4.1.2 Alkanes

Question 1

What’s an example of a mixture of hydrocarbons, that the majority of are alkanes

Answer 1

Crude oil

Question 2

What are alkanes

Answer 2

Saturated hydrocarbons with no free spaces

Question 3

What’s the general formula of alkanes

Answer 3

CnH2n+2

Question 4

How can crude oil be separated into different fractions

Answer 4

Fractional distillation

Question 5

How reactive are alkanes

Answer 5

Very unreactive due to very strong bonding between carbons (sigma bond)

Question 6

3 reasons alkanes make good fuels

Answer 6

• release large amount of energy when burnt (exothermic)
• readily available at reasonable cost
• convenient to transport + store
• generally no toxic products

Question 7

2 reasons alkanes are problematic as fuels

Answer 7

• fossil fuels are being consumed faster than replaced (non-renewable)
• can produce CO2, CO + C pollutants

Question 8

A large quantity of what is produced when an alkane is completely combusted
Why is this bad

Answer 8

CO2

Is a greenhouse gas that contributes to global warming

Question 9

What’s completely combusted

Answer 9

When plenty of oxygen is present to convert all hydrogen to H2O and all carbon to CO2

Question 10

What situation may an alkane be incompletely combusted

Answer 10

If an engine is old/poorly maintained so oxygen is limited and less able to reach each alkane molecule

Question 11

What’s produced in incomplete combustion

Answer 11

Carbon monoxide and carbon

Question 12

Problem with carbon monoxide

Answer 12

Toxic - silent killer

Question 13

Problem with carbon particles (soot)

Answer 13

Harmful by inhalation and can cause cancer, global dimming/asthma

Question 14

What molecules are more susceptible to incomplete combustion

What flame do they burn with

Answer 14

Larger

Sooty flame

Question 15

What happens to boiling point of alkanes as chain length increases
Why

Answer 15

Increases

Longer the chain more electrons + SA, more strength/number of IMFs, more IDDI, more surface contact

Question 16

What happens to boiling point if there are branches

Answer 16

More branches
Less surface contact
Fewer IDDIs
Lower BP

Question 17

Whats formed when an alkane reacts with a halogen

Answer 17

A halogenoalkane (haloalkane)

Question 18

What does a chemical mechanism show

Answer 18

The movement of electrons during a reaction

Question 19

2 ways electrons move in a mechanism

What’s the way we’re looking at

Answer 19

Singly/in pairs

We’re focusing on electrons moving singly

Question 20

What mechanism is happening if electrons are moving singly

Answer 20

Free radical mechanism

Question 21

What’s a free radical

Answer 21

An atom/molecule/ion with unpaired valence electrons

Question 22

When do free radicals form

How

Answer 22

In homolytic fission

2 electrons in a covalent bond are returned to the atoms from which they originated during covalent bond formation

Question 23

What’s formed if a Cl2 molecule is split homylitically

What’s their reactivity like and why

Answer 23

2 chlorine radicals

Very reactive as no longer have full outer shells and will react with many species to regain a full outer shell

Question 24

What’s homolytic fission the reverse of

Answer 24

Covalent bond formation

Question 25

What does a fishhook arrow represent

Answer 25

Movement of only 1 electron (singly)

Question 26

What’s the mechanism for the reaction of an alkane + halogen

Answer 26

The free radical substitution mechanism

Question 27

What does the free radical substitution mechanism involve

Answer 27

Free radicals

Swapping of a H atom in Alkane by a halogen atom

Question 28

What are the 3 steps to this mechanism

Answer 28

Initiation
Propagation
Termination

Question 29

What is used in the initiation to split the molecule to make free radicals
What does this provide

Answer 29

UV light

Provides energy to break bonds

Question 30

What happens during propagation

Answer 30

Free radicals used up
Products are formed
More free radicals are regenerated

Question 31

What’s dangerous about the propagation steps

Answer 31

Explosive

Question 32

What steps of this mechanism are repeated explosively

Answer 32

2x Propagation steps

Question 33

What happens in termination

Answer 33

2 free radicals colliding will cancel each other out and form a stable molecule

Question 34

What is the result of adding up the 2 propagation Equations

Answer 34

The overall equation

Question 35

Why can side products be made from this mechanism (further substitution)

Answer 35

As free radicals are so reactive

Question 36

Why is free radical substitution not a reliable way of making haloalkanes
What’s the general way of making haloalkanes

Answer 36

Due to the many side products

By alkenes