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Chemistry > Unit 3: Section 2 - Alkanes and Halogenoalkanes > Flashcards

Unit 3: Section 2 - Alkanes and Halogenoalkanes Flashcards

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1
Q

What is an alkane?

A

A saturated hydrocarbons containing C-H bonds only

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2
Q

What is the general formula of an alkane?

A

CnH2n+2

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3
Q

Are alkane’s bonds polar? Why/why not?

A

Non polar - carbon and hydrogen have similar electronegativities

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4
Q

Which intermolecular forces do they have? Why?

A

Only van der Waals forces of attraction - bonds are non-polar

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5
Q

Are they soluble in water? Why?

A

Insoluble because hydrogen bonds in water are stronger than alkanes’ van der Waals forces of attraction

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6
Q

How reactive are alkanes?

A

Very unreactive

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7
Q

How reactive are alkanes?

A

Very unreactive

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8
Q

Which reactions will alkanes undergo?

A

Combustion and reaction with halogens

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9
Q

What is crude oil? How is it formed? Is it renewable? Why?

A
  • Mixture of fractions (hydrocarbons with similar boiling points and properties)
  • Formed at high temperatures and pressures deep below earth’s surface over millions of years -> therefore non-renewable
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10
Q

Name the fractions from high to low boiling point

A
  1. Gases - fuel on site
  2. Gasoline/petrol/naphtha - cars
  3. Kerosene/paraffin - jet fuel, lighting
  4. Diesel oil - lorries/taxis
  5. Lubricating oil/waxes - candles, engine oil
  6. Fuel oil - ships, power stations
  7. Tar/bitumen - roads/roofing
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11
Q

What is fractional distillation/how does it work?

A
  1. Crude oil heated until mostly vaporised
  2. Passed into a fractionating tower that is cooler at the top than the bottom
  3. Liquid fractions are piped off at the bottom
  4. Vapours rise up the column and - via trays and buble caps - condense when temperature < their boiling point
  5. Shortest chain hydrocarbons condense at the top as they have the lowest boiling points
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12
Q

What is fracking and how is it done?

A
  • Natural gas held with shale rock
  • Drill into shale, force pressurised water and sand into rock to fracture it, collect gas
  • HCl and methanol added to beak up shale and prevent corrosion
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13
Q

Pros/cons of fracking?

A

Advantages - gas supply for many year, reduces imported gas and electricity
Disadvantages - lots of traffic to local area, concern about amount of water used, chemical additives can pollute water supplies, can cause small earthquakes, combust Ch4 -> CO2 -> global warming

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14
Q

Why are alkanes cracked?

A

To turn a long chain alkane, with is not very economically valuable, into a shorter chain alkane (more economically valuable as can be used as a fuel) and an alkene (more reactive, starting point for many products)

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15
Q

What are the conditions for thermal cracking?

A

700-1200K temperature
Up to 7000 kPa pressure

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16
Q

What is the intermediate for the reaction?

A

Free radicals

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17
Q

What are the main products of thermal cracking?

A

Alkenes

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18
Q

What are the conditions for catalytic cracking?

A

Lower temp (720K)
Lower pressure (but above atmospheric)
Zeolite catalyst (SiO2 and Al2O3) with a honeycomb structure to give a large surface area

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19
Q

What are the main products of catalytic cracking?

A

Cycloalkanes, aromatic hydrocarbons, branched alkanes

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20
Q

Write an equation for the combustion of propane?

A

C3H8 + 5O2 -> 3CO2 + 4H2O

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21
Q

What is a fuel?

A

Something which releases heat energy when combusted

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22
Q

What are the five main fuels comprising of alkanes?

A

Methane, butane, propane, petrol (about C8), paraffin (C10-C18)

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23
Q

What is incomplete combustion and what products are formed in the case of alkanes?

A

Combustion in a limited supply of oxygen
CO - carbon monoxide - poisonous
C - carbon - particulates - soot - global dimming

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24
Q

Which type of hydrocarbons are most likely to undergo incomplete combustion?

A

Longer chains

25
Q

What is the environmentla impact of carbon monoxide?

A

It is toxic/poisonous

26
Q

What is the environmental impact of nitrogen oxides?

A

form nitric acid -> acid rain
photochemical smog

27
Q

What is the environmental impact of sulfur impurities/sulfur dioxide?

A

form sulphuric acid -> acid rain

28
Q

What is the environmental impact of soot (carbon)?

A

asthma, cancer, global warming

29
Q

What is the environmental impact of unburnt hydrocarbons?

