12.Alkanes

Question 1

What are alkanes

Answer 1

Saturated hydrocarbons

Question 2

What are saturated hydrocarbons

Answer 2

A compound containing only H and C with only C-C and C-H bonds, no C=C

Question 3

Describe the physical properties of alkanes: polarity

Answer 3

• non polar

- as electronegativity between C and H are so similar

Question 4

What are the intermolecular forces that act between alkanes

Answer 4

only van der Waals

Question 5

Describe the physical properties of alkanes: boiling points

Answer 5

• as chain length increases
• so does van der Waal forces strength
• as more electrons present
• so boiling point increases

Question 6

Why do alkanes with branched chains have lower melting points than straight chain alkanes with the same number of carbon atoms

Answer 6

• cannot pack together as closely
• less interaction of van der Waal forces
• weaker van der Wall forces
• lower boiling point

Question 7

Describe the physical properties of alkanes: solubility

Answer 7

• insoluble in water
• water is held by much stronger hydrogen bonds than the van der Waals
• lipid soluble

Question 8

How do alkanes react

Answer 8

• strong H-C bonds so relatively unreactive
• react with halogens
• burn in oxygen

Question 9

What is crude oil made of

Answer 9

Branched and unbranched alkanes and impurities

Question 10

How are alkanes in crude oil separated

Answer 10

Fractional distillation in a fractionating tower

Question 11

Describe the process of fractional distillation

Answer 11

• crude oil heated in furnace
• mixture of liquid and vapour passes into a tower that is cooler at the top than at the bottom
• vapour pass up tower via a series of trays containing bubble caps until they arrive at a tray that is sufficiently cool and condense
• shorter chains condense in towers near the top (lower boiling points)
• thick residue collects at base called tar

Question 12

What is cracking

Answer 12

The breaking of C-C bonds in a long hydrocarbon chain to make shorter chains

Question 13

What is useful about cracking

Answer 13

• produces shorter, more useful chains (especially petrol)

- some of the products are alkenes which are more reactive than alkanes

Question 14

What does cracking produces

Answer 14

• shorter alkanes

- high % of alkenes

Question 15

What are the two types of cracking

Answer 15

• thermal

- catalytic

Question 16

Describe thermal cracking conditions

Answer 16

• high temperature (700-1200 K)

- high pressure (7000 kPa).

Question 17

What is produced during thermal cracking

Answer 17

A mixture of alkanes, alkenes and sometimes hydrogen

Question 18

Describe catalytic cracking

Answer 18

• lower temperature (720 K) (still high)
• lower pressure
• zeolite catalyst

Question 19

What is produced during catalytic cracking

Answer 19

• branched alkanes
• cycloalkanes
• aromatic compounds

Question 20

What commercial product is produced by catalytic cracking

Answer 20

Motor fuel

Question 21

Name alkane fuels

Answer 21

• methane
• propane
• butane
• petrol
• paraffin

Question 22

What is incomplete combustion

Answer 22

A combustion reaction in which there is insufficient oxygen for all the carbon in the fuel to burn to carbon dioxide. Carbon monoxide and/or carbon are formed

Question 23

List pollutants hydrocarbon based fuels produce

Answer 23

• CO (poisonous gas)
• nitrogen oxides (produced when there is enough energy for O2 and N2 to react, happens in engines)
• sulfur dioxide (produced from sulfur containing impurities in oil)
• CO2 (climate change)
• unburned hydrocarbons

Question 24

What are contributors to acid rain

Answer 24

NOx and SO2

Question 25

Name two methods of flue gas desulfurisation

Answer 25

- reacting it with calcium oxide to form gypsum | - reacting it with calcium carbonate

Question 26

What is the equation for the reaction of flue gas and calcium oxide

Answer 26

CaO (s) + 2H2O (l) + SO2 (g) + 1/2O2 (g) ----> CaSO4.2H2O (s)

Question 27

What is the equation for the reaction of flue gas and calcium carbonate

Answer 27

CaCO3 (s) + 1/2O2 (g) +SO2 (g) ----> CaSO4 (s) + CO2 (g)

Question 28

What do catalytic converters

Answer 28

Reduce the output of CO, NOx and unburnt hydrocarbons in the exhaust gas mixture

Question 29

Describe structure of a catalytic converter

Answer 29

Honeycomb made of a ceramic material coated with platinum and rhodium metals. Honeycomb shape provides an enormous surface area

Question 30

Write the two reactions that occur in catalytic converters

Answer 30

- 2CO (g) + 2NO (g) ---> N2 (g) + 2CO2 (g) | - hydrocarbons + nitrogen oxide ---> nitrogen + carbon dioxide + water

Question 31

Name the stages of free radical substitution

Answer 31

- initiation - propagation - termination

Question 32

How do Cl-Cl bonds break under UV light

Answer 32

- UV light energy is greater than Cl-Cl bond so it breaks - both atoms are the same so it breaks homiletically - forms two free radicals

Question 33

Write the initiation step of free radical sub

Answer 33

Cl-Cl ----> 2Cl• | ............uv

Question 34

Write the propagation steps

Answer 34

1. Cl• + CH4 ---> HCl + •CH3 | 2. •CH3 + Cl2 ---> CH3Cl + Cl•

Question 35

Write possible termination steps

Answer 35

1. Cl•+ Cl• ----> Cl2 2. •CH3 + •CH3 ---->C2H6 3. Cl• + •CH3 ----> CH3Cl

Question 36

What is the overall reaction of free radical substitution

Answer 36

CH4 (g) + Cl2 ---> CH3Cl (g) + HCl

Question 37

Write how dichloromethane can be made in the propagation stage

Answer 37

1. CH3CL + Cl• ----> •CH2Cl +HCl | 2. •CH2Cl +Cl2 --->CH2Cl2 + Cl•

Question 38

Why are chain reactions important

Answer 38

Chlorine free radicals formed from CFCs are destroying the ozone layer which protects the earth from UV

Question 39

What is the reaction for the breaking down of the ozone layer

Answer 39

Cl• + O3 ---->ClO• + O2 ClO + O3 ----> 2O2 + Cl• 2O3 ---> 3O2