General formulae
4 marks · NormalThe table shows the first four members of the alkene homologous series:
| Name | Formula |
|---|---|
| Ethene | C2H4 |
| Propene | C3H6 |
| Butene | C4H8 |
| Pentene | C5H10 |
State the general formula for alkenes.
Name the next member of this series.
Give the molecular formula for hexene, the sixth member of the series.
Explain the pattern you see in the number of hydrogen atoms for each new member of the series.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) CnH2n · must show CnH2n
(b) Hexene · must show hexene
(c) C6H12 · must show C6H12
(d) Each new member has 2 more hydrogen atoms (increases by –CH2–). · must show increases by 2 H for each C, –CH2– group difference, hydrogen increases by 2
Common mistakes: Giving an alkane formula; Mixing the names (e.g. hexene vs. hexane); Omitting a hydrogen atom
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States general formula CnH2n.
B2 (b) · 1 mark · Gives name 'hexene' as the next member (accept '1-hexene').
B3 (c) · 1 mark · Gives formula C6H12.
B4 (d) · 1 mark · States that each new member has two more hydrogens; or increases by a –CH2– group.
Worked solution
a) The general formula for alkenes is CnH2n.
b) The next member is hexene.
c) The formula for hexene is C6H12.
d) Each new member has two more hydrogen atoms than the one before—because each differs by a –CH2– group.