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chemistry > AS - Alkanes > Flashcards

AS - Alkanes Flashcards

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1
Q

What are alkanes?

A

Saturated hydrocarbons.

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2
Q

What does it mean when a hydrocarbon is saturated?

A

It only contains carbon-carbon single bonds.

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3
Q

What is the general formula for alkanes?

A

CnH2n+2

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4
Q

What is a cycloalkane?

A

A ring of carbon atoms with two hydrogen atoms attached to each carbon. They are saturated.

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5
Q

What is another word for crude oil?

A

Petroleum.

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6
Q

What is petroleum mainly made up of?

A

Alkanes.

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7
Q

How can crude oil be separated?

A

By fractional distillation.

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8
Q

Explain the process of fractional distillation.

A
  1. Crude oil is first heated in a furnace to about 350 degrees C.
  2. The vaporised crude oil passes into the fractioning column and rises up through the trays as the temperature gets cooler.
  3. The largest hydrocarbons dont vaporise because their bp are too high so run off the bottom as a thick residue.
  4. Different allane molecules have different chain lengths so different bp. Each fraction condensed at a different temperature and are drawn off at different levels in the column.
  5. The hydrocarbons with the lowest bp do not condense and are drawn off as gases at the top of the column.
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9
Q

What substance is drawn off as the thick residue at the bottom of a fractioning column? What can this be used for?

A

Tar/bitumen.

Used for road surfacing.

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10
Q

How and why are heavier fractions modified?

A

They can be cracked to form smaller molecules as these are much higher in demand and value.

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11
Q

What does cracking of an alkane involve?

A

Breaking long-chain alkanes into smaller hydrocarbons (which can include alkenes) by breaking the C-C single bonds.

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12
Q

What does thermal cracking involve and what does it produce?

A

Takes place at a high temperature (up to 1000 degrees C) and high pressure (up to 70atm).

Produces a high percentage of alkenes which can be used to make lots of valuable products like polymers (plastics).

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13
Q

What does catalytic cracking involve? What does it produce?

A

Takes place at a slight pressure, high temperature (about 450 degrees C) and in the presence of a zeolite catalyst.

It is used mainly to produce motor fuels and aromatic hydrocarbons.

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14
Q

What are the advantages of using catalytic cracking?

A

Using a catalyst cuts costs because the reaction can be done at a low pressure and LOWER temperature. The catalyst also speeds up the reaction saving time therefore saving money.

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15
Q

Briefly describe what an aromatic compound is.

A

Compounds that contain benzene rings.

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16
Q

Why do alkanes make good fuels?

A

Burning a small amount of alkanes releases a large amount of energy.

17
Q

Describe what happens with complete combustion of alkanes.

A

If alkanes are burnt (oxidised) with plenty of oxygen, the products are carbon dioxide and water.

18
Q

Describe what happens with the incomplete combustion of alkanes.

A

Incomplete combustion occurs when there is not enough oxygen present. It can produce carbon monoxide instead of or as well as carbon dioxide. It can also produce carbon particles (soot).

19
Q

Why is carbon monoxide poisonous? How can carbon monoxide be removed from exhaust gases?

A

CO molecules bind to the same sites on haemoglobin molecules as oxygen meaning oxygen can no longer be carried around the body.

It can be removed from exhaust gases by catalytic converters on cars.

20
Q

Why is soot dangerous?

A

It is thought to cause breathing problems and it can build up in engines meaning they don’t work properly.

21
Q

An internal combustion engine produces a number of pollutants. Name some of these.

A
  1. NOx (nitrogen oxides)
  2. CO
  3. C
  4. Unburned hydrocarbons
  5. CO2 (greenhouse gas)
  6. Water vapour (greenhouse gas)
  7. SO2
22
Q

What is a fuel?

A

A substance that release heat energy when they undergo combustion.

23
Q

How can NOx, CO, C and unburned hydrocarbons be removed from internal combustion engines?

A

Using catalytic converters.

24
Q

How are nitrogen oxides produced in car engines?

A

When the high pressure and temperature in a car engine cause nitrogen and oxygen atoms from the air to react together.

25
Q

How do unburned hydrocarbons and nitrogen oxides contribute to smog? What consequences does this have?

A

They react in the presence of sunlight to form ground level ozone (O3) which is a major component of smog. O3 irritates peoples eyes, aggravates respiratory problems and can cause lung damage.

26
Q

What happens when hydrocarbons containing sulfur are burned? What are the consequences of this?

A

They produce sulfur dioxide gas which can cause air pollution. SO2 dissolves in the moisture of the atmosphere and is converted into sulfuric acid causing acid rain.

Acid rain destroys trees and vegetation, as well as corroding buildings and statues and killing fish in lakes.

27
Q

Other than sulfuric acid, name another substance that can cause acid rain and how.

A

Nitric oxide can escape into the atmosphere and nitric acid can be produced contributing to acid rain.

28
Q

How can sulfur dioxide be removed from power station flue gases before reaching the atmosphere?

A

Powdered calcium carbonate (limestone) or calcium oxide is mixed with water to make an alkaline slurry. When the flue gases mix with the slurry, the acidic sulfur dioxide gas reacts with calcium compounds to form a harmless salt (calcium sulfate).

29
Q

What is a free radical?

A

A particle with an unpaired electron.

30
Q

When does a free radical form?

A

When a covalent bond splits equally, giving one electron to each atom.

31
Q

Why are free radicals very reactive?

A

Because of the unpaired electron.

32
Q

What is a photochemical reaction?

A

A reaction started by ultraviolet light.

33
Q

What is the overall equation for the reaction between chlorine and methane?

What conditions are needed for this reaction to take place?

A

CH4 + Cl2 —> CH3Cl + HCl

UV light.

34
Q

The reaction mechanism between chlorine and methane has three stages. Name these.

A

Stage 1: initiation reactions

Stage 2: propagation reactions

Stage 3: termination reactions.

35
Q

What happens during the initiation stage of the reaction between chlorine and methane?

A
  1. Sunlight provides enough energy to break the Cl-Cl bond (this is photodissociation).
  2. The bond splits equally producing two highly reactive chlorine free radicals.
36
Q

What happens during the propagation stage of the reaction between chlorine and methane?

A
  1. The chlorine free radical attacks a methane molecule producing a methyl free radical and HCl.
  2. The new methyl free radical can attack another Cl2 molecule producing another Cl free radical and chloromethane (CH3Cl).
  3. The new Cl free radical can attack another methane molecule and so on until all of the chlorine and methane molecules are used up.
37
Q

What happens during the termination stage of the reaction between chlorine and methane?

A

If two free radicals join together, they form a stable molecule. The two unpaired electrons form a covalent bond.

There are lots of different termination reactions. Here are the three examples for this reaction:

Cl• + CH3• —> CH3Cl

CH3• + CH3• —> C2H6

Cl• + Cl• —> Cl2

38
Q

What happens after the termination stage in the reaction between chlorine and methane?

A

It depends on whether chlorine and/or methane are in excess.

If chlorine is in excess, Cl• will start attacking chloromethane, producing dichloromethane, trichloromethane, and even tetrachloromethane.

If methane is in excess, the product will mostly be chloromethane.

chemistry (34 decks)

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