Alkanes and Halogenoalkanes

Question 1

What are alkanes?

Answer 1

Alkanes are saturated hydrocarbons.

Question 2

Crude Oil is mainly alkanes, fractional distillation is used to separate the different hydrocarbons. State how fractional distillation works:

Answer 2

1) Crude oil is vaporised at 350° C
2) Vaporised crude oil goes into a fractioning column and rises up through the trays. The largest hydrocarbons don’t vaporise at all, because the boiling points are too high - they just run off the bottom and form a gooey residue.
3) As the crude oil vapour goes up the fractioning column, it gets cooler. Because the alkane molecules have a different chain lengths, they have different boiling points, so each fraction condenses at a different temperature. The fractions are drawn one off at different levels in the column.
4) The hydrocarbons with the lowest boiling point don’t condense. They’re drawn off as gases at the top of the column.

Question 3

What are the two types of cracking you can get?

Answer 3

Thermal cracking

Catalytic cracking

Question 4

Give the products of thermal cracking and state the conditions:

Answer 4

It takes place at a high temperature (up to 1000°C) and high-pressure (up to 70 atm)
It produces a lot of alkenes
These alkenes are used to make valuable products, like polymers (plastics). A good example is poly(ethene), which is made from ethene.

Question 5

Give the products of catalytic cracking and state the conditions:

Answer 5

Catalytic cracking uses a zeolite catalyst (hydrated aluminosilicate), at a slight pressure and high temperature (about 450°C)
Produces aromatic hydrocarbons and motor fuels
Using a catalyst cuts costs, because the reaction can be done at a lower pressure and a lower temperature. The catalyst also speeds up the reaction, saving time.

Question 6

State the products are complete combustion:

Answer 6

Water

Carbon dioxide

Question 7

State the products of incomplete combustion:

Answer 7

Carbon monoxide
Water
Carbon dioxide
Carbon

Question 8

What are the issues with incomplete combustion?

Answer 8

Copper monoxide gas is released and is poisonous to humans.
Carbon particles are produced, this can cause breathing problems and build up and engines meaning they don’t work properly.

Question 9

What causes smog?

Answer 9

Unburnt hydrocarbons

Oxides of nitrogen

Question 10

What causes acid rain and what are the implications of it?

Answer 10

Sulfur dioxide

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

Question 11

How do catalyst converter’s work?

Answer 11

Catalyst stick converter is fitted on cars to reduce the emissions of carbon monoxide and oxides of nitrogen in.

Most catalytic converter is a three way; three reactions occur simultaneously. Oxides of nitrogen oxide carbon monoxide to carbon dioxide and and are themselves reduced to nitrogen.

2CO + 2NO —> 2CO2 + N2

Question 12

What are catalytic converter is made up of?

Answer 12

Transition metals. A mixture of platinum, rhodium and palladium is used.

Question 13

How is sulfur dioxide removed from power station flue gases?

Answer 13

How does calcium carbonate/calcium oxide is mixed with water to make an alkaline slurry.
When the flue gases mix with the alkaline slurry, the acidic sulfur dioxide gas reacts with calcium compounds to form a harmless salt (calcium sulphate).

Question 14

What is a free radical?

Answer 14

A free radical is a particle with an unpaired electrons.

Question 15

Halogens react with alkanes to form:

Answer 15

Halogens react with alkanes from halogenoalkanes.

Halogens react with alkanes in a photochemical reaction (reactions started by UV light)
Hydrogen atom is substituted by Cl or Br. This is free radical substitution.

Question 16

Describe stage one: initiation reactions -

Answer 16

Free radicals are produced.

For example:
Sunlight provides enough energy to break the Cl-Cl bond (photodissociation)
Cl2 —UV—> 2Cl•
The bond split equally and each atom gets to keep one electron.

Question 17

Describe stage two: propagation reactions -

Answer 17

Free radicals are used up and created in a chain sequence.

For example:
Cl• attacks a methane molecule:
Cl• + CH4 —> CH3• + HCl
The new methyl free radical, CH3•, can attack another Cl2 molecule:
CH3• + Cl2 —> CH3Cl + Cl•
The new Cl• can attack another CH4 molecule, and so on, until all of the Cl2 or CH4 molecules are used up.

Question 18

Describe stage three: termination reactions -

Answer 18

Free radicals are locked up.

For example:
If 2 free radicals join together, they make a stable molecule. The two unpaired electrons form a covalent bond.
Cl• + CH3• —> CH3Cl
CH3• + CH3 —> C2H6
Cl• + Cl• —> Cl2

Question 19

What is ozone?

Answer 19

O3

It is the upper atmosphere and acts as a chemical sunscreen. It absorbs a lot of ultraviolet radiation from the sun.

Question 20

How is ozone formed?

Answer 20

Ozone’s formed naturally when oxygen molecule is broken down into 2 free radicals by ultraviolet radiation. The free radicals attack the oxygen molecules forming ozone.
O2 —UV—> O• + O•
O2 + O• —UV—> O3

Question 21

How do you CFC’s create holes in the ozone layer?

Answer 21

Cl• are formed in the upper atmosphere when C-Cl bonds in CFCs are broken down by ultraviolet radiation.
CCl3F —UV—> CCl2F• + Cl•
These free radicals are catalysts. They react with ozone to form an immediate ClO• and oxygen molecule.
Cl• + O3 —> O2 + ClO•
ClO• + O3 —> 2O2 + Cl•
The Cl• in the bottom reaction can then be reused in the top reaction.
The overall reaction is:
2O3 —> 3O2

Question 22

What’s the alternative to CFC’s?

Answer 22

HFC’s

Hydrocarbons

Question 23

The carbon-halogen bond in halogenoalkanes is:

Answer 23

Polar

Halogens are much more electronegative than carbon, so carbon-halogen bonds are polar.
The positive charge in the carbon makes it prone to attack from nucleophiles.

Question 24

Halogenoalkanes under go a _______ reaction.

Answer 24

Nucleophilic substitution

Question 25

Halogenoalkanes react with hydroxides to form…

Answer 25

Alcohols.

For example:
Bromoethane can react form ethanol. You use warm aqueous sodium or potassium hydroxide.

Question 26

Halogenoalkanes and cyanide form:

Answer 26

Nitriles

Warm a halogenoalkane with ethanolic potassium cyanide (potassium cyanide dissolved in ethanol). You get a nitrile.

Question 27

Halogenoalkanes and ammonia forms:

Answer 27

Amines

Warm a halogenoalkane with excess ethanolic ammonia, the ammonia swaps places with the halogen.

Question 28

What is the fastest reacting halogenoalkane?

Answer 28

Iodoalkanes
Because the C-I has the lowest bond enthalpy, it’s easier to break. This means that iodoalkanes are substituted more quickly.

Question 29

What is the slowest reacting halogenoalkane?

Answer 29

Flouroalkanes
Because the C-F has the highest bond enthalpy, it’s difficult to break. This means that flouroalkanes are substituted more slowly.

Question 30

Halogenoalkanes undergo elimination reactions, state the conditions needed.

Answer 30

Under reflux

Hydroxide ions dissolved in ethanol.

Question 31

If you react the halogenoalkane under reflux with ethanol what type of reaction is it?

Answer 31

Elimination reaction.

Anhydrous conditions

Question 32

If you react the halogenoalkane under reflux with water what type of reaction is it?

Answer 32

Nucleophilic substitution

Aqueous conditions