3.2- alkanes

Question 1

what is crude oil

Answer 1

-mixture of hydrocarbons, mainly alkanes

Question 1

stages of fractional distillation (4)

Answer 1

1. crude oil heated until vapourised
2. vapour passed into fractionating column- hot at bottom, cool at top
3. different fractions condense at different levels due to different BPs
   4.fractions are pumped away separately

Question 2

what are fractions

Answer 2

compounds/mixtures with a similar BP/carbon chain length

Question 3

why do short chain alkanes have lower BPs than long chain alkanes

Answer 3

weaker VDW forces between molecules so less energy needed to overcome forces

Question 4

what is fractional distillation (2)

Answer 4

-used to separate crude oil- alkanes have different BPs so the mixture can be separated
-its a physical process-no bonds are broken

Question 5

properties of smaller molecules (5)

Answer 5

-short chain alkanes
-low BP
-very volatile
-flows easily
-ignites easily

Question 6

what is cracking (3)

Answer 6

-thermal decomposition reaction
-breaking a C-C bond in large alkanes to give smaller molecules like alkenes and branched alkanes
-chemical process- covalent bonds are broken

Question 7

thermal cracking: conditions, products, use of products

Answer 7

-high temp (≈1000°C), high pressure
-produces alkenes and some short chain alkanes
-products used to make polymers (plastics)

Question 8

catalytic cracking: conditions, products, use of products

Answer 8

-low pressure, high temp (≈450°C), zeolite catalyst
-produces branched chain alkanes, cycloalkanes and aromatic hydrocarbons
-products used as motor fuel

Question 9

why are shorter chain alkanes more useful

Answer 9

more useful as they are in higher demand therefore more money is made

Question 10

complete combustion equation

Answer 10

hydrocarbon+oxygen->carbon dioxide+water

Question 11

incomplete combustion equation

Answer 11

hydrocarbon+oxygen->carbon monoxide/carbon+water

Question 12

what is sulfur dioxide (2)

Answer 12

-produced during combustion
-environmental problem- when combined with water and air it forms sulfuric acid->acid rain

Question 13

what is flue gas desulfurisation (5)

Answer 13

-used in power stations to remove sulfur dioxide from flue gases
-flue gases are reacted with calcium oxide
-sulfur reacts with calcium oxide to form sulfite salts (CaCO3)
-calcium sulfite salt is removed from the flue gas stream before emission
-this prevents emission of sulfur dioxide, so it reduces acid rain

Question 14

what 3 pollutants do petrol engines produce

Answer 14

-carbon monoxide and carbon
-nitorgen oxides (NO and NO2)
-unburned hydrocarbons

Question 15

why is carbon monoxide a pollution problem + how is it formed

Answer 15

-affects amount of greenhouse gases
-incomplete combustion: hydrocarbon+O2->CO+H2O

Question 16

why is carbon a pollution problem + how is it formed

Answer 16

-traps heat in atmosphere, leading to global warming
-incomplete combustion: hydrocarbon+O2->C+H2O

Question 17

why is nitrogen monoxide a pollution problem

Answer 17

reacts with other pollutants to form smog which damages plants and lungs

Question 18

which 3 catalysts are used in catalytic converters

Answer 18

platinum
palladium
rhodeum