OCR A CHEMISTRY AS UNIT F322

BASIC CONCEPTS
1. empirical formula is the simplest whole number ratio of atoms of each element present in a compound
2. molecular formula is the actual number of atoms of each element present in a compound
3. general formula is the simplest algebraic formula of a member of a homologous series, ie for an alkane: CnH2n + 2,
4. structural formula is the minimal detail formula that shows the arrangement of atoms in a compound e.g. for butane: CH3CH2CH2CH3
5. displayed formula is the diagram displaying the positioning of atoms in a chain e.g. for ethanol you have a chain of 2 carbons with all carbon bonds being attached to hydrogen except for the end of one of the carbons which has the OH functional grow for alcohols
6. skeletal formula displays the chains of compounds without showing all the bounds e.g. hexane is /\/\/
7. homologous series is a series of organic compounds having the same functional group but with each successive member differing by CH2
  1. eg for alkanes the first 10 compounds are methane, ethane, propane, butane, pentane, hexane, septane, octane, nonane, decane
8. functional group is a group of atoms responsible for the characteristic reactions of a compound
9. systematic naming is based on the longest carbon chain, any branches on that chain, any functional groups and the homologous series of the compound
ISOMERISM
1. structural isomers are compounds with the same molecular formula but different structural formulae
2. stereoisomers are compounds with the same structural formula but with a different arrangement in space
3. E/Z isomerism is an example of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for two different groups to be attached to each carbon atom of the C=C group,
  1. cis-trans isomerism is a special case of EIZ isomerism in which two of the substituent groups are the same
4. REACTION MECHANISMS
  1. covalent bond fission
    1. homolytic
      1. forms two radicals
    2. heterolytic
      1. forms a cation and an anion
  2. a ‘curly arrow’ is the movement of an electron pair, showing either breaking or formation of a covalent bond
    1. Curly arrows should start from a bond, a lone pair of electrons or a negative charge
5. PERCENTAGE YIELDS AND ATOM ECONOMY
  1. atom economy is the molecular mass of desired product divided by molecular mass of all products x 100%
    1. addition reactions have an atom economy of 100%, whereas substitution reactions are less efficient
      1. benefits of developing chemical processes with a high atom economy in terms of fewer waste materials
      2. a reaction may have a high percentage yield but a low atom economy
HYDROCARBONS
1. ALKANES
2. ALKENES
3. hydrocarbons are compounds made of hydrogen and carbon ONLY.
4. crude oil is a mixture of hydrocarbons and can be separated by physical methods, in this case by fractional distillation, because they have different boiling and condensation points. The liquids must also be completely soluble in each other, that is they must all be miscible liquids. When the temperature is high enough, the kinetic energy of a particular hydrocarbon molecule will be sufficient for it to escape the intermolecular forces in the liquid and become a gas. The intermolecular forces are much weaker than the strong carbon - carbon bonds in the hydrocarbon molecule, so it vaporises without decomposes. At the bottom of the fractionating column the crude oil is heated to vapourise it (evaporated or boiled) and the vapour passed into the fractionating column.
  1. A fractionating column acts in the same way as a fractional distillation apparatus in the school/college laboratory but on an industrial scale! In an oil refinery the fractionating columns are very tall with huge surface area to give the best chance of separating the dozens of hydrocarbons in the crude oil (see diagram on the left. This is a continuous process (not a batch process). The fractionating column works continuously with heated–vapourised crude oil piped in at the bottom and the various fractions condensed and constantly tapped off from various levels, each with a different condensation temperature range.
    1. Up the fractioning column the temperature gradually decreases (temperature gradient), so the highest boiling (least volatile) molecules tend to be at the bottom and the lowest boiling (most volatile) hydrocarbons go to the top. The rest of the hydrocarbon molecules then condense out in narrow temperature range i.e. the different fractions condense out in a gradual way from top to bottom depending on their boiling point. In other words the most volatile fraction, i.e. the molecules with the lowest boiling points (shortest hydrocarbon molecules), boil or evaporate off first and go higher up the column and condense out at the higher levels in the fractionating column at the lowest temperature.
      1. The rest of the hydrocarbon molecules separate out according to their boiling/condensation point so that the highest boiling fraction, i.e. the less volatile molecules with higher boiling points (longest hydrocarbon molecules), tend to condense more easily lower down the column, albeit at the higher temperatures. The process is perhaps more correctly called fractional condensation but it is still referred to as fractional distillation.
        The bigger the molecule, the greater the intermolecular attractive forces between the molecules, so the higher the boiling point or condensation poin

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OCR A CHEMISTRY AS UNIT F322

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