Ethene reacts readily with bromine at room temperature. Fig. 4.1 shows the displayed formula of ethene, the molecule that reacts. Fig. 4.1 (a) Bromine can b...
Question 1 Report
Ethene reacts readily with bromine at room temperature. Fig. 4.1 shows the displayed formula of ethene, the molecule that reacts.
Fig. 4.1
(a) Bromine can be added as bromine water. Describe what you would see when ethene is bubbled through bromine water. [2]
(b) State the type of reaction that occurs between ethene and bromine. [1]
(c) Name the product formed when ethene reacts with bromine. [1]
(d) Give the molecular formula of this product. [1]
(e) Draw the displayed formula of the product, showing every atom and every bond. [2]
(f) State whether the product is saturated or unsaturated, and explain your answer. [2]
(g) State what the fast reaction of ethene with bromine tells you about the ethene molecule. [1]
Answer Details
This question is about the addition reaction between ethene and bromine, testing the observation, reaction type, product, its formula and displayed structure, and what the reaction reveals about ethene.
(a) The orange (yellow-brown) colour of the bromine water [1] disappears; the mixture turns colourless, that is the bromine water is decolourised [1]. This colour change is the standard test for an unsaturated compound.
(b) The reaction is an addition reaction [1]: the bromine adds across the double bond and nothing is given off.
(c) The product is 1,2-dibromoethane (dibromoethane) [1].
(d) Its molecular formula is \( C_2H_4Br_2 \) [1]: the two bromine atoms simply add to the ethene, \( C_2H_4 \).
(e) The displayed formula shows the C=C double bond opened to a single bond, with one bromine atom added to each carbon and every bond drawn in full:
One mark for the correct single C-C bond with a bromine on each carbon, one mark for the correct four hydrogen atoms with all bonds shown [2].
(f) The product is saturated [1]: the C=C double bond has opened, leaving only single bonds, so no more atoms can add on [1].
(g) The fast reaction tells you that ethene is unsaturated, that is it contains a reactive carbon-carbon double bond that readily takes part in addition [1].
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