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Organic chemistry (Alkanes (Cracking (The thermal decomposition of long…
Organic chemistry
Alkanes
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Cracking
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- Shorter chain alkanes are in very high demand as fuels but longer alkanes are in less demand.
- This means that there is a surplus of the longer alkanes from the fractional distillation of crude oil
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- The main use of alkanes from crude oil is a fuels.
- Alkanes are good fuels because they release a lot of energy when they burn.
- Complete combustion:
hydrocarbon + oxygen → carbon dioxide + water
- Abundant supply of oxygen
- Both Hydrogen and Carbon are oxidised.
- Incomplete combustion: hydrocarbon + oxygen → carbon monoxide + water
- Incomplete combustion: hydrocarbon + oxygen → carbon + water.
- Poor supply of oxygen.
- Carbon monoxide formed is toxic and the soot causes smoky flame.
- As the chain length increase:
- Boiling point increases
- Viscosity increases
- Flammability decreases
- Cleanliness of flames decreases.
This means shorter alkanes are more in demand as fuels because they flow more easily, are re flammable and burn with a cleaner flame
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Crude oil
- It is a fossil fuel.
- It is found underground in rocks.
- It was formed over millions of years from the remains of sea creatures.
- It is a form of ancient biomass.
- It is a finite source
- It is a mixture of many different compound
- Most of these compounds are hydrocarbons
- Hydrocarbons are compounds that contain hydrogen and carbon atoms only.
- Most of the hydrocarbons in crude oil are alkanes.
- Plankton died and fell onto the sea floor.
- Dead plankton were covered in mud.
- Over millions of years, more and more sediments built up.
- The enormous heat and pressure turned the dead plankton into oil and gas.
- Today we drill down through rock to reach the oil and bring it up to the surface.
- Crude oil is an important source of:
- Fuels such as petrol, diesel, kerosene, heavy fuel oil and liquefied petroleum gases.
- Feed-stock for the petrochemical industry.
- Other useful substances made from compounds found in crude oil include:
- Solvents
- Lubricants
- Detergents
- Polymers
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- Crude oil is heated and vaporized.
- The vaporised crude oil enters the fractionating tower that is hotter at the bottom and cooler at the top.
- The hydrocarbon cool as they rise up the tower and condense at different height because they have different boiling points.
- Hydrocarbons with large molecules are collected as liquids near the bottom of the tower while those with small molecules collect at the top.
- Small hydrocarbon molecules have weak intermolecular forces, so they have low boiling points.
- They do not condense, but leave the column as gases.
- Long hydrocarbon molecules have stronger intermolecular forces, so they have high boiling points.
- They leave the column as hot liquid bitumen.
Alkenes
- The alkenes are a homologous series of unsaturated alkenes
- They are unsaturated because they have a carbon carbon double bond
- There is no alkene with one carbon atom
- CnH2n
- Alkenes are more reactive than alkanes because they contain a C==C double bond
- Functional group of C==C double bond
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Polymers
Addition polymers
- Molecules containing a C=C double bond can react with each other in addition reactions.
- They join onto each other to create a long chain molecule called a polymer.
- The C=C double bond opens up and the molecule join onto each other to make a long chain molecule.
- Lots of ethene molecule join up to make poly(ethene), better known as polythene.
- There is no other product formed.
- The polymer is made up of a unit that repeats many times- this is known as the repeating unit
Condensation polymer
- Condensation polymerisation involves monomers with two functional groups joining together to make a polymer.
- You need two different types of monomer, each with two of the same functional groups.(diol and dioic)
Polyesters
- Made when molecules with two carboxylic acid functional groups react with two alcohol functional groups
- Both the groups react to from ester linkages(-COO-) and gives off water
Polyamides
- Made when molecules with two carboxylic acid functional groups react with two amine functional groups
- Both the groups react to from amide linkages(-CONH-) and gives off water
They are long molecules formed when lots of small molecules called monomers join together. The small molecules that we join together to make a polymer are called monomers.
Carboxylic acids
- The functional group in the carboxylic acids is the carboxyl group, -COOH
- They dissolve in water to form an weak acidic solution, H+ ions partially ionise in water.
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Homologous series
- Is a family of compounds with:
- The same general formula
- The same functional group
- Similar chemical properties
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- Reaction requires to happen in the presence of a strong acid catalyst
- Often concentrated sulfuric acid is used as a catalyst
- Functional group -COO
- Made from a condensation reaction
- The smell and flavour of fruits
- The smell of perfumes
- Fats- ester of glycerol and fatty acid
- Used as:
- adhesives
- solvents
- plasticizers
- The flavours and fragrances of different esters are widely used to produce food flavourings