Two stoppered test-tubes are set up as shown in Fig. 1.1. Test-tube A contains ethane and test-tube B contains ethene. The same volume of orange bromine wat...
Question 1 Report
Two stoppered test-tubes are set up as shown in Fig. 1.1. Test-tube A contains ethane and test-tube B contains ethene. The same volume of orange bromine water is added to each tube and both are shaken.
Fig. 1.1
(a) State the colour of the bromine water before the hydrocarbons are added. [1]
(b) After shaking, tube B turns colourless while tube A stays orange. State which tube contains the alkene. [1]
(c) Complete the word equation for the reaction in tube B. ethene + bromine → .......... [1]
Answer Details
This question tests the bromine water test, the standard way to tell a saturated alkane from an unsaturated alkene. Bromine adds across a carbon-carbon double bond, so only an alkene removes the colour.
(a) Before any hydrocarbon is added, bromine water is orange (yellow-orange or red-brown is also accepted) [1].
(b) Tube B decolourises the bromine water, so tube B contains the alkene, ethene [1]. Ethane in tube A is saturated, so it leaves the bromine water orange.
(c) Bromine adds across the double bond of ethene, so the word equation is: ethene + bromine \(\rightarrow\) dibromoethane (1,2-dibromoethane is accepted) [1]. The balanced formula equation is \(C_2H_4 + Br_2 \rightarrow C_2H_4Br_2\).
Exam tip: "orange to colourless" means unsaturated (has C=C); "stays orange" means saturated. Say colourless, not "clear".
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