12 - Alkanes

Question 1

Are alkanes saturated or unsaturated?

Answer 1

Saturated

Question 2

Physical Properties of alkanes

Answer 2

• polarity
• boiling points
• solubility
• reactivity

Question 3

Polarity

Answer 3

Alkanes are almost non polar because the electronegativities of carbon and hydrogen are similar

The intermolecular forces between their molecules are weak van dear Waals forces

The larger the molecule, the larger the vdW force

Question 4

Solubility of alkanes

Answer 4

Insoluble in water

Water molecules are held together by hydrogen bonds which are much stronger than the vdW forces that act between alkane molecules,

Question 5

Boiling points of alkanes

Answer 5

As the length of the alkane chain increases, so does the boiling point. This is because the intermolecular forces increase meaning more energy is required to break the bonds.

Question 6

Reactivity of alkanes

Answer 6

Alkanes are relatively unreal it. They have strong carbon-carbon and carbon-hydrogen bonds

They don’t react with: acids, bases, oxidising agents or reducing agents.

However, they burn and react with halogens

Question 7

Products of complete combustion

Answer 7

Carbon dioxide + Water

Question 8

Products of incomplete combustion

Answer 8

Carbon/Carbon monoxide + water

Question 9

When to know if carbon monoxide or carbon is produces

Answer 9

Carbon monoxide: a poisonous gas is produces
Carbon: black product (soot)

Question 10

What are alkanes used as

Answer 10

Fuels

Question 11

Why are alkanes good fuels

Answer 11

Combustion reactions give out heat and have negative enthalpy changes. Which means lots of heat is given out basically so it is a good fuel

Question 12

Different pollutants produced as a results of burning alkanes

Answer 12

Carbon monoxide
Nitrogen oxides
Sulfur dioxide
Carbon particulates
Unburnt hydrocarbons
Carbon dioxide
Water vapour

Question 13

Why is carbon monoxide a pollutant

Answer 13

Poisonous gas

Question 14

Why are nitrogen oxides a pollutant

Answer 14

Oxides react with water vapour and oxygen in the air to form nitric acid —> acid rain

Question 15

How are nitrogen oxides produced?

Answer 15

Under extreme heat and pressure

N2 + O2 -> 2NO

Question 16

How is Sulfur dioxide produced?

Answer 16

Produced from Sulfur containing impurities present in crude oil

Question 17

Negativity of Sulfur dioxide

Answer 17

Combines with water vapour and oxygen in the air to form Sulfuric acid —> acid rain

Question 18

Negatives of carbon particles

Answer 18

Very small, easy to enter the body

Question 19

Negatives of unburnt hydrocarbons

Answer 19

Easy for them to enter the atmosphere and they are significant greenhouse gases.
They contribute to photochemical smog which can cause a variety of health problems

Question 20

Negatives of CO2

Answer 20

Greenhouse gas -> climate change etc

Question 21

Negative of water vapour

Answer 21

Greenhouse gas

Question 22

Sulfur dioxide gas ‘nickname’

Answer 22

Flue gases

Question 23

Equation for how sulfuric acid is produced

Answer 23

SO2 + 0.5O2 + H2O —> H2SO4

Question 24

How can calcium oxide remove flue gas?

Answer 24

Calcium oxide (lime) and water are sprayed into the flue gas which forms calcium sulphite
This is further oxidised to calcium sulphate aka gypsum

Question 25

Equation for calcium sulphide flue gas desulfurisation

Answer 25

CaO + 2H20 + SO2 + 0.5O2 -> CaSO4.2H20

Question 26

Equation for the calcium carbonate (limestone) flue gas desulfurisation

Answer 26

CaCo3 + 0.5O2 + SO2 —> CaSO4 + CO2

Question 27

What is a catalytic converter?

Answer 27

A honeycomb,b made of a ceramic material coated with ceramic and rhodium metals They reduce the output of carbon monoxide, nitrogen oxides and unburnt hydrocarbons in the exhaust gas mixture

Question 28

Equation for how a catalytic converter removes carbon monoxide and nitrogen oxide

Answer 28

2CO + NO —> N2 +2CO2

Question 29

Equation for how a catalytic converter removes hydrocarbons and nitrogen oxide

Answer 29

Hydrocarbons + Nitrogen oxide —> Nitrogen + Carbon Dioxide + Water Eg C9H18 + 25NO —> 12.5N2 + 8CO2 + 9H20

Question 30

What is a free radical?

Answer 30

A free radical is any species with an unpaired electron

Question 31

What is needed for free radical substitution to occur

Answer 31

UV light

Question 32

Initiation step with free radical substitution

Answer 32

First, you need to break the covalent bond between the halogens Eg: Cl—Cl—> 2Cl•

Question 33

Why does the Cl bond break?

Answer 33

The chlorine molecule absorbs the energy of a single quantum of UV light. The energy of one quantum of UV light is greater than the Cl—Cl bond energy, so the bond will break.

Question 34

Propagation step 1, what happens?

Answer 34

The chlorine free radical takes a hydrogen atom from the alkane to form hydrogen (halogen) CH4 + Cl• —> CH3• + HCl

Question 35

Propagation step 2, what happens?

Answer 35

The methyl free radical is now very reactive and reacts with a chlorine e molecule. This produces another chlorine free radical and a molecule of chloromethane •CH3 + Cl2 —> CH3Cl + Cl•

Question 36

What is the effect of propagation?

Answer 36

To produce hydrogen chloride, chloromethane and a new Cl• free radical. This is ready to react with more methane and repeat the 2 steps.

Question 37

What happens in termination

Answer 37

The free radicals are removed This can happen in 3 ways: Cl. + Cl. —> Cl2 .CH3 + .CH3 —> C2H6 Cl. + .CH3 —> CH3Cl

Question 38

What substances are destroying the ozone layer?

Answer 38

Chlorofluorocarbons (CFCs)

Question 39

Why is the ozone layer important?

Answer 39

It protects the Earth from the harmful exposure to too many UV rays.

Question 40

How are chlorine free radicals formed?

Answer 40

Chlorine free radicals are formed from CFCs because the C—Cl bond breaks homolytically in the presence of UV radiation to produce chlorine free radicals, Cl. These then attack the ozone molecules

Question 41

Equation for how Cl free radicals attack the ozone:

Answer 41

Cl. + O3 —> ClO. + O2 ClO. + O2 —> 2O2 + Cl. The chlorine free radical isn’t destroyed, it acts as a catalyst in the breakdown of ozone to oxygen 2O3 —> 3O2