12 - Alkanes

Question 1

What affects boiling point of alkanes

Answer 1

• chain length
• branching

Question 2

How does chain length effect bp

Answer 2

• LF act between mols which are in close surface contact
• as chain length increases, molecules have a larger surface area, so more surface contact is possible between molecules
• LF between mols is greater and so a larger amount of energy is required to overcome, resulting in larger bp.

Question 3

How else does chain length increase bp

Answer 3

• larger chain length means more atoms
• larger molar mass and more electrons
• we know the more electrons, greater the LF
• becomes stronger and so with more LF and intermolecular, more energy required to overcome resulting in higher bp

Question 4

How are alkanes used in everyday life

Answer 4

• gas and domestic fuel
• petrol
• kerosene
• diesel

Question 5

Why are alkanes unreactive

Answer 5

• C-C and C-H (sigma bonds) are very strong
• C-C are non polar
• the electro negativity are similar so C-H are considered non-polar

Question 6

How does initiation start

Answer 6

The energy for this homolytic bond fission is provided by UV radiation

Question 7

Why is initiation important

Answer 7

• causes radicals to be formed which are highly reactive and able to react

Question 8

What is the first step of propagation known as

Answer 8

Known as abstraction - when the halogen takes the H to form H-Br
E.g Br abstracts H

Question 9

Why does termination take place

Answer 9

When the concentration of reactants lessen (get used up) or no radicals left for reaction to cycle

Question 10

Limitations of radical substitution

Answer 10

• may react differently or with the products (CH3Br or HBr) because they are so high in concentration and accumulated
• this can continue until all hydrogen atoms have been substituted
• substitution can occur in different parts of the chain
• termination can occur between any two radicals

Question 11

Alkanes general formula

Answer 11

CnH2n+1

Question 12

What is a sigma bond

Answer 12

Overlap of two orbitals of each C atom directly between the bonding atoms

Question 13

Shape of alkanes

Answer 13

• bond angle of 109.5
• sigma bonds acts as an axis around which the atoms can rotate and so are not rigid

Question 14

How does branching affect mp/bp

Answer 14

• more branching reduces the points of surface contact between molecules of the branched alkanes giving less Lf.
• branches get in the way and prevent getting close to gas a straight chain mol.
• reduces sa and so less LF