Organic Chemistry: Alkanes Flashcards
Alkanes
- Saturated (no double C-C bond)
- Formula - Cn H2n+2
Crude Oil
- Formed from animal and plant remains which are subjected to high pressures and temperatures under the ocean
- Finite resource
- Mix of hydrocarbons of different chain lengths
How and Why does Chain Length impact melting and boiling point of Alkanes?
- Longer chain length = higher melting/boiling point
- This is because more atoms means more more Van Der Waals forces which need more energy to break
Fractional Distillation Process
- Crude oil is heated in a furnace
- A mixture of vapor and liquid is passed through a chamber that is hotter at the bottom and cooler at the top
- Vapour passes up the chamber via a series of trays containing bubble caps until it reaches a tray that is sufficiently cool where it condenses
- The mixture of condensed liquids on each tray is piped off
- Shorter chain hydrocarbons condense in trays near the top of the chamber (cooler, they have lower bp)
- Bitumen collects at the base of the chamber
Names and Uses of Fractions
- Bitumen - road surfacing and asphalt for roofs
- Oil - domestic central heating and ship fuel
- Kerosene - jet fuel and paint solvent (oil paint)
- Petrol - fuel
- Naphtha - used to make drugs, plastics, dyes, explosives and paints
- LPG (Liquified Petroleum Gas) - used in heating
Cracking
The process of breaking down longer chain hydrocarbons into shorter chain hydrocarbons
Why do we need to crack Hydrocarbons?
The supply of short chain hydrocarbons isn’t meeting the demand
Types of Cracking
- Thermal Cracking
- Catalytic Cracking
Thermal Cracking
- High Temperature (700-1200K)
- High Pressure (up to 700 kPa)
- Produces high proportion of alkenes
Catalytic Cracking
- High Temperature (around 700K)
- Low Pressure
- Zeolite catalyst
- Produces mainly branched alkanes, cycloalkanes and aromantic compounds
Advantages and Disadvantages of Thermal Cracking
Advantages:
- Produces a high proportion of alkenes
- Can be used for hydrocarbons of any chain length
Disadvantage:
- High temperature and pressure means it’s expensive and uses more energy
Advantages and Disadvantages of Catalytic Cracking
Advantages:
- Lower temperature and pressure means it’s cheaper and uses less energy
- Produces more arenes and branched alkanes
Disadvantage:
- Can’t be used for all fractions (e.g bitumen, which there is loads of)
Why are Alkanes relatively unreactive?
- The bonds in alkanes are relatively strong (van der waals)
- They are saturated - no double bonds to breaks
- Non-Polar as carbon and hydrogen have similar electronegativities
Products of Complete Combustion
CO₂ and H₂O
Why are combustion of alkanes always exothermic?
The amount of energy needed to break bonds is less than the and amount of energy given out when bonds are formed
Fuels
Substances that release heat energy when they undergo combustion
Incomplete Combustion
A limited supply of oxygen during combustion which results in carbon monoxide (CO) and water being formed
Carbon Monoxide
- Formed during incomplete combustion
- CO is a poisonous gas - toxic
Nitrogen Oxides
- Produced in engines
- High temperatures in the engine cause nitrogen to react with oxygen to form nitrogen dioxide
- The NO₂ then reacts with water vapours and oxygen to form nitric acid
- Contributes to acid rain and smog
Sulfur Dioxide
- Produced from sulfur impurities in crude oil
- Combines with water vapour and O₂ in the air to form sulfuric acid
- Contributes to acid rain
Carbon Particulates
- Formed during incomplete combustion
- Exacerbates asthma and causes cancer (carcinogen)
Unburnt Hydrocarbons
- Left over from combustion
- Greenhouse gases
- Contribute to smog
Carbon Dioxide
- Complete combustion
- Greenhouse gas
- Always produced when hydrocarbons burn
How do you reduce levels of Sulfur Dioxide?
- Flue Gas desulphurisation
- SO₂ reacts with CaO to form hydrated calcium sulfate
- CaO(s) + 2H₂O(l) + SO₂(g) + ½O₂(g) –> CaSO₄*2H₂O
How do you reduce levels of Carbon Monoxide, Nitrogen Oxides and Unburnt Hydrocarbons?
- Catalytic Converter
- Honeycomb shape to increase surface area available for reaction
- 2NO + 2CO → N₂+ 2CO₂
Substitution
A reaction in which a substituent group is swapped with a new atom
Stages of Free Radicalisation
- Initiation
- Propagation
- Termination
Initiation
- Mixture of Br₂ and CH4
- UV light is shone at the mixture to break the covalent bonds between Br₂ molecule
- Br₂ –> 2Br*
Propagation
- Br. reacts with CH4 where the Br. bonds with a H atom and takes the electron from the H atom with it to form HBr and CH3.
- Br. + CH4 –> CH3. + HBr
- CH3. reacts with Br2 to form CH3Br and Br.
- CH3. + Br2 –> CH3Br + Br.
Termination
- 2 Br. react to form Br2
- Br. + Br. –> Br2
OR - 2 CH3 . react to form C2H6 (ethane)
- CH3. + CH3. –> C2H6
OR - A CH3. and a Br. react to form CH3Br (bromomethane)
- CH3. + BR. –> CH3Br
What is the issue with Free Radical Substitution?
- Loads of side products can be formed
- E.g dibromomethane, tribromomethane and tetrabromomethane
What causes the breakdown of Ozone?
Ozone reacts with trichlorofluorocarbon (CFCl₃) free radicals and is broken down into oxygen rather than staying as ozone
Ozone Breakdown: Initiation
CFCl₃ –> CFCl₂ + Cl*
Ozone Breakdown: Propagation
- O₃ + Cl* –> ClO* + O₂
- ClO* + O₃ –>Cl* + 2O₂
What are the purpose of the Cl Free Radicals?
They act as catalysts
