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A Level Chemistry: Organic Chemistry > Chapter 12: Alkanes > Flashcards

Chapter 12: Alkanes Flashcards

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1
Q

What are intermolecular forces

A
  • Forces in between molecules
  • 3 types: Hydrogen Bonds, Van der Waals, Dipole- dipole forces
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2
Q

What are Van der Waals forces

A
  • At any moment there could be more electrons at 1 end just through chance
  • This results in the formation of a slight negative charge at 1 end of the atom and other side slight positive charge - instantaneous dipole
  • This instantaneous dipole can cause the formation of an instantaneous dipole in other atoms - causes a weak attractive force between different atoms
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3
Q

What are alkanes

A
  • Saturated Hydrocarbons
  • Used as fuels, lubricants and starting materials
  • Main source of alkanes is crude oil
  • Among the least reactive groups
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4
Q

What are dipole- dipole forces

A
  • The arrangement of polar bonds can result in a molecule becoming polar overall
  • Dipoles on different molecules can interact
  • This causes an attractive force between different molecules. Dipoles are permanent in a molecule making them stronger Van der Waals
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5
Q

What are the properties of Alkanes

A
  • Polarity - Almost non-polar because the electronegatives of carbon and hydrogen are the similar. Only van der Waals forces.
  • Boiling Points - higher intermolecular forces, boiling point increases
  • Solubility - Alkanes are insoluble in water because water is held by hydrogen bonds which are stonger than Van der Waals forces
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6
Q

How does Fractional Distillation work

A
  • Crude oil is vaporised by heating in a furnace
  • The vapour is passed into a tower which is hot at the bottom and cold at the top
  • As the vapour rises it cools
  • Molecules will condense at different heights in the tower as they have different boiling points
  • The larger the molecule, the lower in the column it condenses
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7
Q

What are some of the fractions

A
  • C1-C3 - gases - fuel on site
  • C1-C12- Gasoline/petrol (naptha) - cars
  • C12 - C16 - Kerosene/Paraffin - Jet fuel, lighting
  • C15 - C18 -Diesel Oil - Lorries and taxis
  • C19-C35 - Lubricating oil/ waxes - candles and waxes
  • C35-C70 - fuel oil - ships/power stations
  • C70< Tar/ Bitumen - Roads and roofing
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8
Q

What are the 2 different cracking methods:

A
  • Thermal Cracking
  • Catalytic Cracking
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9
Q

What is cracking

A
  • When long chain hydrocarbons are broken into smaller molecules or compounds
  • An alkane is always a product of cracking
  • Small chain molecules are more useful to industry and so have a higher demand and value
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10
Q

What is Thermal Cracking

A
  • Involves heating alkanes at high temperatures (700-1200k) and high pressures (7000kPa)
  • Initially 2 shorter chains are produced, each ending in a carbon atom with an unpaired electron (free radicals). They are highly reactive intermediates and react to form shorter chain molecules
  • Products: Short alkanes and alkenes
  • Use of products: Producing polymers and other chemicals
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11
Q

What is Catalytic Cracking

A
  • Takes place at lower temperatures (720K) and slight/ lower pressure (more than atmospheric)
  • Catalyst: Zeolite consisting of silicon dioxide and aluminium oxide - have a honeycomb structure with an enormous surface area
  • Products: Branched alkanes, cycloalkanes and aromatic hydrocarbons
  • Use of products: Produce motorfuels
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12
Q

What are the pollutants from burning alkanes

A
  • Carbon monoxide
  • Carbon (soot)
  • Sulfur dioxide
  • Nitrogen Oxides
  • Unburned Hydrocarbons
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13
Q

Carbon monoxide

Chemical Formula, How it is formed, problem caused, ways to reduce the problem

A
  • CO
  • Incomplete Combustion
  • Binds irreversibly to haemoglobin - results in suffocation
  • Provide more oxygen
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14
Q

Carbon (soot)

How it is formed, chemical formula, Problem caused, ways to reduce the problem

A
  • C
  • Incomplete Combustion
  • They are dangerous as they pass deep into lungs and aren’t removed. Can block engine parts, less energy given out, less powerful, engine needs to burn more fuel - increased cost. Can cause global dimming
  • Provide more oxygen
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15
Q

Sulfur dioxide

Chemical Formula, How it is formed, problem caused, ways to reduce the problem

A
  • SO2
  • During combustion of the fuels sulfur dioxide is formed
  • This is an acidic oxide and can react with water to form acid rain
  • This is toxic and cause respiratory problems
  • Sulfur is usually removed from the fuel before burning ; or SO2 is removed from fumes after burning (flue gas desulfurization)
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16
Q

