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Chemistry > Alkanes > Flashcards

Alkanes Flashcards

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1
Q

What is crude oil’s natural state?

A

A thick, tarry substance difficult to ignite

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2
Q

What does crude oil contain?

A
  • it is a mixture of about 150 different hydrocarbons
  • the majority of the hydrocarbons are unbranched alkanes
  • the mixture also contains cycloalkenes and arenes
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3
Q

What are crude oil fractions and how are they obtained?

A
  • mixtures of hydrocarbons with similar boiling points

* these are obtained by fractional distillation

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4
Q

How does fractional distillation work?

A
  • the crude oil is heated until it vaporises.
  • the gases pass into a fractionating column
  • a temperature gradient is created between the bottom of the column (350 °c) and the top (60°c) and there is a gradual cooling as vapours pass up the column
  • any hydrocarbons that remain liquid at temperatures as high as 340° fall to the bottom of the column and are removed as residue
  • the smaller hydrocarbons rise up the column in their gaseous state
  • as a particular hydrocarbon reaches the level in the column where the temperature is equal to it’s boiling point it condenses and is collected in trays.
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5
Q

What does the primary distillation of crude oil do?

A

Doesn’t separate individual hydrocarbons but instead seperates them into fractuons containing a mix of hydrocarbons that have boiling points within a particular range

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6
Q

What is secondary distillation

A

After primary distillation when fractions are further seperated to obtain less complex mixtures

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7
Q

How is the fraction with a boiling point above 350° distilled?

A

By vacuum distillation under reduced pressure which lowers its temperature

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8
Q

What are the most useful fractions of hydrocarbons?

A

The fractions that contain hydrocarbons of shorter chain lengths from C6-C10

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9
Q

What is cracking?

A

A process that breaks doen some carbon bonds in long-chain alkanes and produces smaller molecules

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10
Q

What is cracking used to produce?

A
  • primarily alkakes for petrol

* Also produces alkenes that are used by the chemicial industry to make a variety of plastics and other products

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11
Q

What is the overall summary of the cracking reaction?

A

High Mr alkane -> smaller Mr alkane + alkene

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12
Q

What are the two types of cracking?

A

Thermal cracking and catalytic cracking

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13
Q

What happens in thermal cracking?

A
  • uses heat to provide the energy required to breal the C-C bonds
  • usually carried out in a steam cracker
  • when long-chain alkanes are heated under pressure and in the absence of air bonds in the molecules vibrate more vigorously
  • this increased vibration can lead to bonds breaking and alkane molecules being split into lower molecules
  • at the lower end of the temperature range carbon chains tend to break in the centre of the molecule
  • at higher temperatures carbon bonds tend to break towards the end of the chain whicb leads to a higher proportion of alkenes with low molecular mass
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14
Q

What is the major alkene produced by thermal cracking?

A

Ethene

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15
Q

Why is ethene particularly reactive?

A

The double bond makes it reactive

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16
Q

What conditions need to be used in thermal cracking?

A
  • high temperatures (between 400 and 900°c)
  • high pressures (up to 7000 kPa)
  • kept at these conditions for only a second (the residence time) to avoid breaking too many C-C bonds
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17
Q

What is the advamtage and disadvantage of steam cracking?

A
  • it requires a lot of energy

* it can be used on all long chain alkane fractions including the residue from the bottom of the fractionating column

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18
Q

What are the products of thermal cracking.

A

Micture of unbranced and branched chain alkanes and a high percentage of alkenea

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19
Q

What catalyst is used in catalytic cracking?

A
  • synthetic zeolite catalysts

* it has a network of holes in which the reactions involved in catalytic cracking take place in

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20
Q

What are the conditions used in catalytic cracking?

A

A slight pressure and temperature of about 450°c

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21
Q

What are the products of catalytic cracking?

A
  • mainly branched alkanes (mainly used to make petrol)

* aromatic hydrocarbons ( with ring structures in their overall structure)

22
Q

What is the general formula for alkanes?

A

CnH2n+2

23
Q

Why aren’t alkanes very reactive?

A
  • the C-C and C-H bonds are very strong bonds and a chemical reaction that involves breaking these requires a lot of energy
  • they are not polar so don’t attract polar molecules or ions so don’t often react with them
24
Q

What happens when alkanes are heated in a plentiful supply of oxygen?

A

Combustion occurs. Initially, energy must be transferred to start the bond-breaking process but once bond breaking has started bond making can begin and energy is released. Some of the energy is released is transferred to break new bonds and the reaction can proceed without any further input of energy

25
Q

Are combustion reactions exothermic or endothermic?

A

Exothermic

26
Q

What is complete combustion and write the equation for the complete combustion of methane

A

When sufficient oxygen is present, alkanes burn to produce carbon dioxide and water

CH4 + 2O2 -> CO2 + 2H2O

27
Q

Why are alkanes valuble fuels?

A

Because they release a lot of energy when they react with oxygen

28
Q

What is incomplete combustion and write the equation for the incomplete combustion of methane

A

• if insufficient oxygen is available, carbon monoxide or carbon and water are produced

  • 2CH4 + 3O2 -> 2CO + 4H2O
  • CH4 + O2 -> C + 2H2O
29
Q

What does petrol contain?

A

A mixture of branched and unbranched alkanes between C6 and C10 pluss aditives such as oxygenates

30
Q

What do oxygenates do?

A

Provide extra oxygen for combustion reactions

31
Q

Why is it potentially lethel running a car engine in a closed space?

