4.1.2 Alkanes

Question 1

What is an alkane?

Answer 1

A homologous series of organic compounds with the same functional group but with each successive member differing by CH2
(Saturated hydrocarbon)

Question 2

Free rotation around single bonds

Answer 2

There is free rotation around all of the single bonds
These are called σ bonds (sigma)

Question 3

How are sigma bonds formed (basic)?

Answer 3

To form a sigma bond there is a overlap of orbitals directly between the bonding atoms

Question 4

What is the shape around each carbon atom?

Answer 4

Tetrahedral shape
109.5°
Electron pairs repel and arrange themselves as far apart as possible

Question 5

Draw a diagram to show the shape of butane (tetrahedral)

Answer 5

See notes

Question 6

Explain boiling points in straight chain alkanes

Answer 6

• in straight chain alkanes boiling point increases as chain length increases
• as chain length increases, number of electrons increases and so there are greater London forces and a higher boiling point

Question 7

Explain boiling points in isomeric alkanes

Answer 7

• in isomeric alkanes, boiling point decreases as isomers become more branched
• branched isomers have a smaller area for London forces to act through
• smaller attraction between molecules and boiling point decreases

Question 8

Why are alkanes not very reactive?

Answer 8

• strong covalent bonds between atoms
• high bond enthalpy
• very low polarity of sigma bonds present

Question 9

Why are alkanes used as fuels?

Answer 9

• flammable and used as fuels

Question 10

When do complete and incomplete combustion occur?

Answer 10

• complete combustion occurs when fuels are burned in a plentiful supply of air
• products are carbon dioxide and water
• if there is insufficient air, carbon monoxide and/or carbon can be formed