Fig. 1 shows part of a teaching display showing the first four members of a homologous series. The cards have been placed in order of increasing number of c...
Question 1 Report
Fig. 1 shows part of a teaching display showing the first four members of a homologous series. The cards have been placed in order of increasing number of carbon atoms. The formulae are molecular formulae, not displayed formulae. A student notices that each successive alkane has one extra CH2 group. The display is used to predict the formula of a later compound before a sample is made in the laboratory. Alkanes are hydrocarbons and are often obtained from crude oil, then separated for fuels and other reactions.
(a) State the general formula of an alkane. [1]
(b) Complete the formula of pentane: C5H__. [1]
(c) Give the difference in molecular formula between ethane and propane. [1]
(d) Name the alkane with molecular formula C6H14. [1]
(e) Explain why all compounds in Fig. 1 have similar chemical reactions. [2]
(f) State two ways in which the physical properties change as the carbon chain becomes longer. [3]
Answer Details
(a) The alkane general formula is \(\mathrm{C_nH_{2n+2}}\). [1]
(b) For pentane, \(H=2(5)+2=12\). The completed formula is \(\mathrm{C_5H_{12}}\). [1]
(c) Propane has one extra \(\mathrm{CH_2}\) unit compared with ethane. [1]
(d) \(\mathrm{C_6H_{14}}\) is hexane. [1]
(e) The compounds belong to the same homologous series, so they have the same functional-group characteristics. They are all saturated hydrocarbons with the same type of bonding, and therefore have similar chemical reactions. [2]
(f) As the carbon chain becomes longer, any three valid changes are: boiling temperature increases, melting temperature generally increases, viscosity increases, and volatility decreases. [3]
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