Alkanes and Halogenalkanes

Question 1

What is the general formula of alkanes?

Answer 1

Alkanes have the general formula of CnH2n+2

Question 2

What are hydrocarbons?

Answer 2

Hydrocarbons are only made up of carbon and hydrogen atoms

Question 3

Why are alkanes considered saturated?

Answer 3

Every carbon atom makes four single bonds, so there are only single bonds in the molecule.

Question 4

What are cycloalkanes?

Answer 4

They are a ring of carbon atoms with two hydrogen atoms bonded to each carbon. They are still saturated.

Question 5

What is the general formula of cycloalkanes?

Answer 5

CnH2n

Question 6

What is petroleum/crude oil?

Answer 6

It is a mixture of hydrocarbons- found as the sticky black stuff they get out of the ground with oil wells. It is mostly made up of Alkanes. They range from small alkanes, like pentane, to massive alkanes of more than 50 carbons.

Question 7

What is the process used to separate crude oil?

Answer 7

Fractional distillation

Question 8

How does the fractional distillation of crude oil work?

Answer 8

1) The crude oil is vaporized at about 350 degrees C
2) The vaporized crude oil goes into a fractionating column and rises up through the trays. Longer chain alkanes don’t vaporize and remain at the bottom as a gooey residue.
3)As the crude oil vapour goes up it gets cooler and different chain-length alkanes condense to a liquid at various levels starting from longest-shortest.
4) The hydrocarbons with the lowest boiling point don’t condense. They are drawn off as gases at the top of the column.

Question 9

What are the uses and number of carbons in the gases fraction?

Answer 9

1-4 carbons
Liquified petroleum gas (LPG), camping gas

Question 10

What are the uses and number of carbons in the Naphtha fraction?

Answer 10

7-14
processed to make petrochemicals

Question 10

What are the uses and number of carbons in the petrol fraction?

Answer 10

5-12
petrol

Question 11

What are the uses and number of carbons in the kerosene (paraffin) fraction?

Answer 11

11-15
Jet fuel, petrochemicals, central heating fuel

Question 12

What are the uses and number of carbons in the Gas oil (diesel) fraction?

Answer 12

15-19
diesel fuel, central heating fuel

Question 13

What are the uses and number of carbons in the Mineral oil (lubricating) fraction?

Answer 13

20-30
Lubricating oil

Question 14

What are the uses and number of carbons in the Residue (fuel oil, wax, grease, bitumen) fraction?

Answer 14

30-40 Ships, power stations
40-50 Candles, Lubrication
50+ Roofing, Road surfacing

Question 15

Why is cracking of alkanes needed?

Answer 15

There is a large demand for shorter-chain alkanes like petrol and naphtha, but there isn’t enough naturally produced. There is also a low demand for longer-chain alkanes like bitumen and a large store so this excess can be tuned into smaller-chain alkanes to fill the need via cracking.

Question 16

What is cracking?

Answer 16

Cracking is breaking bonds in long-chain alkanes into smaller hydrocarbons. You get a different combination each time you crack alkanes.

Question 17

What is Thermal cracking?

Answer 17

• takes place at high temperatures of up to 1000 degrees
• takes place at high pressure of 70 atm
• produces a lot of alkenes
• alkenes produced are used to make heaps of valuable products, like polymers. A good example of poly(ethene) which is made from ethene.

Question 18

What is Catalytic cracking?

Answer 18

• uses a zeolite catalyst (hydrated aluminosilicate)
• occurs at slight pressure
• occurs at high temperatures of around 450 degrees
• it mostly produces aromatic compounds and motor fuels
• using a catalyst cuts costs as the reaction can be done at low pressure and temperature. Catalyst also speeds up the reaction, saving time and thus money.

Question 19

What is a combustion reaction?

Answer 19

A combustion reaction is when you burn (oxidize) hydrocarbons with sufficient oxygen and you get carbon dioxide and water as products. (complete combustion)

Question 20

Why do alkanes make good fuels? (Pos and Neg)

Answer 20

Burning just a small amount releases a humongous amount of energy. They’re burnt in power stations, central heating systems, and of course, to power car engines.
The downside is that burning alkanes produces a lot of pollutants.

Question 21

What are the two products of incomplete combustion and the problems involved with them?

Answer 21

Carbon monoxide:
- carbon monoxide gas is poisonous and binds to hemoglobin molecules which prevent your red blood cells from carrying oxygen. Carbon dioxide can be removed from exhaust gases by catalytic converters
Carbon soot:
- Carbon soot can contribute to smog and also build up in engines, meaning they don’t work properly.

Question 22

How does burning fossil fuels contribute to global warming?

Answer 22

• burning fossil fuels produces carbon dioxide which is a greenhouse gas
• it is really good at absorbing infrared energy and traps hot air around the earth (greenhouse effect)
• by increasing the percentage of carbon dioxide in the atmosphere more heat is trapped and therefore the earth is warmer
• this process is called global warming

Question 23

What contributes to smog and why?

Answer 23

Unburnt hydrocarbons and oxides of nitrogen.
They both react in the presence of sunlight to form ground-level ozone which is a major component of smog.
Ground-level ozone also irritates people’s eyes, aggravates respiratory problems, and causes lung damage.
(catalytic converters remove unburnt hydrocarbons and oxides of nitrogen from the exhaust)

Question 24

How are oxides of nitrogen formed?

Answer 24

Oxides of nitrogen are formed with atmospheric nitrogen and oxygen at high pressure and temperature in car engines.

Question 25

How is sulphur dioxide formed and what are the problems associated with it?

