Alkanes

Question 1

What is crude oil?

Answer 1

⦾ Mixture of different HCs

Question 2

How can you separate crude oil?

How can this be possible

Answer 2

⦾ Fractional Distillation

⦾ Different lengths of HCs have different bp.s

Question 3

State the method for separating crude oil?

Answer 3

1 ⦾ Mixture vaporised + fed into fractionating column
2 ⦾ Vapors rise, cool + condense
3 ⦾ Products siphoned off for different uses

Question 4

How are the short and long HC chains differentiated between?

Answer 4

⦾ Short C chains: lower bp.s - rise higher in column before reaching their bp - collected at top of column
⦾ Long C chains: higher bp.s - don’t rise v. high before reaching their bp. - condense - collect at bottom of column

Question 5

What are the differences between short and long HC chains?

Answer 5

Short:
⦾ Low bp.
⦾ Reactive (easily ignited)
⦾ Runny

Long:
⦾ High bp.
⦾ ~Unreactive (not easily ignited)
⦾ Viscous

Question 6

What is the general order in length for long HCs to short HCs?
What are their uses?

Answer 6

LONGEST to SHORTEST:
⦾ Gases - LPG, camping gas
⦾ Petrol (gasoline) - Petrol
⦾ Naphtha - processed to make petrochemicals
⦾ Kerosene (paraffin) - Jet fuel, Central Heating
⦾ Gas oil - Disel, Central Heating
⦾ Mineral oil - Lubricating oil
⦾ Fuel oil - Ships, Power stations
⦾ Wax, Grease - Candles, Lubrication
⦾ Bitumen - Roofing, Road surfacing

Question 7

Why are long chain molecules cracked?

Answer 7

⦾ Not useful

Question 8

What are the 2 main types of cracking?

Answer 8

1 ⦾ Thermal

2 ⦾ Catalytic

Question 9

What is Thermal Cracking?

Answer 9

⦾ Produces alkanes + alkenes
⦾ High temps ~1200K
⦾ High Pa ~700kPa

Question 10

What is Catalytic Cracking?

Answer 10

⦾ Produces aromatic compounds (with C rings)
⦾ Lower temps ~720K
⦾ Normal Pa
⦾ Zeolite catalyst

Question 11

What is complete combustion?

Complete the equation for e.g. CH4?

Answer 11

⦾ React with sufficient Oxygen
⦾ Produce Carbon dioxide and Water
⦾ CH4 + 2O2 → CO2 + 2H2O

Question 12

What is incomplete combustion?

Complete the equation for e.g. CH4?

Answer 12

⦾ React with insuffient Oxygen
⦾ Produce Carbon monoxide and Water (even C)
⦾ 2CH4 + 3O2 → 2CO + 4H2O

Question 13

Why is Carbon monoxide dangerous?

Answer 13

⦾ Toxic gas
⦾ Odorless
⦾ Colourless

Question 14

What can remove dangerous gaseous products?

Answer 14

⦾ Catalytic converters

Question 15

How do catalytic converters work?

Answer 15

⦾ Uses rhodium catalyst

⦾ Convert harmful products into more stable products e.g. CO2 or H2O

Question 16

What else can incomplete combustion produce?

And why is this dangerous?

Answer 16

⦾ Carbon particulates

⦾ Respiratory problems as they pollute air

Question 17

Why are Sulfur impurities dangerous in the atomosphere?

Answer 17

⦾ Lead to acidification of water in Earth’s atm

⦾ React to form Sulfuric acid - H2SO4

Question 18

How can Sulfur impurites be removed from waste products?

Answer 18

⦾ Flue gas desulfurisation
⦾ Use powdered CaCO3 or CaO and gypsum
⦾ Flue gases mix with alkaline slurry + acidic SO2 gas reacts with Ca compounds to form harmless salt
⦾ CaO(s) + SO2(g) → CaSO3(s)

Question 19

What are Nitrogen Oxides?

Answer 19

⦾ Series of toxic + poisonous molecules

⦾ NOx

Question 20

How is Nitrogen monoxide produced?

Answer 20

⦾ High Pa
⦾ High temp
⦾ In car engine
⦾ N2 and O2 in air react together

Question 21

What are the series of Nitrogen oxide reaction that can occur?

Answer 21

⦾ N2(g) + O2(g) → 2NO(g)

⦾ 2NO(g) + O2(g) → 2NO2(g)

Question 22

Draw out the reaction that occur when catalytic converters remove pollutants?
Start with: C3H8(g)

Answer 22

⦾ C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)
⦾ 2NO(g) → N2(g) + O2(g)
⦾ 2NO(g) + 2CO(g) → N2(g) + 2CO2(g)

Question 23

What is the Chlorination of Alkanes?

Answer 23

⦾ Alkanes react with Halogen in presence of UV light to produce halogenoalkanes

Question 24

What is a free radical?

Answer 24

⦾ Particle with an unpaired electron

Question 25

How do freed radicals form?

Answer 25

⦾ Covalent bond splits equally

⦾ Giving each species 1 electron

Question 26

Why are free radicals reactive?

Answer 26

⦾ Unpaired electron

Question 27

What is photochemical reaction?

Answer 27

⦾ Started by UV light

Question 28

What is a Free Radcial Substitution reaction?

Answer 28

⦾ H atom is substituted by Cl or Br

Question 29

What are the series of reactions that result for the synthesis of Chloroalkanes?

Answer 29

1 ⦾ Initiation
2 ⦾ Propagation
3 ⦾ Substitutions
4 ⦾ Terminations

Question 30

Explain Initiation?

Answer 30

⦾ Photodissociation
⦾ Sunlight provides (J) to split Cl-Cl bonds equally
⦾ Each gets 1 e-
⦾ Cl• - highly reactive free radical

                           Cl2 + (UV) → 2Cl•

Question 31

Explain Propagation?

Answer 31

⦾ Free radicals used up + made again
⦾ Cl• attacks methane
⦾ New methyl free radical attacks another Cl2
⦾ New Cl• can attack another CH4 until all Cl• or CH4 used up
Cl• + CH4 → •CH3 + HCl
•CH3 + Cl2 → CH3Cl + Cl•

Question 32

Explain Substitutions?

Answer 32

⦾ If Cl's in excess, H atoms on CH4 eventually replaced by Cl
⦾ You'l get: dichloromethane - CH2Cl2, 
trichloromethane - CHCl3
tetrachloromethane - CCl4
CH4 + Cl2 → CH3Cl + HCl
CH3Cl + Cl2 → CH2Cl2 + HCl
CH2Cl + Cl2 → CHCl3 + HCl
CHCl3 + Cl3 → CHCl4 + HCl
⦾ But if CH4 in excess, Cl will be used up quickly + products mostly chloromethane

Question 33

Explain Termination?

Answer 33

⦾ 2 radicals join to end chain reaction + form stable product:
•CH3 + Cl• → CH3Cl
•CH3 + •CH3 → C2H6
Cl• + Cl• → Cl2