3.2 Alkanes

Question 1

what is a petroleum fraction

Answer 1

• a mixture of hydrocarbons with a similar chain length and boiling point range

Question 2

how does fractional distillation work?

Answer 2

• oil is preheated then passed into column
• the fractions condense at different heights
• temp of column decreases upwards
• seperation depends on boiling point
• boiling points depend on size of molecules
• similar molecules condense together
• small molecules condense at the top at lower temps
• big molecules comsense at the bottom at higher temps

Question 3

what is cracking?

Answer 3

• conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds

Question 4

what are the economic reasons for cracking

Answer 4

• petroleum fractions with shorter C chains are more in demand than larger fractions
• to make use of excess larger hydrocarbons and to supply demand for shorter ones
• products of cracking are more valuable than the starting materials

Question 5

what are the conditions for thermal cracking

Answer 5

• high pressure ( 7000kPa )
• high temperature ( 400 to 900 )

Question 6

what are the conditions for catalytic cracking

Answer 6

• slight or moderate pressure
• high temperature ( 450 )
• Zeolite catalyst

Question 7

what are the products of thermal cracking

Answer 7

• produces mostly alkenes
• sometimes produces hydrogen

C8H18 > C6H14 + C2H4

Question 8

what are the products of catalytic cracking

Answer 8

• produces branched and cyclic allanes and aromatic hydrocarbons

Question 9

why is catalytic cracking cheaper than thermal cracking?

Answer 9

• saves energy as lower temperatures and pressures used

Question 10

what are the products for complete combustion of alkanes

Answer 10

• carbon dioxide and water

Question 11

what are the products for incomplete combustion of alkanes

Answer 11

CO and/or C and water

Question 12

whats the impact of carbon soot

Answer 12

• causes global dimming

Question 13

whats the impact of SO2 produced

Answer 13

• dissolves in atmospheric water and can produce acid rain

Question 14

how can SO2 be removed from waste gases from furnaces

Answer 14

• by flue gas desulfurisation
• gases pass through scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide
• SO2 + CaO > CaSO3

Question 15

how do nitrogen oxides form

Answer 15

• from the reaction between N2 and O2 inside the car engine
• the high temp and spark in engine provides enough energy to break strong N2 bond

Question 16

give the equations for production of NO2

Answer 16

N2 + O2 > 2NO

N2 + 2O2 > 2NO2

Question 17

give the environmental consequence for carbon monoxide

Answer 17

• toxic

Question 18

give the environmental consequence for carbon dioxide

Answer 18

• contributes towards global warming

Question 19

give the environmental consequence for unburnt hydrocarbons

Answer 19

• contributes towards formation of smog

Question 20

give the environmental consequence of soot

Answer 20

• causes global dimming and respiratory problems

Question 21

give the environmental consequence of nitrogen oxides

Answer 21

• toxic and can form acidic gas
• NO2 is toxic and acidic and forms acid rain

Question 22

what are the roles of catalytic converters

Answer 22

• they remove CO, NOx and unburned hydrocarbons from exhaust gases, turning them into ‘harmless’ CO2, N2 and H20

Question 23

give the equation for catalytic converters

Answer 23

2 CO + 2 NO > 2CO2 + N2

C8H18 + 25 NO > 8 CO2 + 12.5 N2 + 9H20

Question 24

why do converters have a ceramic honeycomb coated with a thin layer of catalyst metals

Answer 24

• to give a large surface area

Question 25

what are the main greenhouse gases

Answer 25

- water, carbon dioxide and methane

Question 26

what is the mechanism of greenhouse effect

Answer 26

- UV wavelength radiation passes through the atmosphere, and heats up the Earths surface - Earth radiates out infrared long wavelength radiation - the C=O bonds in CO2 absorb infrared radiation so the IR radiation does not escape from the atmosphere - this energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed

Question 27

why have carbon dioxide levels risen significantly in recent years

Answer 27

- due to increasing burning of fossil fuels - carbon dioxide has though to be largely responsible for

Question 28

what happens when alkanes react with chlorine in the presence of UV light?

Answer 28

- mixture of products with halogens substituting hydrogen atoms

Question 29

what happens in initation step?

Answer 29

- condition is UV light - UV light supplies the energy to break the Cl-Cl bond - it is broken in preference to the others because it is the weakest - the bond has broken in a process called homolytic fission

Question 30

what happens in homolytic fission

Answer 30

- a bond broken in this method will form free radicals - free radicals do not have a charge - each atom gets one electron from the covalent bond

Question 31

what is the definition of a free radical

Answer 31

- a free radical is a reactive species which possesses an unpaired electron

Question 32

what happens in propagation step

Answer 32

- the halogen free radicals are very reactive and remove a H from the methane leaving a methyl free radical - the methyl free radicals reacts with a halogen molecules to produce the main product and another halogen free radical

Question 33

what happens in the termination step?

Answer 33

- collision of two free radicals does not generate further free radicals - chain is terminated

Question 34

why is free radicals a chain reaction?

Answer 34

- because the free radical is regenerated, it can react with several more alkane molcls