Nitrogen Oxides

Chemical Formula, How is it formed, Problem Caused, Ways to reduce the problem

A
  • NOx
  • There are sparks at high temperatures in an engine. This causes N2 from the air to react with O2. This can form NO and NO2 (NOx)
  • NOx can react with water and oxygen to form acid rain
  • Catalytic Converters in cars can reduce the amount NOx
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17
Q

Unburned Hydrocarbons

Chemical Formula, How is it formed, Problem Caused, Ways to reduce the problem

A
  • CnH2n+2
  • Some hydrocarbons fuels pass through the engine unburnt
  • Wastes fuel and is also harmful as they are carcinogenic and air pollutants. Act as greenhouse gases
  • Careful mixing of air/ fuel reduces unburnt hydrocarbons. Can be removed in catylytic converters
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18
Q

What are the products of complete combustion

What are the products if incomplete combustion

A

Carbon dioxide and water

Carbon monoxide and with even less is carbon (soot) happens when longer chains need more oxygen to burn

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19
Q

What is flue gas desulfurization

A
  • The process of removing sulfur dioxide
  • A slurry of calcium oxide (lime) and water is sprayed into the flue gas
  • Water reacts with the calcium oxide to form calcium sulfite, which can be further oxidised to form calcium sulfate also known as gypsum
  • Gypsum can be used to make plasterboards
  • An alternative process uses calcium carbonate
20
Q

What are catalytic converters

A
  • Petrol engines are equipped with catalytic converters in their exhause engines
  • Reduce output of carbon monoxide, nitrogen oxides, unburnt hydrocarbons
  • Is a honeycomb - enormosus surface area coated with platinum, palladium or rhodium
  • As polluting gases pass over the catalyst they produce less harmful gases
21
Q

What are the types of green house gases

A
  • Carbon dioxide
  • Hydrocarbons
  • Water vapour
  • Methane
  • Nitrous Oxide
22
Q

The green house effect of a given gas - how much it heats up the atmosphere is dependent on…

A
  • Its atmospheric circulation: the greater the concentration of the gas, the more molecules there are to absorb IR radiation
  • Its ability to absorb IR radiation: some gases absorb and re-emit more strongly than others
23
Q

What is carbon neutral

A
  • Refers to “an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere”
24
Q

What are Biofuels

A
  • A biofuel is a fuel made from a living things or the waste produced by them, and so is renewable and potentially carbon neutral
  • The living things that can produce biofuels
    • Wood
    • Biogas (methane) from animal excrements
    • Ethanol and diesel made form plants and plant waste
25
Q

What is carbon Footprint

A
  • A measure of the impact of our activities have on the environment in terms of the amount of greenhouse gases we produce
  • It is measured in units of carbon dioxide
26
Q

What are the different ways to control Global Warming

A
  • Carbon Capture and storage
    • Removal of waste CO2
    • The CO2 can either be captured post - combustion from power stations by removal from flue gases
  • Encouraging the more economical use of fuels
  • Use of alternative fuels: hydrogen, wind/ solar/wave/tidal power
  • By planting more vegetation there would be increased photosynthesis which would remove CO2 from the atmosphere
27
Q

What is the Kyoto Protocol

A
  • An agreement signed in 1997 by at least 55 devloping countries
  • Pledged to cut greenhouse emissions to below previous levels by set dates
28
Q

What does a Substitution mean in a reaction mechanism

A
  • One or more of the hydrogen atoms in the alkane have been replaced by a bromine atom and hydrogen bromide is given off
  • C6H14(g)+Br2(l)—-> C6H13Br(l) + HBr(g)
  • Bromo hexane is a halogenoalkane
29
Q

What are the 3 main steps of a reaction mechanism

A
  • Initiation
  • Propagation
  • Termination
30
Q

What is the Initiation step for ….

CH4(g)+Cl2(g)—–>CH3Cl(g) + HCl(g)

A
  • Breaking the Cl-Cl bonds. UV breaks chlorine to form chlorine free radicals
  • Cl2 —-> 2Cl•
  • The C-H bonds require more energy to break so only the halogen is part of the initiation step
31
Q

What is the Propagation step 1 for….

