Chemical reactions of the alkenes
4 marks · NormalA student compares the combustion reactions of three alkenes. The table shows the number of carbon and hydrogen atoms in each.
| Alkene | Number of carbon atoms (n) | Number of hydrogen atoms |
|---|---|---|
| Ethene | 2 | 4 |
| Propene | 3 | 6 |
| Pentene | 5 | 10 |
State the general formula for alkenes.
Explain the pattern between the number of carbon atoms and hydrogen atoms in these alkenes.
Write the molecular formula for hexene.
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.
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(a) CnH2n
(b) The number of hydrogen atoms is always twice the number of carbon atoms for alkenes.
(c) C6H12
Common mistakes: Using alkane formula for (c); Describing pattern unclearly in (b)
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States: CnH2n.
B2 (b) · 1 mark · Recognises hydrogen atoms = 2 × carbon atoms or gives a specific data-based example.
B3 (b) · 1 mark · Correct full statement relating n and number of hydrogens for all alkenes, eg 'number of H = 2n.'
B4 (c) · 1 mark · Writes the molecular formula for hexene: C6H12.
Worked solution
a) CnH2n
b) The number of hydrogen atoms is always double the number of carbon atoms. For example, if n = 3, hydrogens = 6.
c) For hexene (n=6), formula is C6H12.