Chapter 12 - alkanes

Question 1

what is an alkane

Answer 1

an alkane is a type of saturated hydrocarbon with the general formula CnH2n+2

Question 2

what sort of bonds are formed within alkanes

Answer 2

alkanes form sigma bonds

sigma bonds are caused by the overlap of orbitals, (usually a p orbital of a carbon with an s orbital of a hydrogen)

Question 3

what property do sigma bonds have

Answer 3

free rotation around the bond (unlike double bonds)

Question 4

shapes of alkanes

Answer 4

the alkanes have a shape of 109.5 degrees around the carbon because there are four bonds so they form a tetrahedral shape

Question 5

how does the boiling point of alkanes change with chain length and why

Answer 5

the boiling point of alkanes increases with chain length because Mr increases and surface contact between molecules increases. This increases the strength of the London forces between molecules thus meaning more energy is required to separate the molecules.

Question 6

how does the boiling point of alkanes change with branching

Answer 6

Branched alkanes have lower boiling points because the molecules can’t get as close, this reduces surface contact meaning there are lower London forces and less energy is required to overcome the inter-molecular forces.

Question 7

how are these boiling point properties used to help separate crude oil.

Answer 7

fractional distillation is used to separate alkanes of different chain length. There is a temperature gradient in the fractionating column, this means that the longest chain molecules condense and drain off first. The shorter chain molecules then rise for longer before condensing. eventually this separates all the fractions.

Question 8

Are alkanes reactive?

Answer 8

No, the only ways that alkanes react are combustion and with the halogens in the presence of UV light.

Question 9

Why are alkanes so unreactive?

Answer 9

the strong sigma bonds have a high bond enthalpy, along with this there is low bond polarity.

Question 10

formula of complete combustion

Answer 10

CH4 + 2O2 —> CO2 + 2H2O

Question 11

formula of incomplete combustion (CO formed)

Answer 11

CH4 + 3/2 O2 —> CO + 2H2O

Question 12

formula of incomplete combustion (C formed)

Answer 12

CH4 + O2 —> C + 2H2O

Question 13

full formula of methane with bromine

Answer 13

CH4 + Br2 —> CH3Br + HBr

Question 14

what are the three stages of free radical substitution

Answer 14

initiation
propagation
termination

Question 15

what is a radical

Answer 15

a radical is a species with an unpaired electron

Question 16

what occurs during initiation

how many radicals?

Answer 16

in initiation the chain is started by UV light breaking the halogen molecule into two radicals through homolytic fission
e.g. Cl2 —> 2Cl*
0 radicals –> 2 radicals

Question 17

what occurs during propagation

and how many radicals

Answer 17

the reactions which keep the chain going, they always come in pairs
e.g.
CH4 + Cl* —> CH3* + HCl
CH3* + Cl2 —> CH3Cl + Cl*
this Cl* can then go back into the top reaction

1 radical —> 1 radical

Question 18

what occurs during termination

Answer 18

these are reactions where radicals are removed from the system
2 radicals ---> 0 radicals
e.g.
2Cl* ---> Cl2
CH3* + Cl* ---> CH3Cl

Question 19

variants of substitution

Answer 19

substitution can occur at any point on the carbon chain

substitution can also occur multiple times to give molecules such as CH2Cl2

Question 20

why are branched chain alkanes considered more useful than straight chain alkanes

Answer 20

their combustion is more efficient