Alkanes
3 marks · NormalExplain why alkanes have the general formula CnH2n+2, making reference to the bonding and structure of these molecules.
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Alkanes have the general formula CnH2n+2 because each carbon atom in a straight chain forms four single covalent bonds. The two end carbons each bond to 3 hydrogens, and each middle carbon to 2 hydrogens. This gives two more hydrogens than double the number of carbons.
Must show: Tetrahedral carbon, Each carbon forms four bonds
Also accept: Describes hydrogens on terminal carbons and internal carbons.
Common mistakes: Confusing formula with alkenes; Incomplete explanation of bonding
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Marking points
B1 · 1 mark · Mentions that every carbon forms four single covalent bonds.
B2 · 1 mark · Describes that two end carbons are bonded to 3 hydrogens, and each internal carbon bonds to 2 hydrogens.
B3 · 1 mark · Explains that this pattern results in the formula CnH2n+2.
Worked solution
Alkanes are saturated hydrocarbons where each carbon atom forms four single covalent bonds.
In a straight alkane chain, the two end carbons are each bonded to three hydrogens, while each middle carbon atom bonds to two hydrogens.
This explains why the general formula is CnH2n+2: there are always two more hydrogens than double the number of carbons.