Alkanes

Question 1

what are alkanes?

Answer 1

alkanes are hydrocarbons that fit the general formula CnH2n+2

Question 2

what are alkanes made up of?

Answer 2

they are made of simple molecules containing C and H atoms held together by covalent bonds

Question 3

what does the name of the alkane show?

Answer 3

• the start of the name shows how many carbon atoms it has
• the end of the name is ‘-ane’ to show its an alkane

Question 4

what is the formula of methane?

Answer 4

CH4

Question 5

how many carbon atoms does methane have?

Answer 5

1

Question 6

what is the formula of ethane?

Answer 6

C2H6

Question 7

how many carbon atoms does ethane have?

Answer 7

2

Question 8

what is the formula of propane?

Answer 8

C3H8

Question 9

how many carbon atoms has propane got?

Answer 9

3

Question 10

what is the formula of butane?

Answer 10

C4H10

Question 11

how many carbon atoms does butane have?

Answer 11

4

Question 12

what is the formula of pentane?

Answer 12

C5H12

Question 13

how many carbon atoms does pentane have?

Answer 13

5

Question 14

what are the names of alkanes after the first five?

Answer 14

the names follow the 2D shapes: hexane, heptane, octane etc.

Question 15

what are the four different formulas of alkanes?

Answer 15

• displayed formula
• structural formula
• molecular formula
• empirical formula

Question 16

what is the displayed formula of an alkane?

Answer 16

a diagram showing the positions of every atom and every bond (C and H’s)

Question 17

what is the structural formula of an alkane?

Answer 17

this shows the structure of the molecule in one line, read from left to right like a word
eg. CH3CH2CH2CH3

Question 18

what is the molecular formula of an alkane?

Answer 18

this only shows how many carbon and hydrogen atoms there are
e.g. C4H10

Question 19

what is the empirical formula of an alkane?

Answer 19

this only shows the simplest whole-number ratio of carbon to hydrogen
e.g. C2H5

Question 20

what is a homologous series?

Answer 20

alkanes are a homologous series of organic molecules - this means they are a family of molecules

Question 21

what do alkanes all have?

Answer 21

• similar chemical reactions
• trends in physical properties (e.g. boiling points, viscosity or colour)
• the same general formula

Question 22

how many series of homologous are there in the IGSCE course?

Answer 22

five:
- alkenes
- alcohols
- carboxylic acids
- esters
- alkanes

Question 23

what are isomers?

Answer 23

isomers are molecules with the same molecular formula but different structural formulae

Question 24

how many isomers does C4H10 have?

Answer 24

two

Question 25

how many isomers does C5H12?

Answer 25

three

Question 26

what makes isomers different?

Answer 26

the branching in the carbon chain - you can’t make a different isomer just by bending the molecule

Question 27

why are most alkanes used as fuels?

Answer 27

they release a lot of energy when combusted

Question 28

what are the two types of combustion that alkanes can undergo, depending on how much oxygen is available?

Answer 28

1. complete combustion
2. incomplete combustion

Question 29

what is complete combustion of alkanes?

Answer 29

• this occurs when there is excess oxygen
• it produces only CO2 and H2O:
  alkane + oxygen → carbon dioxide + water

Question 30

what is the incomplete combustion of alkanes?

Answer 30

• this occurs when there isn’t enough oxygen
• instead of CO2 this produces a nasty mixture of:
• carbon monoxide (CO) - a toxic gas which lowers oxygen-carrying capacity of blood
• soot (C) - which causes lung diseases
• it also releases less energy so it makes the alkane less useful as a fuel

Question 31

what are the three significant pollutants of the combustion of alkanes into the atmosphere?

Answer 31

• carbon dioxide (CO2)
• nitrogen oxides (NOx)
• sulphur dioxide (SO2)

Question 32

how does alkane release carbon dioxide (CO2) into the atmosphere?

Answer 32

• this forms directly from the combustion of the alkane fuel
• it is a greenhouse gas that traps heat in the atmosphere and causes climate change

Question 33

how does alkane release nitrogen oxides (NOx) into the atmosphere?

Answer 33

• this family of compounds form when nitrogen from the air combusts
• usually nitrogen doesn’t combust but car engines are really, really hot
• they dissolve in rainwater to form acid rain, which corrodes structures and is harmful to plant and aquatic life

Question 34

how does alkane release sulphur dioxide (SO2) into the atmosphere?

Answer 34

• this forms when sulphur impurities in the fuel combust
• like nitrous oxides, it also dissolves in rainwater to form acid rain

Question 35

how do alkanes react with halogens?

Answer 35

alkanes react with halogens by swapping one hydrogen atom for one halogen atom
e.g. for example, when ethane (C2H6) reacts with bromine (Br2):
C2H6 + Br2 → C2H5Br + HBr

Question 36

does alkane reactions with halogens work all the time?

Answer 36

• these reactions would work with any alkane (e.g. pentane) any halogen (e.g. chlorine)
• all that happens is a hydrogen swaps with a halogen

Question 37

what does a halogen and alkane reaction require?

Answer 37

this reaction requires UV light - because the alkane is pretty unreactive

Question 38

what type of reaction is a halogen and alkane reaction?

Answer 38

its a substitution reaction - because the H swaps with the Br