Chemistry - 3.3.2: Alkanes

Alkanes

Saturated hydrocarbons = they contain only carbon-carbon bonds and carbon-hydrogen single bonds

Why might it not be environmentally beneficial to remove flue gases (SO2) with CaCO3? (2)

1. Would need to dispose of large quantities of CaSO3 / CaSO4

2. Produces CO2

3 uses of alkanes

1. Fuels
2. Lubricants
3. Starting materials for a range of different compounds

Why are alkanes important?

They have many uses in industry

General formula for alkanes

CnH2n+2

General formula for cycloalkanes

CnH2n

3 types of alkanes

Unbranched chains, branched chains and ring

Do all alkanes have isomers?

Methane, ethane and propane don't - the rest do

3 physical properties of alkanes

1. Polarity: almost non-polar - C and H have similar electronegativities (so only have weak VdW forces)
2. Boiling point: increases with chain length (stronger VdW forces), decreases with chain branching (pack together less well so smaller SA and weaker VdW forces)
3. Solubility: insoluble in water (hydrogen bonds in water stronger than VdW), but soluble with other non-polar liquids

Reactivity of alkanes

Relatively unreactive (due to strong C-C and C-H bonds)

Where do alkanes come from?

Crude oil / petroleum

Why is crude oil a fossil fuel?

It was formed millions of years ago by the breakdown of animal and plant remains at high pressures and temperatures deep below the Earth's surface

Crude oil / Petroleum

A mixture consisting mostly of alkane hydrocarbons

Why is crude oil non-renewable?

It forms very slowly

7 pollutants produced by the combustion of crude oil and how?

1. Carbon dioxide: during complete combustion
2. Water vapour: during combustion
3. Sulfur dioxide: when sulfur impurities in the hydrocarbon react with oxygen, causes acid rain if it reacts water vapour and oxygen to form sulfuric acid
4. Carbon monoxide: during incomplete combustion
5. Nitrogen oxides: when nitrogen in the air reacts with oxygen at very high temperatures, causes acid rain if it reacts with water vapour and oxygen to form nitric acid
6. Soot: during incomplete combustion
7. Unburnt hydrocarbons

What are the effects of sulfur dioxide?

Acid rain and breathing problems

Fractions

A group of hydrocarbons with similar boiling points

How do you separate crude oil into useful fractions?

Fractional distillation, based on boiling point

How does fractional distillation work? (6)

1. Crude oil heated to very high temperature in a furnace
2. Mixture of liquid and vapour passes through fractionating column - as you go up, temperature decreases
3. Vapour passes up tower via series of trays until it reaches a tray at a temperature lower than its boiling point
4. It cools and condenses, then is collected
5. Shorter chain hydrocarbons have lower boiling points, so condense in trays towards the top of the tower
6. Thick residue at the bottom (tar/bitumen) has a very high boiling point, so collect at the bottom and may be further processed to give more valuable products

Why is cracking alkanes necessary?

To produce substances which are (more) in demand / produce products with a high value / products worth more

2 products of cracking

Alkanes with shorter chains (more useful) and alkenes (more reactive than alkanes)

What is petrol a mixture of?

Mainly alkanes containing between 4 and 12 carbon atoms

What are alkenes used for?

Chemical feedstock - used to make different products

2 types of cracking

Thermal cracking and catalytic cracking

How does thermal cracking work?

Alkanes heated to a high temperature (700-1200 K) and are under high pressure (7000 kPa)

What does thermal cracking produce?

A high percentage of alkenes

Catalytic cracking

The breaking of long-chain alkane molecules (with the aid of a catalyst) into shorter chain hydrocarbons

How does catalytic cracking work?

Takes place at a high temperature (720 K) and at a slight pressure, in the presence of zeolite (a catalyst)

What does catalytic cracking produce?

Motor fuels and aromatic hydrocarbons

Complete combustion and its products

When a substance is burnt in a plentiful supply of oxygen - produces CO2 (g) and H2O (g)

Incomplete combustion and its products

When a substance is burnt in an insufficient supply of oxygen for all the carbon to burn to CO2 - produces C (s) and CO (g)

Is combustion endothermic or exothermic?

Exothermic - becomes more exothermic as the number of carbon atoms increases

Fuels

Substances that release heat energy when they undergo combustion

Paraffin

A mixture of hydrocarbon of chain lengths C10 to C18

What are the effects of CO?

