Chemistry- Unit 1- Alkanes

  • Created by: FireDwarf
  • Created on: 23-11-13 11:35
General formula for an alkane?
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Not very reactive, they burn and are used as fuels, they have van der waal forces between molecules
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Ring alkane formulas?
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How do their boiling points increase?
As molecular size increases, force of van der waals increases therefore causing stronger dipole induced attaction and therefore more energy required to break them apart.
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C5 C17
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C18 onwards
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Why do branched chains have diffrent boiling points?
They are lower- the chains are not as packed closley together and therefore fewer points of contact between molecules and weaker van der waals.
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What does crude oil contain?
Branched and unbranched alkanes, cycloalkanes,arenes, impurities (sulphur).
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Where is the fractional distilation collumn cooler?
At the top, heat decreases as we move up.
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State some facts about it?
350 degrees heated and then pumped into collumn near the bottem. Most fractions are gases, liquids fall to the bottem.
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What are bubble caps?
they force hot vapours to mix with the liquid that has condensed therefore aiding the seperation.
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How does it work?
Alkanes with a boiling point at each fraction will condense there. If their boiling point is not that temperture, they wll continue to rise through the collumn.
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What are alkanes in water?
Insouble - water molecules are held together by strong hydrogen bonding which is stronger then van der waals.
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What is cracking?
When long hydrocarbon chains are broken into more useful smaller hydrocarbon chains. Often done because the shorter chains have more economic value.
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Why do we crack?
To produce more useful and therefore economicly valuable shorter chains & alkenes which are more reactive then alkanes.
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How do we complete thermal cracking?
Heating alkanes to a high tempertures under high pressure, causing carbon carbon bonds to break and producing alkane and alkene
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How high is the temp? How high the pressure?
700-1200K & 7000kpa.
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How do we form the products, what occurs?
The bonds break, causing the convalently bonded electrons to become unpaired, becoming free radicals. These are highly reactive and react to form a variety of short chains.
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Why is an alkene formed?
because there is not enough hydrogens to form an alkane, and therefore alkene is formed.
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What does it usually produce?
Hydrogen (sometimes), higher proportition of alkenes.
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where can the chain break?
At any point, not always the middle.
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What is catalytic cracking?
Lower temp & lower pressure, using a catalyst ( zeolite) to break the carbon carbon bonds.
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What are the products usually?
branched alkanes, cycloalkanes, aromatic compounds. Gases mostly.
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Catalytic , lower temps & lower pressure, less cost. More alkenes in thermal, more alkanes in catalytic.
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Temp & pressure?
720k & higher then 100kpa but lower then 700kpa.
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Combustion of alkanes? 2 Types?
Complete & uncomplete.
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Short chain alkanes burn in o2 to give co2 and h20. (+ heat)
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Why are they good for fuels?
More carbon, greater heat output. Therefore, give a large amount of heat energy when they undergo combustion and large amount of E to weight.
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Limited amount of o2, therefore mono dioxide or just carbon produced.
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Why is the co bad?
poisonous, occupies hameogobin and decreases o2 absorbed.
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c610 + o2 --> Co + H20.
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How are ntriogen oxides produced?
When there is enough energy for the nitrogen and oxygen to combine. This is occurs in the engine at hot tempertures when the spark ignites the fuel.
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N2 + O2 ---> No2
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What occurs next?
The nitrogen oxide combines with water vapour and oxygen in the air to form nitric acid.
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What does nitric acid contribute towards?
Acid rain & photochemical smog
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Sulfur dioxide?
produced from sulphur impurities in the crude oil. Forms a oxide which combies with water vapour and oxygen to make sulfuric acid, contributing to acid rain.
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S + o2 --> so2 ---> so2 + H20--->Hzso3 + 1/2 o2 ---> h2so4
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What are carbon particles called?
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What do they contribute towards?
Asthma & cancer
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Caused by?
Incomplete combustion
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Unburnt hydrocarbons?
Not buring fules fully,produces ethanes and methanes which contribute towards greenhouse gases & photochemical smog.
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Complete combustion, contribute towards global warming,
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Water vapour is a..?
Greenhouse gas.
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What is photochemical smog?
hotochemical smog is a unique type of air pollution which is caused by reactions between sunlight and pollutants like hydrocarbons and nitrogen oxides.
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Removing sulphur?
Use flue gas desulphurisation- use calcium oxide in the flus to absorb the sulphur dioxide causing caso4 to be formed, which can be used as gypsum (plaster).
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Catalystic converter? What is it? What metals are in it?
Honey come of ceramic matieral coated with platnium and phodium.
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Whats the point of honeycomb?
Large surface so minimal gas can escape and not under go the catalytic convertor proccess.
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What occurs as they pass over the catalyst?
They react with each other, the catalyst lowers the activation energy, and this causes less harmful products to be formed.
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Co + No --> N2 + 2CO3
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How is global warning caused?
Greenhouse gases forming a layer. Light passes in as visible light but is trapped as infa-red by this layer. Heats earth up.
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Role of water?
Water vapour is a greenhouse gas. As earth gets warmer, the amount of water vapour will increase.
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Card 2




Not very reactive, they burn and are used as fuels, they have van der waal forces between molecules

Card 3


Ring alkane formulas?


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Card 4


How do their boiling points increase?


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