A student added a portion of orange bromine water to four different hydrocarbons and shook each mixture. The observations are shown in Table 3.1. hydrocarbo...
Question 1 Report
A student added a portion of orange bromine water to four different hydrocarbons and shook each mixture. The observations are shown in Table 3.1.
| hydrocarbon | formula | observation with bromine water | saturated or unsaturated |
|---|---|---|---|
| methane | CH4 | stays orange | saturated |
| ethene | C2H4 | ? | unsaturated |
| propane | C3H8 | stays orange | ? |
| butene | C4H8 | orange to colourless | ? |
(a) Complete Table 3.1 by filling in the three boxes marked with a question mark. [3]
(b) Name the type of reaction that takes place when bromine reacts with an alkene. [1]
(c) State which two compounds in the table are alkenes. [1]
Answer Details
This question tests the link between structure (saturated alkane vs unsaturated alkene) and the bromine water result. An alkene has a C=C double bond, so it decolourises bromine water; an alkane does not.
(a) The completed table is:
| hydrocarbon | formula | observation with bromine water | saturated or unsaturated |
|---|---|---|---|
| methane | CH4 | stays orange | saturated |
| ethene | C2H4 | orange to colourless | unsaturated |
| propane | C3H8 | stays orange | saturated |
| butene | C4H8 | orange to colourless | unsaturated |
Ethene decolourises the bromine water (orange to colourless) because it is unsaturated [1]; propane fits the general formula \(C_nH_{2n+2}\) and leaves the colour, so it is saturated [1]; butene fits \(C_nH_{2n}\) and decolourises the bromine water, so it is unsaturated [1]. [3]
(b) When bromine reacts with an alkene, bromine atoms add across the double bond, so the reaction type is addition [1].
(c) The two alkenes are ethene and butene [1] (their names end in "-ene" and their formulae fit \(C_nH_{2n}\)).
Exam tip: a formula fitting \(C_nH_{2n}\) (twice as many H as C) is an alkene; \(C_nH_{2n+2}\) is an alkane.
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