Module 4: Alkanes (Chapter 12)

Question 1

Define sigma bonds

Answer 1

Overlap of orbitals on the line directly between the bonding atoms. This is end on overlap. Free rotation is possible

Question 2

Bond angle in alkanes

Answer 2

109.5 = 4 bonded pairs around each carbon atom meaning electrons repel as far as part as possible. Leads to a tetrahedral arrangement.

Question 3

Reason for high bp with increasing chain length

Answer 3

More points of surface contact due to increased surface area. Means stronger London forces so more energy needed to overcome the intermolecular forces.

Question 4

Reasons for low bp with more branching

Answer 4

Fewer points of surface contact so weaker London forces, so less energy needed to overcome intermolecular forces.

Question 5

Why are alkanes so unreactive?

Answer 5

High bond enthalpy due to sigma bonds, non-polar between C-C and negligible electronegativity between C-H means they are considered non-polar

Question 6

Why does CO kill?

Answer 6

Combines irreversibly with Hb in red blood cells to form carboxyhaemoglobin preventing oxygen being transported around the body. Odourless and colourless so unknown.

Question 7

3 stages of radical substitution

Answer 7

Initiation
Propagation
Termination

Question 8

Reactant needed for initiation

Answer 8

UV radiation

Question 9

How does termination differ from other steps?

Answer 9

Involves 2 radicals colliding to form a stable species, whereby no more free radicals are produced

Question 10

Limitations of radical substitution

Answer 10

Further substitution = until all H atoms removed. Results in a mixture.
Substitution at different points along the carbon chain = results in isomers