Chemistry - Alkanes And Halogenalkanes

Front 1

How does fractional distillation work?(6)

Back 1

1) The crude oil is vaporised at about 350°C.

2) The vaporised crude oil goes into the bottom of the fractionating column and rises up through the trays.

3) The largest hydrocarbons don't vaporise at all, because their boiling points are too high - they just run to the bottom and form a gooey residue.

4) As the crude oil vapour goes up the fractionating column, it gets cooler, creating a temperature gradient.

5) Because boiling points of alkanes increase as the molecules get bigger, each fraction condenses at a different temperature. The fractions are drawn off at different levels in the column.

6) The hydrocarbons with the lowest boiling points don't condense. They're drawn off as gases at the top of the column.

Front 2

What is cracking?

Back 2

Cracking is breaking long-chain alkanes into smaller hydrocarbons (which can include alkenes). It involves breaking C-C bonds.

Front 3

What are the uses of gas crude oil fractions?(2)

Back 3

Liquefied petroleum gas (LPG), camping gas.

Front 4

What are the uses of petrol (gasoline) crude oil fractions?(1)

Back 4

Petrol (fuel).

Front 5

What are the uses of naphtha crude oil fractions?(1)

Back 5

Processed to make petrochemicals.

Front 6

What are the uses of kerosene (paraffin) crude oil fractions?(3)

Back 6

Jet fuel, petrochemicals, central heating fuel.

Front 7

What are the uses of gas oil (diesel) crude oil fractions?(2)

Back 7

Diesel fuel, central heating fuel.

Front 8

What are the uses of mineral oil (lubricating) crude oil fractions?(1)

Back 8

Lubricating oil.

Front 9

What are the uses of fuel oil crude oil fractions?(2)

Back 9

Ships, power stations.

Front 10

What are the uses of wax and grease crude oil fractions?(2)

Back 10

Candles, lubrication.

Front 11

What are the uses of bitumen crude oil fractions?(2)

Back 11

Roofing, road surfacing.

Front 12

What are the two types of cracking?

Back 12

Thermal cracking and catalytic cracking.

Front 13

What is thermal cracking?

Back 13

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 lots of valuable products, like polymers.

Front 14

What is catalytic cracking?

Back 14

Catalytic cracking uses something called a zeolite catalyst (hydrated aluminosilicate), at a slight pressure and high temperature (about 500°C). This mostly produces aromatic hydrocarbons and the alkanes needed to produce motor fuels.

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

Front 15

What is complete combustion?

Back 15

If you burn (oxidise) alkanes (and other hydrocarbons) with plenty of oxygen, you get only carbon dioxide and water.

Front 16

What is incomplete combustion?

Back 16

If there's not enough oxygen around when you burn a hydrocarbon, incomplete combustion happens. This produces particulate carbon (soot) and carbon monoxide gas instead of, or as well as, carbon dioxide.

Front 17

Why is carbon monoxide gas poisonous?

Back 17

Carbon monoxide molecules bind to the same sites on haemoglobin molecules in red blood cells as oxygen molecules. So oxygen can't be carried around the body.

Front 18

How is carbon monoxide removed from exhaust gases?

Back 18

By catalytic converters on cars.

Front 19

Why does soot cause problems?(2)

Back 19

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

Front 20

What are nitrogen oxides?

Back 20

Nitrogen oxides are a series of toxic and poisonous molecules which have the general formula NOx. Nitrogen monoxide is produced when the high pressure and temperature in a car engine cause the nitrogen and oxygen atoms from the air to react together. Nitrogen monoxide can react further to produce nitrogen dioxide:

N2(g) + O2(g) ➡️ 2NO(g).

2NO(g) + O2(g) ➡️ 2NO2(g).

Front 21

How is ground-level ozone (O3) formed?

Back 21

Engines don't burn all the fuel molecules, and some of these come out as unburnt hydrocarbons. These hydrocarbons react with nitrogen oxides in the presence of sunlight to form ground-level ozone.

Front 22

What problems does ground-level ozone cause?

Back 22

Ground-level ozone is a major component of smog, and irritates people's eyes, aggravates respiratory problems and even causes lung damage.

Front 23

What are the three main pollutants from vehicle exhausts?

Back 23

Nitrogen oxides, unburnt hydrocarbons and carbon monoxide.

Front 24

What are the equations for catalytic converters removing pollutants from exhausts in cars?(3)

Back 24

•C3H8(g) + 5O2(g) ➡️ 3CO2(g) + 4H2O(g).

•2NO(g) ➡️ N2(g) + O2(g).

•2NO(g) + 2CO(g) ➡️ N2(g) + 2CO2(g).

Front 25

What causes acid rain?

Back 25

Some fossil fuels contain sulfur. When they are burned, the sulfur reacts to form sulfur dioxide gas (SO2). If sulfur dioxide gets into the atmosphere, it dissolves in the moisture and is converted into sulfuric acid, which causes acid rain. The same process occurs when nitrogen dioxide escapes into the atmosphere - nitric acid is produced.

Front 26

What problems does acid rain cause?

Back 26

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

Front 27

How is sulfur dioxide removed from power station flue?

Back 27

Powdered calcium carbonate (limestone) or 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 the calcium compounds to form a harmless salt (calcium sulfate).

Front 28

What is the equation for the reaction of calcium oxide with sulfur dioxide?

Back 28

CaO(s) + SO2(g) ➡️ CaSO3(g).

Front 29

What is the greenhouse effect?

