AlkenesIB MYP Chemistry: Subtopic test

1. 1
   A student compares two colourless gases, ethane, C₂H₆, and ethene, C₂H₄. Both gases burn, but they behave differently when she bubbles each through bromine water.

   (a)

   Which is the general formula of the alkenes?

   [1 mark]

   • ACₙH₂ₙ
   • BCₙH₂ₙ₊₂
   • CCₙH₂ₙ₋₂
   • DCₙH₂ₙ₊₁

   (b)

   Which feature of an ethene molecule makes it an alkene?

   [1 mark]

   • AAn –OH group
   • BOnly single bonds between its carbon atoms
   • CA carbon–carbon double bond
   • DA –COOH group

   (c)

   Explain why ethene is described as an unsaturated hydrocarbon.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician has two unlabelled colourless liquids. One is pentane, an alkane, and the other is pentene, an alkene. She adds 2 cm³ of orange-brown bromine water to 2 cm³ of each liquid in separate test tubes and shakes each tube.

   (a)

   What is the molecular formula of pentene?

   [1 mark]

   • AC₅H₁₂
   • BC₅H₁₀
   • CC₅H₈
   • DC₅H₁₁

   (b)

   Why does bromine water react with pentene but not with pentane?

   [1 mark]

   • APentene has more hydrogen atoms than pentane
   • BPentane contains oxygen atoms
   • CPentane has a longer carbon chain than pentene
   • DPentene has a carbon–carbon double bond, which opens so that bromine atoms can add on

   (c)

   Describe what the technician sees in each test tube and how she can tell which liquid is pentene.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student has four colourless liquids labelled P, Q, R and S. Each is either an alkane or an alkene. She adds 2 cm³ of bromine water to 2 cm³ of each liquid in a test tube, shakes it and records the colour of the mixture after 1 minute. P and R stay orange-brown. Q and S become colourless.

   (a)

   Identify the independent variable, the dependent variable and one control variable in the student's investigation.

   [3 marks]

   (b)

   Identify which liquids are alkenes and give a reason. State one safety precaution she should take when using bromine water. Write the balanced symbol equation for the reaction of ethene, C₂H₄, with bromine, Br₂.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A food company in Malaysia makes a spread by reacting liquid vegetable oil with hydrogen gas at about 150 °C in the presence of a nickel catalyst. The molecules in the oil contain carbon–carbon double bonds. The product is a solid fat that is spreadable at room temperature. Some health campaigners want the process to be restricted.

   (a)

   Discuss the benefits and drawbacks of hardening vegetable oils by reacting them with hydrogen.

   [6 marks]

   (b)

   A technician says: "If bromine water stays orange-brown when we add it to the product, all the double bonds have reacted." Explain what happens to the double bonds during the reaction and evaluate the technician's claim.

   [6 marks]

   Total for question 4: 12 marks

End of questions