A

Photochemical smog

30
Q

What is the environmental impact of carbon dioxide?

A

Greenhouse gas -> global warming, increases global temperatures, speeds up climate change

31
Q

What is the environmental impact of water vapour?

A

Greenhouse gas -> global warming, increases global temperatures, speeds up climate change

32
Q

What are flue gases?

A

Gases given out by power stations

33
Q

What are the equations for 2 different ways of desulfurising flue gases?

A

CaO(s) + 2H2O(l) + SO2(g) + 1/2O2(g) -> CaSO4.2H2O(s)
OR
CaCO3(s) + SO2(g) + 1/2O2(g) -> CaSO4 + CO2(g)

34
Q

What are catalytic converters made up of?

A

Ceramic honeycomb coated with platinum, palladium and rhodium

35
Q

What do catalytic converters catalyse (equations)?

A

These reactions of products from car exhausts
* 2CO(g) + 2NO(g) -> N2(g) + 2CO2(g)
* Hydrocarbons + NO -> N2 + C)2 + H2O

36
Q

What are greenhouse gases?

A

Gases which trap infrared radiation, making the earth act like a greenhouse

37
Q

What is the greenhouse effect an how doesit contribute to global warming?

A
  1. Greenhouses gases trap infrared radiation in the atmosphere
  2. Atmosphere heats up
38
Q

What is the definition of carbon neutral activities?

A

Activities that produce no net/overall carbon dioxide emissions

39
Q

How are halogenoalkanes formed from alkanes?

A

Free radical substitution reaction

40
Q

What are the 3 stages of free radical substitution?

A
  1. Initiation - breaking halogen bond to form free radicals
  2. Propagation - chain part of the reaction where products are formed but free radicals remain
  3. Termination - free radicals removed, stable products formed
41
Q

What are the conditions needed for the formation of a free radical chlorine atom?

A

UV light

42
Q

What is the ozone’s layer function?

A

Protects the earth from harmful exposure to too many UV rays

43
Q

How do CFCs break down the ozone layer?

A

Free radical substitution

44
Q

What is the equation for the overall decomposition of ozone into oxygen?

A

2O3 -> 3O2

45
Q

What are the free radical substitution equations that show how Cl free radicals catalyse the break down of O3?

A

Cl2 -> 2Cl⋅ (in the presence of UV)
Cl⋅ + O3 -> ClO⋅ + O2
ClO⋅ + O3 -> 2O2 + Cl⋅

46
Q

Are halogenoalkanes soluble in water?

A

Insoluble as C-H bonds are non-polar, not compensated for enough by C-X bond polarity

47
Q

Do halogenoalkanes have a polar bond? Why?

A

Yes polar, as halogen has a higher electronegativity than C (halogen is δ-, carbon is δ+)

48
Q

What intermolecular forces do halogenoalkanes have? Why?

A

Permanent dipole-dipole and van der waals forces of attraction
C-X bond polarity creates permanent dipoles

49
Q

When would halogenoalkanes have higher boiling points?

A
  • Increase carbon chain length
  • Halogen further down group 7
50
Q

How would the mass of a halogenoalkane compare with the mass of an alkane of the same chain length

A

Greater as mass of halogen > mass of H

51
Q

What is the most important factor in determining a halogenoalkane’s reactivity

A

Carbon-halogen bond enthalpy

52
Q

What is the order of reactivity of halogenoalkanes?

A

Although C-F is the most polar bond, the bond enthalpy of C-X decreases down the group, so reactivity increases down the group

53
Q

What is a nucleophile?

A

A negatively charged ion/δ- atom with a lone pair of electrons which can be donated to an electron deficient atom

54
Q

What is nucleophilic substitution?

A

A reaction where a nucleophile donates a lone pair of electrons to δ+ C atom, δ- atom leaves molecule (replaced by nucleophiles)

55
Q

What are CFCs?

A

Chlorine-fluoro-carbons - halogenoalkanes containing C, F and Cl only (no H)

56
Q

What is the problem with CFCs?

A

Although unreactive under normal conditions, they catalyse the breakdown of ozone in the atmosphere via free radical substitution

57
Q

What are CFCs being replaced with?

A
  • HCFCs (hydrogen, chlorine, fluorine, carbon)
  • HFCs (hydrogen, fluorine, carbon)
58
Q

What are the conditions/reactants needed for the elimination reaction of halogenoalkanes?

A

NaOH or KOH dissolved in ethanol (no water present)
Heated

59
Q

What is formed from the elimination reaction of halogenoalkanes?

A

An alkene, water and halogen ion

Chemistry (16 decks)

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