ElectrolysisIB MYP Chemistry: Subtopic test

1. 1
   A technician in a school laboratory in Nairobi electrolyses molten lead(II) bromide, PbBr₂, using inert graphite electrodes inside a fume cupboard.

   (a)

   Why must solid lead(II) bromide be melted before it will conduct electricity?

   [1 mark]

   • AMelting makes electrons in the compound free to move
   • BMelting turns the compound into a metal
   • CThe ions are free to move when the compound is molten
   • DMelting creates ions that were not there before

   (b)

   Which row shows the products formed at the electrodes?

   [1 mark]

   • ACathode: lead; anode: bromine
   • BCathode: bromine; anode: lead
   • CCathode: hydrogen; anode: bromine
   • DCathode: lead; anode: oxygen

   (c)

   Write the half-equation for the change at the anode.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student electrolyses blue copper(II) sulfate solution for 20 minutes using inert graphite electrodes. A pink-brown solid coats the cathode, bubbles of a colourless gas form at the anode and relight a glowing splint, and the blue colour of the solution fades slightly.

   (a)

   What is the gas formed at the anode?

   [1 mark]

   • AHydrogen
   • BChlorine
   • CCarbon dioxide
   • DOxygen

   (b)

   Why does the blue colour of the solution fade?

   [1 mark]

   • AWater is evaporating from the solution
   • BCopper(II) ions are being removed from the solution as copper is deposited
   • CSulfate ions are turning into copper
   • DThe graphite electrodes are dissolving in the solution

   (c)

   The student now plans to investigate how the size of the current affects the mass of copper deposited at the cathode in 20 minutes. Identify the independent variable and the dependent variable.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemical plant in Jubail, Saudi Arabia, electrolyses concentrated sodium chloride solution (brine) using inert electrodes. The process makes three useful products.

   (a)

   Explain why hydrogen, and not sodium, forms at the cathode.

   [3 marks]

   (b)

   Name the product formed at the anode and outline one use of each of the three products of this process.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A jewellery workshop in Jaipur coats cheap brass bracelets with a thin layer of silver by electroplating, using a solution of silver nitrate. A nearby factory also electroplates metal parts with chromium, and its waste solutions contain toxic metal ions. Local residents have asked the council whether electroplating should be allowed to continue in the area.

   (a)

   Describe how a brass bracelet can be silver plated by electroplating, including the half-equation at the cathode.

   [6 marks]

   (b)

   Discuss whether the council should allow electroplating to continue in the area.

   [6 marks]

   Total for question 4: 12 marks

End of questions