4.1.2 Alkanes

Question 1

What are alkanes and cyclohexanes examples of

Answer 1

Saturated hydrocarbons

Question 2

How are sigma bonds formed in alkanes

Answer 2

One sp3 orbital from each carbon overlap to form a single C-C bond
Sigma bond

Question 3

What can occur around a sigma bond

Answer 3

Free rotation

Question 4

Shape of alkanes

Answer 4

Tetrahedral
Angle: 109.5 degrees
Four bonded pairs repelling equally

Question 5

What effects boiling points of alkanes

Answer 5

Size

Shape (branching)

Question 6

How does size of alkanes effect boiling point

Answer 6

Increases as size increases
Increased number of electrons in larger molecules
Increased size of London forces between molecules
Increased energy to break London forces

Question 7

How does the shape of alkanes effect boiling point

Answer 7

Increased branching decreases the boiling point
Less points of contact as alkanes can’t pack as closely together so decreased London forces
Decreased boiling point to break London forces

Question 8

Reactivity of alkanes

Answer 8

Low reactivity
High bond enthalpies of C-C and C-H bonds
Low polarity of sigma bonds

Question 9

Definition of fuel

Answer 9

Releases heat energy when burnt

Question 10

How are alkanes useful as fuels

Answer 10

Readily burn in presence of oxygen
Highly exothermic
Explains use as fuels

Question 11

Complete combustion of alkanes

Answer 11

In excess oxygen will completely combust to form carbon dioxide and water

Question 12

Incomplete combustion of alkanes

Answer 12

In limited oxygen
Produces toxic carbon monoxide
Produces carbon which is soot

Question 13

What can production of carbon in incomplete combustion contribute to

Answer 13

Global dimming

Reflection of the suns light

Question 14

Which type of combustion releases more energy per mole

Answer 14

Complete combustion

Question 15

What can formation of carbon monoxide cause

Answer 15

Death if built up in enclosed environment

Bonds to haemoglobin so prevents oxygen binding

Question 16

Definition of cracking

Answer 16

Conversion of large hydrocarbons into smaller molecules by breaking C-C bonds
High mr alkanes -> smaller mr alkanes + alkenes + hydrogen

Question 17

Economic reasons for catalytic cracking

Answer 17

Petroleum fractions with shorter carbon chains are in greater demand
Make use of larger hydrocarbons and supply demand for shorter ones
Products are more valuable than the starting materials

Question 18

Conditions of catalytic cracking

Answer 18

Slight pressure
High temperature
Zeolite catalyst

Question 19

Catalytic cracking

Answer 19

Turns straight chain alkanes into branched and cyclic alkanes and aromatic hydrocarbons

Because they burn more cleanly and give fuels a higher octane number

Question 20

Why does cracking require high temperatures

Answer 20

Involves splitting of strong covalent bonds so needs high temperatures

Question 21

Synthesis of haloalkanes from alkanes

Answer 21

Uses mechanism of free radical substitution

Presence of UV light

Question 22

Three steps of free radical substitution

Answer 22

Initiation
Propagation
Termination

Question 23

Condition of free radical substitution

Answer 23

UV light

Question 24

Why is UV light required in free radical substitution

Answer 24

Supplies energy To break the halogen bond

Question 25

Free radical substitution (using methane and chlorine)

Answer 25

Initiation (using UV light): Cl2 -> 2Cl ⏺
Propagation: CH4 + Cl⏺ -> HCl + CH3⏺
⏺CH3 + Cl2 -> CH3Cl + Cl⏺
Termination: ⏺CH3 + Cl⏺ -> CH3Cl
⏺CH3 + ⏺CH3 -> CH3CH3
(Also regenerates Cl from radicals)

Question 26

What do propagation steps include

Answer 26

A free radical in the reactants and the products

Question 27

Further substitution

Answer 27

Excess Cl2 could produce products such as:
CH3Cl2
CHCl3
CCl4