Toxic (colourless and odourless) + flammable

What are the effects of nitrogen oxides?

Acid rain, breathing problems and smog

What are the effects of soot?

Breathing problems, exacerbates asthma, smog and global dimming

What are the effects of unburnt hydrocarbons?

Greenhouse gases and smog

What are the effects of CO2?

Greenhouse gas - contributes to global warming and climate change

What are the effects of water vapour?

Greenhouse gas - contributes to global warming and climate change

Flue gases

The gases given out by power stations

Flue gas desulfurisation

The process of removing the sulfur dioxide from flue gases

Reaction for forming sulfuric acid

SO2 (g) + 1/2 O2 (g) + H2O (l) -> H2SO4 (l)

How does flue gas desulfurisation take place? (2)

1. Slurry of CaO (lime) and water is sprayed on the flue gases, forming calcium sulfite, can then further be oxidised to form calcium sulfate:
CaO (s) + 2H2O (l) + SO2 (g) + 1/2 O2 (g) -> CaSO4.2H2O (s)
2. Alternatively, CaCO3 can be used instead:
CaCO3 (s) + 1/2 O2 (g) + SO2 (g) -> CaSO4 (s) + CO2 (g)

Catalytic converters function

Reduces the output of CO, nitrogen oxides and unburnt hydrocarbons in the exhaust gas mixture of a car

How does a catalytic converter work?

1. Honeycomb coated with platinum / rhodium / iridium / palladium metals (large surface area due to shape) so large surface area
2. Polluting gases pass over catalyst, react to form less harmful products:
3. 2CO (g) + 2NO (g) -> N2 (g) + 2CO2 (g)
hydrocarbons + nitrogen oxide -> nitrogen + carbon dioxide + water

What do free-radical substitution reactions need in order to start?

Ultraviolet light (or at high temperatures)

3 steps of free radical substitution reactions:

1. Initiation: Covalent bond between the halogen breaks after absorbing energy from UV light - it breaks homolytically, forming 2 free radicals (Cl2 -> 2Cl•)
2. Propagation: Cl• takes a hydrogen atom from methane to form HCl (stable), leaving CH3•. CH3• is very reactive, so then reacts with another Cl2, forming CH3Cl and Cl• - chain part of the chain reaction, repeats until termination
Cl• + CH4 -> HCl + •CH3 / •CH3 + Cl2 -> CH3Cl + Cl•
3. Termination: Free radicals removed, react to form a stable compound with no unpaired electrons
Cl• + Cl• -> Cl2 OR •CH3 + •CH3 -> C2H6 OR Cl• + •CH3 -> CH3Cl

Why does the C-H bond not break during initiation?

It needs more energy

Why is the chain reaction of the chlorination of alkanes not useful?

They produce a mixture of products: ethane, dichloromethane and many other isomers if longer-chain alkanes are used

Ozone

O3, decomposes to oxygen

Why is the ozone layer important?

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

How do chlorine free radicals form from CFCs and what happens after?

C-Cl bond breaks homolytically in the presence of UV radiation, forming Cl• - these then attack the ozone, acting as a catalyst in the breakdown of ozone to oxygen

CFCs

Chlorofluorocarbons

2 reactions caused by Cl• and ozone

Cl• + O3 -> ClO• + O2
ClO• + O3 -> 2O2 + Cl•

Overall reaction with ozone?

2O3 -> 3O2

Overall reaction for chlorination of alkanes

Alkane + nCl2 -> Chloroalkane + nHCl

What is the mechanism for the chlorination of alkanes called?

Free-radical substitution

What metals are used as catalysts in catalytic converters?

Platinum / rhodium / iridium / palladium

Cracking

The process where large molecules / long chain hydrocarbons are broken down into smaller molecules / smaller chain hydrocarbons

On honeycomb catalytic converters, why is a thin layer of metal used as a catalyst?

Increases the surface area of the catalyst to reduce the amount of metal needed

2 equations for catalytic converters

2CO (g) + 2NO (g) -> N2 (g) + 2CO2 (g)
hydrocarbons + nitrogen oxide -> nitrogen + carbon dioxide + water

How does carbon dioxide cause global warming?

The C=O bonds absorb infrared radiation, so the IR radiation emitted by the earth does not escape

What does the ozone absorb?

It absorbs UV and infrared radiation