Back 29

Burning fossil fuels produces carbon dioxide. Carbon dioxide is a greenhouse gas. Greenhouse gases in our atmosphere are very good at absorbing infrared energy (heat). They emit some of the energy they absorb back towards the Earth, keeping it warm.

Front 30

What are photochemical reactions?

Back 30

Reactions started by ultraviolet (UV) light.

Front 31

How do halogens react with alkanes?

Back 31

In photochemical reactions to form halogenoalkanes.

Front 32

What is a free radical substitution reaction?

Back 32

A hydrogen atom is substituted (replaced) by chlorine or bromine.

Front 33

What is a free radical?

Back 33

A free radical is a particle with an unpaired electron?

Front 34

How do free radicals form?

Back 34

Free radicals form when a covalent bond splits equally, giving one electron to each species. The unpaired electron makes them very reactive.

Front 35

How do you show something is a free radical in a mechanism?

Back 35

By putting a dot next to it, e.g. Cl• or •CH3. The dot represents the unpaired electron.

Front 36

What is the equation for the photochemical reaction of methane and chlorine?

Back 36

UV

CH4 + Cl2 ➡️ CH3Cl + HCl.

Front 37

What is a reaction mechanism?

Back 37

A reaction mechanism shows each step in the synthesis of a chemical. The reaction mechanism for the synthesis of chloromethane by a photochemical reaction has three stages - initiation, propagation and termination.

Front 38

What happens in the initiation step in the reaction mechanism for the synthesis of chloromethane by a photochemical reaction?

Back 38

In the initiation step, free radicals are produced. Sunlight provides enough energy to break some of the Cl-Cl bonds - this is photodissociation.

UV

Cl2 ➡️ 2Cl•

The bonds splits equally and each atom gets to keep one electron. The atom becomes a highly reactive free radical, Cl•, because of its unpaired electron.

Front 39

What happens in the propagation step in the reaction mechanism for the synthesis of chloromethane by a photochemical reaction?

Back 39

During propagation, free radicals are used up and created in a chain reaction. First, Cl• attacks a methane molecule:

Cl• + CH4 ➡️ •CH3 + HCl.

The new methyl free radical, CH3•, can then attack another Cl2 molecule:

•CH3 + Cl2 ➡️ CH3Cl + Cl•.

The new Cl• can attack another CH4 molecule, and so on, until all the Cl2 or CH4 molecules are used up.

Front 40

What happens during substitutions in the reaction mechanism for the synthesis of chloromethane by a photochemical reaction?

Back 40

If the chlorine's in excess, the hydrogen atoms on methane will eventually be replaced by chlorine atoms. This means you'll get dichloromethane CH2Cl2, trichloromethane CHCl3, and tetrachloromethane CCl4.

CH4 + Cl2 ➡️ CH3Cl + HCl.

CH3Cl + Cl2 ➡️ CH2Cl2 + HCl.

CH2Cl2 + Cl2 ➡️ CHCl3 + HCl.

CHCl3 + Cl2 ➡️ CCl4 + HCl.

But if the methane's in excess, then the chlorine will be used up quickly and the product will mostly be chloromethane.

CH4 + Cl2 ➡️ CH3Cl + HCl.

Front 41

What happens in the termination step in the reaction mechanism for the synthesis of chloromethane by a photochemical reaction?

Back 41

In the termination step, free radicals are mopped up. If two free radicals join together, they make a stable molecule - the two unpaired electrons form a covalent bond. This terminates the chain reaction. Here are three possible termination reactions from the synthesis of chloromethane:

•CH3 + Cl• ➡️ CH3Cl.

•CH3 + CH3• ➡️ C2H6.

Cl• + Cl• ➡️ Cl2.

Front 42

What happens in a nucleophilic substitution reaction?

Back 42

A nucleophile attacks a polar molecule, kicks out a functional group and settles itself down in its place. The general equation for the nucleophilic substitution of a halogenoalkane is:

CH3CH2X + Nu- ➡️ CH3CH2Nu + X-.

How it works:

H H

| | _

H - C - C+ - X- :Nu

| |

H H

The lone pair of electrons on the nucleophile attacks the slightly positive charge on the carbon - this is shown by a black curly arrow. In mechanisms, curly arrows always show the movement of an electron pair. The lone pair of electrons creates a new bond between the nucleophile and the carbon.

(PHOTO)

The carbon can only be bonded to four other atoms so the addition of the nucleophile breaks the bond between the carbon and the halogen - this is shown by another curly arrow. The pair of electrons from the carbon-halogen bond are taken by the halogen and become a lone pair.

Front 43

What happens when halogenalkanes react with hydroxides?

Back 43

Halogenalkanes will react with hydroxides to produce alcohols.

In the equation for the reaction, R represents an alkyl group. X stands for one of the halogens (F, Cl, Br or I). As it's a nucleophilic substitution reaction, the nucleophile (-OH) kicks out the halogen (X) from the R-X molecule and takes its place. So the overall reaction can just be written as:

R-X + -OH ➡️ ROH + X-.

Front 44

What are nitriles?

Back 44

Nitriles have CN groups. The carbon atom and nitrogen atom are held together with a triple bond, e.g. hydrogen cyanide, ethanenitrile.

Front 45

What are chlorofluorocarbons (CFCs)?

Back 45

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

Front 46

What is a halogenoalkane?

Back 46

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

Front 47

What is the polarity of halogenalkanes?

Back 47

Halogens are generally much more electronegative than carbon, so most carbon-halogen bonds are polar.