Answer 25

Some fossil fuels contain sulfur which when burnt reacts with the air to form sulfur dioxide gas. when it gets into the atmosphere it dissolves in moisture in the air to form sulphuric acid. This causes acid rain. Acid rain destroys trees and vegetation as well as corroding limestone buildings and statures, also killing fish in lakes.

Question 26

How can sulfur dioxide be removed from power station flue gases before it gets into the atmosphere?

Answer 26

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

Question 27

What are free radicals?

Answer 27

A free radical is a particle with an unpaired electron. Free radicals form when a covalent bond splits equally, giving one electron to each atom. (Homolytic fission) The unpaired electron makes them very reactive

Question 28

How do halogens react with alkanes to form halogenalkanes?

Answer 28

Halogens react with alkanes in photochemical reactions- meaning they are started with UV light. A hydrogen atom is substituted by chlorine or bromine etc. This is a free radical substitution reaction.

Question 29

What are the three stages in Free radial substitution reactions?

Answer 29

Initiation- radials only in products Propagation-radicals on either side of the equation Termination- radicals only in reactants

Question 30

What happens after a termination reaction?

Answer 30

It depends on which compound is in excess

Question 31

What are chlorofluorocarbons?

Answer 31

Chlorofluorocarbons (CFCs) are halogenoalkane molecules where all of the hydrogen atoms have been replaced by chlorine and fluorine atoms.

Question 32

How are chlorine atoms destroying the ozone layer?

Answer 32

Ozone in the upper atmosphere acts as a protectant (chemical suncream). it absorbs a lot of UV radiation from the sun, stopping it from reaching us (skin cancer and sunburn). Ultraviolet radiation can cause sunburn or even skin cancer. Ozone is formed naturally when an oxygen molecule is broken down into two free radicals by UV radiation. The free radicals then attack other O2 molecules making ozone. The problem is that chlorine radicals are catalyzing the conversion of ozone to oxygen (essentially destroying the ozone layer)

Question 33

What is the process of chlorine radicals and oxygen to destroy ozone?

Answer 33

1) Chlorine free radicals are formed in the upper atmosphere when C-Cl bonds in CFCs are broken down by UV radiation. 2) The chlorine free radicals act as catalysts to react with ozone to form an intermediate ClO radical and an oxygen molecule that breaks down ozone.

Question 34

Why were CFCs banned?

Answer 34

They are pretty unreactive, non-flammable, and non-toxic. They used to be used as a coolant gas in fridges, as solvents, and as propellants in aerosols. In the 1970s research by several people demonstrated that CFCs were causing damage to the ozone layer. The advantages of CFCs couldn't outweigh the environmental problems they were causing, so they were banned. HFCs and hydrocarbons are a much safer alternative.

Question 35

What is a haloalkane?

Answer 35

A haloalkane is an alkane with at least one halogen atom in place of a hydrogen atom.

Question 36

Why is the carbon-halogen bond in haloalkanes polar?

Answer 36

1) halogens are much more electronegative than carbon, so carbon-halogen bonds are polar 2) the slight positive charge on the carbon makes it prone to nucleophiles 3) a nucleophile is an electron pair donor. it donates an electron pair to somewhere without enough electrons. 4) OH-, CN-, and NH3 are all nucleophiles that can react with haloalkanes.

Question 37

How can haloalkanes undergo nucleophilic substitution reactions?

Answer 37

1) a nucleophile can react with a polar molecule by kicking it out of the functional group and taking its place 2) this is called a nucleophilic substitution reaction 3) the product of these reactions depends on what the nucleophile is

Question 38

What do haloalkanes react with to form alcohols?

Answer 38

They react with hydroxides, you will have to use warm aqueous sodium or potassium hydroxide to have a nucleophilic substitution reaction.

Question 39

What do haloalkanes react with to form nitriles?

Answer 39

They react with cyanide. If you warm a little haloalkane with ethanolic potassium cyanide (potassium cyanide dissolved in ethanol) you get a nitrile. It is another nucleophilic substitution reaction. the cyanide ion CN- is the nucleophile.

Question 40

What do halogenalkanes react with to form amines?

Answer 40

They react with ammonia. If you warm a haloalkane with excess ethanolic ammonia the ammonia swaps place with the halogen. It's another nucleophilic substitution reaction. The amine group in the product still has a lone pair of electrons. This means that it can also act as a nucleophile sit it may react with haloalkanes molecules.

Question 41

Which haloalkanes react fastest and why?

Answer 41

The bond enthalpy decides reactivity and for any reaction to occur the carbon halogen bond needs to break. The C-F bond is the strongest- it has the highest bond enthalpy. So fluoroalkanes undergo nucleophilic substitution reactions more slowly than other haloalkanes. The C-I bond has the lowest bond enthalpy, so it's easier to break. this means that Idoalkanes are substituted more quickly.

Question 42

How do haloalkanes undergo elimination reactions?

Answer 42

1) if you warm a haloalkane with hydroxide ions dissolved in ethanol instead of water, an elimination reaction occurs and you end up with an alkene. (anhydrous conditions) 2) you have to heat the mixture under reflux or you'll lose volatile stuff.

Question 43

How can we change whether nucleophilic substitution or elimination occurs with haloalkanes?

Answer 43

You can change the conditions: water- nucleophilic substitution ethanol- elimination If you use a mixture of water and ethanol as the solvent, both reactions will happen and you will get a mixture of the two products.

Question 44

What is the catalyst in catalytic converters?

Answer 44

Rhodium catalyst

Question 45

What are the catalytic converter equations?

Answer 45

CO + O2----> CO2 unburnt hydrocarbons are oxidised to water and carbon dioxide NO + CO2 ---> N2 and CO2