Ch. 12.1 : Properties of Alkanes

Question 1

Describe the structure and bonding in alkanes

Answer 1

They are saturated hydrocarbons : contain only carbon and hydrogen atoms joined together by single covalent bonds.

Each carbon atom is joined to four other atoms by a type of covalent bond called a sigma bond. Either C-C or C-H.

Question 2

What is a sigma bond?

Answer 2

The result of the overlap of two orbitals, one from each bonding atom.

Each overlapping orbital has one electron, so the sigma bond has two electrons that are shared between the two bonding atoms.

Question 3

Explain the bond angles in alkanes. What shape do they form?

Answer 3

• each carbon atom is surrounded by four electron pairs in four sigma bonds.
• repulsion between the electron pairs results in a 3D tetrahedral arrangement around each carbon atom.
• each bond angle is 109.5°

Question 4

How does the sigma bond affect the shape of alkanes?

Answer 4

• the bond acts as axes around which atoms can freely rotate
• so the shapes aren’t rigid
• eg butane can be a zig zag, but also a u shape

Question 5

What are London forces?

Answer 5

• weak intermolecular forces
• hold molecules together in solids and liquids
• once broken, molecules move apart -> alkane turns into a gas

Question 6

How does chain length affect boiling point?

Answer 6

• as chain length increases
• surface area increases
• amount of contact points increases between molecules
• amount of London forces increase
• amount of energy required to overcome these forces increases
• boiling point increases

Question 7

How does branching affect boiling point?

Answer 7

• as number of branches increase
• surface area + contact points decreases
• amount of London forces decreases
• amount of energy needed to overcome forces decreases
• boiling point decreases

Also, the branches get in the way and prevent molecules getting close together, decreasing IMF further.