes with the same number of carbon atoms are isomers of each other.

Question 24

What is a functional group?

Answer 24

A functional group is an atom, group of atoms or types of bonds within a molecule that gives the compound its characteristic chemical properties.
e.g.
The functional group of an alkene is the double carbon to carbon covalent bond.

Question 25

What are the rules for naming branched alkanes?

Answer 25

1. Identify and name the longest chain. (This will form the main part of the name.)
   2.Identify the branche(s) and number the carbon atoms in the longest chain, starting from the end nearest to the branch.
   3.Name and number the branche(s). (each branch will have to be named and numbered)
2. Seperate numbers by commas and letters and numbers by a hyphen.
   use prefixes di-,tri-,etc when a branch appears more than once.

Question 26

What are the rules for naming alkenes and branch chained alkenes?

(note these rules apply to alkenes and molecules containing other functional groups (such as alcohols)

Answer 26

1. The position of the double bond must be indicated by a number. If the double bond can appear in more than one different place. The carbon nearest the functional group is numbered the first carbon atom.
2. If a functional group, such as a double bond, and a branch is present, the longest chain is numbered from the end nearest the functional group and the branch(es) allocated accordingly.

Question 27

What is meant by viscosity?

Answer 27

A measure of how thick or runny a substance is. A substance that is very viscous is thick like syrup, a substance that is not very viscous is runny like water.

Question 28

What is meant by flammability?

Answer 28

A measure of how easily a substance will ignite. (high flammability means the substance will ignite easily.)

Question 29

What is meant by volatility?

Answer 29

A measure of how easily a substance will evaporate. (Low volatility means it will not evaporate easily)

Question 30

Which physical properties show a trend as you go down a homologous series i.e as the carbon chain increases?

Answer 30

The following physical properties all show a trend as you go down a homologous series i.e. as the carbon chain length increases:
boiling point
volatility
flammability
viscosity
colour (becomes darker as you descend a homologous series)

Question 31

Describe the trend in boiling point as you go down a homologous series.

Answer 31

The boiling point increases as you go down a homologous series.

Question 32

Why does the boiling point increase with molecular size?

Answer 32

The boiling point increases as you go down a homologus series because as the size of a molecule increases there are more attractive forces between molecules. These attractive forces are weak and are called intermolecular forces or Van der Waals forces of attraction. The intermolecular forces are greater, the bigger the molecule, requiring more energy to separate larger molecules, thus the boiling point increases with molecular size.

Question 33

Describe the trend in volatality in a homologus series.

Answer 33

Volatility decreases as you go down a homologous series. This is because as molecules get larger there are more intermolecular forces that need to break in order to evaporate, requiring more energy.

Question 34

Describe the trend in flammability in a homologous series.

Answer 34

Flammability decreases as you go down a homologous series. This is because molecules have to evaporate and mix with oxygen before they can burn. We have already seen that larger molecules require more energy to evaporate due to stronger intermolecular forces.

Question 35

Describe the trend in viscosity in a homologous series.

Answer 35

Viscosity increases down a homologous series as the longer chains become entangled making it harder for the molecule to move past each other and so they so do not flow so freely as smaller molecules.

Question 36

What are the two important reactions with hydrocarbons?

Answer 36

Two important reactions that alkanes, cycloalkanes and alkenes do are combustion and reacting with bromine.

Question 37

Describe combustion of hydrocarbons.

Answer 37

All hydrocarbons undergo complete combustion to produce carbon dioxide and water.

Question 38

Describe incomplete combustion.

Answer 38

Incomplete combustion, takes place in a limited air or oxygen supply. The products of incomplete combustion include carbon monoxide and/or unburnt carbon (soot).

Question 39

Which hydrocarbons are more reactive?

Answer 39

Saturated compounds such as the alkanes are quite unreactive as the carbon atoms are bonded to the maximium number of atoms allowed by the valency and one or more of these atoms would have to be removed or substituted to allow reaction with another substance. Unsaturated compounds such as alkenes are more reactive as the carbon atoms can bond to other atoms across the double bond.

Question 40

What is the addition reaction of bromine with alkenes used for?

Answer 40

The addition reaction of bromine with alkenes is used to distinguish between saturated and unsaturated compounds such as alkanes and alkenes. Shake the compound with bromine solution, if the bromine solution decolourises rapidly (goes from orange to colourless), the compound is unsaturated but if the orange colour remains the compound is saturated.

Question 41

What can alkenes undergo addition reactions with?

Answer 41

Halogens (group 7 elements)
Hydrogen
Hydrogen halides
Water

Question 42

When naming carbon compounds containing halogens what is the functional group and how does the name change?

Answer 42

When naming carbon compounds containing halogens the halogen becomes the functional group and the name ending changes as follows:
Fluorine becomes fluoro
Chlorine becomes chloro
Bromine becomes bromo
Iodine becomes iodo

Question 43

Describe the addition reaction of a halogen (halogenation) with an alkene.

Answer 43

Aswell as bromine, other halogens (group 7 elements) can add across the double to produce a halogenoalkane.

Question 44

Describe the addition of hydrogen (hydrogenatation) with an alkene.

Answer 44

The addition of hydrogen to an alkene produces the corresponding alkane.

Question 45

Describe the addition of a hydrogen halide.

Answer 45

A hydrogen halide is a compound of hydrogen with any halogen e.g. HBr. They add across the double bond to form halogenoalkanes. The hydrogen halide can give two products (isomers) as it can add across the double bond in two different ways if the double bond is not in the middle of the alkene molecule.

Question 46

Describe the addition of water (hydration) with an alkene.

Answer 46

When water is added to an alkene (e.g. ethene) an alcohol is formed. This reaction is carried out using a catalyst and water (in the form of steam).