12.2 Chemical reactions of alkanes

Question 1

Explain alkanes lack of reactivity

Answer 1

-C-C and C-H σ bonds are stronger
-C-C bonds are non polar, the charge of the electrons is evenly spread out so no δ charges are formed
-The electronegativity of carbon and hydrogen is so similar, so the C-H bond is considered non-polar

Question 2

Why are alkanes used as fuels?

Answer 2

Give out heat, readily available, easy to transport, burn in plentiful supply of oxygen without releasing toxic products

Question 3

What conditions are required for alkanes to undergo complete combustion/

Answer 3

Plentiful supply of oxygen

Question 4

What are the products of complete combustion?

Answer 4

CO2 and H2O

Question 5

What is the equation for the complete combustion of methane?

Answer 5

Question 6

What is the equation for the complete combustion of hexane (in petrol)?

Answer 6

Question 7

What is the algebraic equation to balance the equation for complete combustion of alkanes?

Answer 7

Question 8

When does incomplete combustion of alkanes occur?

Answer 8

When there is a limited supply of oxygen, they will burn incompletely. They hydrogen will oxidise to form water but the combustion of C will remain incomplete

Question 9

What are the products of incomplete combustion of alkanes?

Answer 9

carbon monoxide CO
OR
soot C

Question 10

Why is carbon monoxide (CO) harmful?

Answer 10

Toxic, binds to haemoglobin in the blood and prevents oxygen from binding

Question 11

Why is soot (C) harmful?

Answer 11

Causes breathing problems or smog

Question 12

What is the equation for complete and incomplete combustion of heptane forming CO2, CO and C?

Answer 12

Question 13

When is incomplete combustion more likely to occurred?

Answer 13

In closed, unventilated paces such as car engines, faulty heating systems, inadequate ventilation in living area

Question 14

What are the 3 stages of radical substitution?

Answer 14

Initiation, Propagation, Termination

Question 15

Define radical

Answer 15

A species with an unpaired electron

Question 16

What conditions are required for the reaction of alkanes with haloalkanes?

Answer 16

high-every ultraviolet light (UV), this produces the initial energy

Question 17

What is the equation for the reaction of methane with bromine?

Answer 17

Question 18

What type of reaction is the reaction of alkanes with halogens?

Answer 18

Substitution reaction

Question 19

What is the mechanism for bromination of alkanes?

Answer 19

Free radical substitution

Question 20

Explain the initiation step for bromination or alkanes?

Answer 20

Covalent bond of a bromine molecule is broken by UV radiation by homolytic fission, each bromine atom takes one electron from the pair to form two highly reactive bromine radicals

Question 21

What is the equation for the initiation step for bromination of methane?

Answer 21

Question 22

What are the two propagation steps for the bromination of methane?

Answer 22

Question 23

What is a chain reaction?

Answer 23

A reaction in which the propagation steps release new radicals that continue the reaction

Question 24

What does the dot show in radical substitution?

Answer 24

An unpaired electron on a radical

Question 25

When does the propagation step stop?

Answer 25

In theory, could continue until all the reactants have been used up. Terminated when two radicals collide

Question 26

What happens in the termination stage?

Answer 26

Two radicals collide and react to from a molecule, both radicals are removed

Question 27

What are the possible termination steps for bromination of methane?

Answer 27

Question 28

What is the overall mechanism for the chlorination of methane?

Answer 28

Question 29

What are the limitations of radical substitution in organic synthesis?

Answer 29

-Further substitution -With a longer carbon chain, we will get a mixture of structural isomers by substitution at different positions on a carbon chain

Question 30

What does a curly arrow show?

Answer 30

The movement of a pair of electrons

Question 31

Why are straight chain molecules converted into branched isomers for the petroleum industry?

Answer 31

Branched chains have more efficient combustion, more efficient fuel

Question 32

Why is H20 always produced during combustion even in incomplete combustion?

Answer 32

Hydrogen atoms in alkane are always oxidised to water

Question 34

30cm3 of hydrocarbon X combusts completely with 240c3 of oxygen. 150cm3 of carbon dioxide is produced. What is the molecular formula of hydrocarbon X?

Answer 34

C5H12