Organic Chemistry

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Organic chemistry is the study of the compounds of Carbon. The simplest compound of Carbon is a hydrocarbon, which contains Hydrogen and Carbon only.

Alkanes are an example of a homologous series, which is a family of compounds in which all

  • obey a general formula
  • differ from one to the next by a CH2 unit
  • have similar chemical properties and gradually changing physical properties

The empirical formula is the simplest whole number ratio of the atoms in a compound.

Eg.          1. Find the empirical formula of a compound which is 82.76% Carbon and 17.24% Hydrogen.

                Carbon:                                82.76/12 = 6.896

                Hydrogen:           17.24/1 = 17.24


                6.896/6.896 = 1                  17.24/6.896 = 2.5              (then multiply by 2)

                = C2H5                   relative molecular mass = 29.



                2. Given that it’s Mr is 58, find its molecular formula.

                58/29 = 2              (C2H5)  x2             = C4H10



3. The compound has two isomers – compounds with the same molecular formula but a different structure. Draw displayed formulae of the isomers.


                This is a straight chain.


This is a branched chain






The melting points and boiling points of alkanes increase with increasing chain length. The liquid state alkanes at room temperature become increasingly more dense.

C1 – C4                   Gases at room temperature

C5 – C19                                 Liquids at room temperature

C20 +                      Solids at room temperature


All hydrocarbons are non-polar, the molecules are held together by Van der Waal forces. The longer the chain, the more Van der Waal forces there are between molecules, so more energy is needed to cause them to reach their boiling points and melting points.


                 C4H10                                                   C8H18

These are examples of skeletal formula.

When liquid molecules pack together tightly, there are more molecules in a smaller space, therefore the density increases.

Alkanes are unreactive because they are non-polar. However, they do burn.

Complete combustion producesCO2 and H2O.

·         CH4 + 2O2 à CO2 + 2H2O

·         C3H8 + 5O2 à 3CO2 + 4H2O

·         C2H6 + ⁷⁄₂ O2 à 2CO2 + 3H2O

·         C8H18 + 25⁄₂ O2 à 8CO2 + 9H2O

Incomplete combustion can produce Carbon Monoxide (CO) and Carbon (soot) (C)

·         C6H14 + 132 O2 à 6CO + 7H2O

·         C4H10 + 52  O2 à 4C + 5H2


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