4.1.2 Alkanes

Question 1

General formula of alkanes.

Answer 1

CnH2n+2

Question 2

What are alkanes?

Answer 2

Saturated hydrocarbons.

Question 3

Describe the bonding in alkanes.

Answer 3

Single covalent bonds, sigma bonds.
Overlap of orbitals form each bonding atom forms a sigma bond with 2 electrons in it.

Question 4

Describe the arrangement and bond angles of alkanes.

Answer 4

Tetrahedral shape around each carbon atom due to 4 sigma bonds.
Each bond angle is approximately 109.5

Question 5

Effect of chain length on boiling point.

Answer 5

Boiling points increase as chain length increases.
As chain length increases, larger surface area of molecules so more surface contact. Therefore greater London forces between molecules so more energy needed to overcome.

Question 6

Effectof branching on boiling point.

Answer 6

Boiling points decrease as branching increases.
As branching increases, fewer points of contact between molecules. Therefore fewer London forces so less energy needed to overcome.
Also shape of branched molecules means they can’t get as close together as straight chain, decreases IMF.

Question 7

Why do alkanes have low reactivity with most common reagents?

Answer 7

C-C and C-H sigma bonds are strong.
C-C bonds are non-polar.
C-H bonds considered non-polar (minimal electronegativity difference).

Question 8

Complete combustion of alkanes.

Answer 8

Burn completely in plentiful supply of oxygen to produce CO2 and H2O.

Question 9

Why are alkanes used as fuels?

Answer 9

They are readily available, easy to transport and burn in plentiful supply of O2 without releasing toxic products.

Question 10

Incomplete combustion of alkanes.

Answer 10

When O2 is limited, H2O is produced but combustion of carbon may be incomplete. Forms CO or C (soot). CO is a toxic gas.

Question 11

Conditions for reaction of alkanes with halogens.

Answer 11

UV radiation.

Question 12

What mechanism is the bromination of methane?

Answer 12

Radical substitution.

Question 13

Initiation stage of the bromination of methane.

Answer 13

Covalent bond in a bromine molecule is broken by homolytic fission.
Two highly reactive bromine radicals formed.
Br-Br -> Br. + Br.

Question 14

Propagation stage of the bromination of methane.

Answer 14

Chain reaction:
Step 1: CH4 + Br. -> .CH3 + HBr
Step 2: .CH3 + Br2 -> CH3Br + Br.

Question 15

Termination stage of the bromination of methane.

Answer 15

Br. + .Br -> Br2
.CH3 + .CH3 -> C2H6
.CH3 + .Br -> CH3Br

Question 16

Limitations of radical substitution.

Answer 16

Further substitution: can occur until all hydrogen atoms have been substituted, therefore mixture of products produced.
Substitution at different points in carbon chain: if carbon chain is longer, mixture of isomers will be produced.