CH4(g)+Cl2(g)—–>CH3Cl(g) + HCl(g)

A
  • Molecule + radical —–> molecule + radical
  • For every H that is replaced, there is 1 pair of propagation reactions
  • The alkane reacts with the halogen radical
  • The H atom is removed from the alkane to produce a hydrogen halide and hydrocarbon free radical
  • Cl•+CH4—–> HCl + •CH3
32
Q

What is the propagation step 2 for ……

CH4(g)+Cl2(g)—–>CH3Cl(g) + HCl(g)

A
  • Molecule + radical ——> molecule + radical
  • The hydrocarbon free radical from step 1 reacts with the halogen to put a halogen atom to put a halogen atom onto a hydrocarbon free radical
  • This also produces another halogen free radical to continue the chain reaction in another step 1
  • •CH3+Cl2—–>Cl•+CH3Cl
33
Q

What is the termination step for…..

CH4(g)+Cl2(g)—–>CH3Cl(g) + HCl(g)

A
  • Termination is any step that does not produce free radicals
  • Cl• + Cl• —–> Cl2
  • •CH3 + •CH3 —–> C2H6
34
Q

What are the 2 ways a covalent bond can be broken

A
  • Heterolytically
  • Homolytically
35
Q

What is Heterolytic Fission

A
  • Energy is added to break the bonds which splits unevenly and 1 atom takes both electrons
  • Bond breaking is an endothermic reaction
  • Ions are made
36
Q

What is Homolytic Fission

A
  • Energy is added (usually UV light) to break the bond which splits evenly
  • This forms an unpaired outer electron also called a free radical
37
Q

What are some other products of chain reactions

A
  • Some ethane is produced in termination stage
  • Dichloromethane is produced in propagation stage with chloromethane that has already formed
  • With longer chain alkanes there are more isomers formed
  • Chain reactions are not very useful because they such a mixture of products. Will occur without light at high temperatures
38
Q

Why are chain reactions so important

A
  • Believed that Chloroflourocarbons in the stratosphere are destroying the ozone layer
  • Ozone is a molecule made of O3. It decomposes to oxygen. Too much ozone at ground level == lung irritation and degradation of paints and plastics
  • Ozone is important as it protects Earth from harmful exposure to ultraviolet rays
  • Chlorine free radicals are formed and reacts with ozone
39
Q

What is a chlorofluorocarbon

A
  • CFCs are halogenoalkane molecules where all of the hydrogen atoms have been replaced by chlorine and fluorine atoms
40
Q

How is ozone formed

A
  • Ozone is formed naturally when an oxygen molecule is broken down into 2 free radicals by ultraviolet radiation
  • O2+hv→O•+O•
  • The free radicals attack other oxygen molecules forming ozone:
  • O2 + O• →O3
41
Q

How is the outer layer being destroyed

A
  • It is being destroyed by CFCs
  • Chlorine free radicals are formed in the upper atmosphere when the C-Cl bonds are being broken down by ultraviolet radiation
  • CCl3F →•CCl2F + Cl•
  • These free radicals are catalysts. They react with ozone to form an intermediate (Cl•) and an oxygen molecule
42
Q

What is the Ozone Layer

A
  • Ozone is an allotrope of oxygen
  • The formula of ozone is O3
  • The ozone layer is a very high area of the atmosphere rich in ozone.
  • The ozone layers absorbs ultraviolet radiation from the sun and prevents it from reaching the surface of the Earth
  • Without this layer, plant and animal life would suffer.
43
Q

What is ozone depletion and how does it happen

A
  • When an ozone molecule absorbs ultraviolet light it dissociates to form an oxygen molecule and an oxygen atom:
  • O3→•O• + O2
  • This is a revesible reaction 2O3 ⇔ 3O2
44
Q

What are CFCs and how are they used

A
  • Chlorofluorocarbons are halogenoalkane molecules
  • In CFCs some of the hydrogens in the alkane have been replaced by chlorine and fluorine atoms
  • UV light can break C-Cl bonds in CFCs to form chlorine free radicals
  • CCl3F→•CCl2F + •Cl
  • The C-F bond is too strong to be broken by UV
45
Q

What do the Chlorine free radicals do (CFCs reacting with UV light)

A
  • The chlorine radicals react with ozone and decompose it
  • •Cl + O3 →ClO• +O2
  • The chlorine oxide radical can react with ozone and decompose it
  • ClO• + O3 →2O2 + •Cl
46
Q

What are the alternatives to CFCs

A
  • HCFCs
    • Contains H, C, F, Cl
    • Contain C-Cl bond that are far less stable than CFCs
    • Spend less time in the upper atmosphere
  • HFCs
    • Contains C, F, H
    • Dont contain chlorine dont contribute to destruction of ozone
  • Both are powerful greenhouse gases

A Level Chemistry: Organic Chemistry (12 decks)

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