Organic Chemistry
7 marks · EasyAnswer all parts about the formulas and physical properties of alkanes and alkenes.
State the general formula for alkanes and for alkenes.
Explain what is meant by a homologous series.
For an alkene and an alkane both with four carbon atoms, compare their physical state at room temperature and explain any similarities or differences.
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(a) Alkane: CnH2n+2; Alkene: CnH2n
(b) A homologous series is a group of compounds with the same general formula, similar chemical properties, and members differing by a CH2 group.
(c) Both butane (C4H10, alkane) and butene (C4H8, alkene) are gases at room temperature because small members of both series are gaseous up to four carbons. Their melting and boiling points increase as the number of carbons increases. · must show identifies physical state for both, explains similarity (series trend), mentions number of carbons effect
Common mistakes: using alkene formula for alkane
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Marking points
B1 (a) · 1 mark · States correct general formula of alkanes.
B1 (a) · 1 mark · States correct general formula of alkenes.
B1 (b) · 1 mark · Mentions compounds have the same general formula or differ by CH2 group.
B1 (b) · 1 mark · Mentions similar properties or gradual change in properties.
B1 (c) · 1 mark · Identifies that both alkanes (butane) and alkenes (butene) are gases at room temp (C2–C4).
M1 (c) · 1 mark · Refers to trend with carbon chain length (properties change gradually in series).
B1 (c) · 1 mark · States that physical states are similar for both due to similar size/mass.
Worked solution
a) The general formula for alkanes is CnH2n+2 and for alkenes is CnH2n.
b) A homologous series is a family of compounds with the same general formula and similar properties. Each member differs from the next by a CH2 group.
c) Butane (C4H10) and butene (C4H8) both have four carbons and are gases at room temperature. This is because alkanes and alkenes with up to four carbons are gases. As the number of carbons increases, the boiling and melting points increase, and members become liquids or solids. The trend is similar for both series.