Chapter 12 - Alkanes

Question 1

What are alkanes? General formula?

Answer 1

Homologous series of saturated hydrocarbons with the general formula C2H2n+2

Question 2

What is the reactivity of alkanes?

Answer 2

Very un reactive but do burn and react with halogens

Question 3

Why aren’t alkanes very reactive?

Answer 3

• C-C and C-H bonds are strong
• C-C and C-H bonds are non polar (similar electronegativity)

Question 4

Which isomer of alkanes I more likely to react E or Z?

Answer 4

Z isomer in E isomers the polar bonds cancel out as they act in opposite directions

Question 5

What is the trend for boiling points in alkanes?

Answer 5

• longer chains means higher boiling point due to stronger van der Waals’ forces between molecules (more electrons in the molecule)
• for isomers the more branched carbon chain has the lower boiling point as they have weaker van der Waals’ forces (molecules are less closely packed together)
  THEN TALK ABT ENERGY IN RELATION TO BP

Question 6

How is petroleum (crude oil) formed?

Answer 6

Slow decay of marine animals and plants over millions of years under heat and pressure

Question 7

How is crude oil separated?

Answer 7

By fractional distillation as different hydrocarbons within crude oil have different boiling points

Question 8

What is the process of fractional distillation of crude oil?

Answer 8

• crude oil is vaporised
• vapour is passed into a tower which is hot at the bottom and cold at the top
• as the vapour rises it cools
• molecules condense at different heights as they have different boiling points
• larger the molecules, lower down it condenses

Question 9

What are the different fractions at what temperatures?

Answer 9

< 40° -> FUEL GASES (C - C4)
40° - 90° -> GASOLINE
90° - 160° -> NAPTHA
160° - 250° -> KEROSENE
250° - 350° -> DIESEL
> 350° -> RESIDUE

Question 10

What is the trend in properties as the C chain gets longer?

Answer 10

• becomes more viscous
• harder to ignite
• less volatile
• higher boiling points

Question 11

What is part of the residue fraction?

Answer 11

Contains fuel oil, lubricating oil, waxes, and bitumen
These decompose at up high temperatures so they’re separated by distillation at lower pressure

Question 12

What is cracking?

Answer 12

Thermal decomposition of longer alkanes to form smaller alkanes

Question 13

What are the conditions for thermal cracking?

Answer 13

TEMPERATURE: 900°
PRESSURE: 70atm
CATALYST: none
PRODUCTS: alkenes

Question 14

What are the conditions for catalytic cracking?

Answer 14

TEMPERATURE: 450°
PRESSURE: 1-2 atm
CATALYST: zeolites
PRODUCTS: motor fuels (aromatics, cyclic alkanes, branched alkanes)

Question 15

What is reforming?

Answer 15

Process where straight chain hydrocarbons are converted into branched chain alkanes and cyclic alkanes. Both these burn more efficiently and are used in petrol for cars

Question 16

What is the complete combustion formula?

Answer 16

Alkane + O2 → CO2 + H2O
Highly exothermic, with enough oxygen

Question 17

What are the 2 formulas for incomplete combustion?

Answer 17

Alkane + O2 -> CO + H2O
Alkane + O2 -> C + H2O

Question 18

What are the problems with CO2, CO, C and SO2 and how can we avoid them?

Answer 18

CO2 - greenhouse gas -> burn less fossil fuels
CO - toxic -> ensure good supply of oxygen when burning fossil fuels
C - respiratory problems, global dimming, blackens building -> ensure good supply of oxygen
SO2 - acid rain -> remove S from fuel before burning (flue gas desulfurisation)