Chapter 12 - Alkanes (MODULE 4)

Question 1

What are alkanes

Answer 1

saturated hydrocarbons

Question 2

what property of alkanes allows crude oil deposits to remain in the Earth for millions of years

Answer 2

they are usually very stable

Question 3

What is the general formula of aliphatic alkanes

Answer 3

C(n) H (2n+2)

Question 4

What are the names of the first ten alkanes

Answer 4

methane,
ethane
propane
butane
pentane
hexane
heptane
octane
nonane
decane

Question 5

What type of covalent bond is present in alkanes
- How many of these does each carbon atom have in an alkane

Answer 5

sigma bonds
- 4

Question 6

What is a sigma bond
- how many electrons are present

Answer 6

an overlap of 2 orbitals
- 2, one from each bonded atom

Question 7

Where is the electron density in a sigma bond

Answer 7

between the two atoms

Question 8

WHat is the shape and bond angle in an alkane

Answer 8

tetrahedral, 109.5 degrees

Question 9

How do oil refineries separate crude oil

Answer 9

fractional distilation

Question 10

How do boiling points of unbranched alkanes change as chain length increases

Answer 10

boiling point increases

Question 11

What are London forces
- what is the relation between amount of London forces and boiling point for alkanes

Answer 11

weak intermolecular forces due to surface contact between molecules
- more London forces correlates to a higher boiling point

Question 12

why does increasing chain length affect boiling point (3)

Answer 12

greater chain length -> greater surface contact
-> more London forces
-> higher boiling point

Question 13

Why does increased branching decrease boiling point (4)

Answer 13

-> more branching -> less surface contact
-> less London forces
-> branching keeps molecules further apart from each other
-> lower boiling point

Question 14

What are the 3 reasons for the lack of reactivity of alkanes

Answer 14

C-C and C-H sigma bonds are strong
- C-C bonds are non-polar
- electronegativity of carbon and hydrogen is so similar that C-H bond is considered non-polar

Question 15

what is the general equation for the combustion of alkanes

Answer 15

alkane + oxygen -> carbon dioxide + water

Question 16

Why are alkanes used as fuels

Answer 16

-> readily available
-> easy to transport
-> burn in a plentiful supply of oxygen without releasing toxic products

Question 17

WHat is the symbol equation for the complete combustion of methane and ethane

Answer 17

methane: CH4 + 2 O2 → CO2 + 2 H2O + energy
ethane: C2H6 + 3.5 O2→ 2CO2 + 3H2O

Question 18

What happens if combustion occurs when supply of oxygen is not plentiful
- what is formed

Answer 18

incomplete combustion
- carbon monoxide, carbon

Question 19

Why is CO harmful

Answer 19

it binds irreversibly with haemoglobin preventing it from transporting oxygen around the body

Question 20

Why is CO hard to detect

Answer 20

colourless and odorless

Question 21

What condition is required for alkanes to react with halogens
- What type of reaction is it

Answer 21

UV
- substitution

Question 22

What is the equation of the reaction between bromine and methane

Answer 22

CH4 + Br2 -> CH3Br + HBr

Question 23

What type of mechanism is the bromination of an alkane

Answer 23

radical substitution

Question 24

What are the three stages of radical substitution

Answer 24

initiation, propagation, termination

Question 25

What happens in the initiation stage of bromination of methane

Answer 25

the Br2 molecule is broken by homolytic fission forming 2 bromine radicals

Question 26

How are radicals represented

Answer 26

with a single dot

Question 27

What happens in the propagation stage of bromination of methane

Answer 27

methane reacts with Br radical forming HBr and methyl radical. methyl radical reacts with Br2 making another Br radical and CH3Br cycle repeats

Question 28

What happens in the termination stage of bromination of methane

Answer 28

one of three things happens stopping the reaction - Br radical + Br radical -> Br2 - CH3 radical + CH3 radical -> C2H6 CH3 radical + Br radical -> CH3Br

Question 29

What is the main issue with using radical substitution to produce organic products

Answer 29

- Further substitution might occur e.g. Br radical may react with CH3Br eventually causing CH2Br2, CHBr3 and/or CBr4 to form

Question 30

What is the problem with radical substitution with alkanes with 3 or more carbons

Answer 30

a mixture of isomers will form as the radical can attach to different carbons causing different products to form