A

Because combustion is never complete in engines so exhaust fumes will also contain some carbon monoxide

32
Q

Why do nitrogen and oxygen react together in engines and what do they form?

A
  • Because petrol burns at temperatures of about 1000 °c in an engine and so they have enough energy to react
  • nitrogen monoxide (NO) is usually formed in engines, but this is rapidly oxidised to nitrogen dioxide (NO2)
33
Q

What is the problem with nitrogen dioxide?

A

• it is brown and under certain weather conditions can build up to give a brown haze over large cities
• nitrogen dioxide also contributes to acid rain. It reacts with water and oxygen to make nitric acid HNO3
4NO2 + 2H2O + O2 -> 4HNO3
• the acid rain can damage buildings, plants and animal life

34
Q

What does a catalyst do?

A

Reduce the rate of a reaction without being used up

35
Q

How have the problems of cars emitting carbon monoxide and oxides of nitrogen been tackled?

A

By fitting catalytic converters into the exhaust pipes

36
Q

How do catalytic converters work?

A

• they are three way: three reactions occur simultaneously
• oxides of nitrogen oxidise carbon monoxide to carbon dioxide whilst they themselves are being reduces
2CO + 2NO -> 2CO2 + N2

37
Q

What is the catalyst used in catalytic converters, why and how?

A
  • a mixture of platinum, rhodium and palladium
  • they are used because transition metals have several oxidation states which makes them good catalysts
  • the catalyst is spread in a very thin layer over the surface of a metal oxide or ceramic honeycomb that has a large surface area
  • carbon monoxide and nitrogen monoxide molecules form temporary weak bonds with the surface of the catalyst. They are adsorbed. While they are held on the surface, old bonds break and new ones form.
38
Q

Why should cars fitted with catalytic converters not use leaded petrol?

A

The lead forms strong bonds with the surface of the catalyst and poisons it

39
Q

What impurities do fossil fuels contain?

A

Sulfur compounds

40
Q

What problems does sulfur dioxide cause?

A
  • acid rain

* irritates asthma sufferers as it irritates the lining of the nose and throat

41
Q

How is sulfuric acid formed from sulfur containing fuels?

A

• they burn to produce sulfur dioxide
S + O2 -> SO2
• sulfur dioxide is oxidised in the atmosphere to make sulfur trioxide
2SO2 + O2 -> 2SO3
• sufur trioxide then dissolves in water to produce sulfuric acid
SO3 + H2O -> H2SO4

overall reaction: SO2 + O2 + H2O -> H2SO4

42
Q

How is sulfurous acid formed?

A

Sulfur dioxide may dissolve in water to form sulfurous acid

SO2 + H20 -> 2H2SO3

43
Q

Where do most sulfur dioxide emissions come from?

A

Power stations and industrial processes

44
Q

Out of sulfuric acid and sulfurous acid which is strong and which is weak?

A

Sulfuric is strong and sulfurous is weak

45
Q

In what process is sulfur dioxide removed from waste gases?

A

Flue gas desulfurisation

46
Q

What happens in flue gas desulfurisation?

A

• the waste gases are treated with either calcium oxide or calcium carbonate as they react with sulfur dioxide to produce calcium sulfate:

• SO2 + CaCO3 -> CaSO3 + CO2
2CaSO3 + O2 -> 2CaSO4

• SO2 + CaO -> CaSO3
2CaSO3 + O2 -> 2CaSO4

47
Q

Methane reacts with chlorine Cl2 in the presence of UV light to produce a mixture of chloroalkanes. What do the chloroalkanes formed depend on and give examples

A

• they depend on the number of hydrogen atoms substituted in the methane molecule:

  • If one hydrogen atom is substituted by chlorine chloromethane forms: CH4 + Cl2 -> CH3Cl + HCl
  • if a second hydrogen atom is substituted then dichloromethane forms

And so ob…

48
Q

What reaction does chlorine react with methane in?

A

A free radical substitution reaction

49
Q

What are the three steps of free radical substitution reactions?

A

Initiation, propagation and termination

50
Q

Describe the initiation step of free radical substitution of chlorine

A
  • only takes place in light (UV)
  • The UV wavelengths of light provide enough energy to split the chlorine molecule into two atoms
  • this is an example of homolytic fission. Homolytic fission occurs when a covalent bond breaks and each atom recieves one electron
  • the chlorine atoms formed are extremely reactive because their unpaired electrons readily pair up with electrons from other molecules.
  • they are called radicals

Cl2 -> (UV) Cl• + Cl•

51
Q

Describe the propogation step of free radical substitution of chlorine

A

• the chlorine radical now reacts with a methane molecule
CH4 + Cl• -> CH3• + HCl
• CH3• is a methyl radical and is also highly reactive.
• when a methyl radical collides with a chlorine molecule, chloromethane is produced along with another chlorine radical
CH3• + Cl2 -> CH3Cl + Cl•
• the chlorine radical is free to react with more methane molecules.
• the above two reactions are propagation steps since they produce radicals that can now take part in another propagation step
• if plenty of oxygen is present further substitution of hydrogen atoms can occur:
CH3Cl +Cl• -> CH2Cl• + HCl
CH2Cl• + Cl2 -> CH2Cl + Cl•
This process can continue until tetrachloromethane is produced

52
Q

Describe the termination step of free radical substitution with chlorine

A

• propagation steps continue until two radicals collide
• when a methyl radical collides with a chlorine radical chloromethane is produced. There is mo radical in the products:
CH3• + Cl• -> CH3Cl
• other termination reactions are possible:
Cl• + Cl• -> Cl2

Chemistry (15 decks)

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