recap of alkanes

Question 1

properties of alkanes

Answer 1

Hydrocarbon
Saturated (single c-c bonds)
All atoms bonded via sigma bonds.
Shape around each carbon is tetrahedral (109.5).

Question 2

BP of straight chain alkanes

Answer 2

BP increases as C chain gets longer.

This is because induced dipole dipole (london) forces get stronger as number of electrons in molecule increases so more energy is needed to overcome intermolecular forces.

Question 3

BP of branched isomers (same no. electrons)

Answer 3

BPs decrease as degree of branches increases.

This is because London forces get weaker. Branching reduces surface contact between molecules.

Question 4

Reactivity of alkanes

Answer 4

alkanes are usually unreactive due to:

• high bond enthalpies in molecules.
• very low polarity in the sigma bonds. C + H have very high electronegativities.

Question 5

Combustion of alkanes

Answer 5

Alkanes combust completely with a lot of O2 supply. (O2 = CO2 + H2O).

Alkanes can be used as fuels.

Alkanes combust incompletely in a limited supply of O2. (O2 = CO/O + H2O)