12.1 -12.2

Question 1

What are alkanes

Answer 1

• saturated hydrocarbons

Question 2

What does a compound being saturated mean

Answer 2

• all the carbon bonds are bonded 4 times

Question 3

What is the general formula of alkanes

Answer 3

• CnH2n+2

Question 4

What is the shape and bond angle of alkanes

Answer 4

• tetrahedral (109.5)

Question 5

Why do alkanes have a specific shape with specific angle

Answer 5

• because the bond repel each other equally

Question 6

If a molecule has no branching what does this mean

Answer 6

• molecules can pack together closely leading to more surface contact

Question 7

Why is the boiling point for longer straight chain high

Answer 7

• they have more London forces so more energy is needed to overcome these forces so a high boiling point

Question 8

What is the boiling point for branched hydrocarbons

Answer 8

• they can’t pack closely together which weakens the London forces between the chain so they have a lower boiling point

Question 9

What do alkanes form when they are completely combusted

Answer 9

• CO2 and H2O

Question 10

What are alkanes good as

Answer 10

• fuels as most burn readily to produce large amounts of energy
• used to power vehicles
• used in most of Britain’s electricity

Question 11

Using combustion equations:
15cm^3 of hydrocarbon ‘A’ burns completely in 120cm^3 of oxygen. 75cm^3 of carbon dioxide is produced what is the molecular formula of hydrocarbon A?

Answer 11

FORMULA: C5H12
1) use info to write a volume ratio equation
2) simplify ratio
3) 8 moles of O2 reacts to form 5 moles of CO2 and the ‘n’ moles of H2O. Any oxygen that isn’t in CO2 must be in H2O
4) combustion equation is
5) to identify ‘A’- all carbon in ‘A’ make CO2 and the number of hydrogens in ‘A’ must be 12 (we have 6H2O)

Question 12

What is incomplete combustion

Answer 12

• alkanes burning with a limited oxygen supply

Question 13

What is produced in incomplete combustion

Answer 13

• carbon monoxide(CO) and carbon(soot)

Question 14

What problems does carbon monoxide cause

Answer 14

• poisonous as it bonds to haemoglobin in the blood preventing oxygen from binding

Question 15

What problems does soot cause

Answer 15

• breathing problems

Question 16

What reaction do alkanes undergo

Answer 16

• free radical reactions

Question 17

What type of bond fission happens in free radical substitution reactions

Answer 17

• homolytic fission

Question 18

What are the three steps of free radical substitution reactions

Answer 18

1) Initiation
2) Propagation
3) Termination

Question 19

What is the catalyst in free radical substitution

Answer 19

• UV light

Question 20

What happens in initiation

Answer 20

• radicals are produced by breaking a covalent bond forming two radical using UV light

Question 21

What happens in propagation

Answer 21

• radicals react with non-radicals to form radicals and non radicals

Question 22

What happens in termination

Answer 22

• 2 radicals react and form a non-radical molecules
• ends the chain reaction

Question 23

Write the free radical substitution reaction for this reaction:
CH4 + Cl2 —> CH3Cl + HCl

Answer 23

Initiation:
UV
Cl2——> Cl . + Cl .

Propagation:
1) Cl . + CH4 —-> . CH3 + HCl
2) . CH3 + Cl2 —> CH3Cl + Cl .

Termination:
1) Cl . + Cl . —> Cl2
2) . CH3 + . CH3 —> C2H6
3) . CH3 + . Cl —> CH3Cl

Question 24

What are usually the products of free radical substitution reaction

Answer 24

• normally a mixture of product
• so it has a low percentage yield

Question 25

What is required to get your desired product in free radical substitution

Answer 25

• further substitution