Electrolysis

1. Name the products formed when the following substances are electrolysed using inert electrodes: molten copper bromide (CuBr2 )  ;  molten zinc oxide (ZnO)  ;  aqueous sodium chloride (NaCl)  ;  aqueous potassium nitrate (KNO3 )

Answer: copper and bromine  ;  zinc and oxygen  ;  hydrogen and chlorine  ;  hydrogen and oxygen


2. Give one reason why a metal would be extracted from its ore using electrolysis rather than heating with carbon.

Answer: It is more reactive than carbon (so carbon is unable to displace it fromits compound). The metal would react with carbon (in an unwanted reaction).


3. Give the half equation for the oxidation of chloride ions to form chlorine gas.

Answer: 2Cl  →  Cl2 + 2e


4. When a solution of sodium sulfate is electrolysed, hydrogen gas is formed at the cathode and oxygen gas is formed at the anode. Which two ions are discharged to form these two gases?

Answer: Hydrogen ions (H+) are discharged to form hydrogen gas, and hydroxide ions (OH) are discharged to form oxygen gas.


5. Balance the following half equations. Write down whether each one shows oxidation or reduction: Cl  →  Cl2 + e  ;  H+ + e  →  H2  ;  OH  →  H2O + O2 + e

Answer: 2Cl  →  Cl2 + 2e (oxidation)  ;  2H+ + 2e  →  H2 (reduction)  ;  4OH  →  2H2O + O2 + 4e (oxidation)


6. This question is about the extraction of aluminium from its ore, bauxite. Bauxite contains aluminium oxide. During the electrolysis process carbon electrodes are used to pass electricity through the molten aluminium oxide, as shown in the figure below.


a) Explain why the aluminium oxide must be molten for electrolysis to occur.

Answer: Electricity is the flow of charged particles. When molten, the ions are free to move and carry the charge.


b) Explain why aluminium oxide is mixed with cryolite before electrolysis takes place.

Answer: Mixing aluminium oxide with cryolite lowers the melting point of the aluminium oxide. Less energy is needed to melt the mixture, so the cost is less.


c) Describe the changes that take place during the electrolysis of aluminium oxide to form aluminium and oxygen gas.

Answer: Aluminium ions (Al3+) are attracted to the negative electrode (cathode). The aluminium ions gain electrons to be discharged at the cathode as aluminium atoms / Al3+ + 3e  →  Al.
Oxide ions (O2−) are attracted to the positive electrode (anode). The oxide ions lose electrons to be discharged at the anode as oxygen gas / 2O2−  →  O2 + 4e.


d) Give the reason why the positive carbon electrodes need to be replaced regularly.

Answer: They gradually burn away.


7. A student investigated the electrolysis of aqueous copper sulfate. The figure below shows the apparatus the student used to carry out the electrolysis reaction, while the table shows the student’s observations.


a) Explain why the student did not need to heat the copper sulfate before it would conduct electricity.

Answer: When dissolved in water the ions are already free to move and carry the charge.


b) Explain why graphite electrodes are able to conduct electricity.

Answer: Graphite contains delocalised electrons.


c) The red-brown solid that formed at the negative electrode was copper metal. Explain why copper metal formed at this electrode.

Answer: Copper ions (Cu2+) are attracted to the negative electrode where they gain electrons to form copper atoms.


d) Identify the gas formed at the positive electrode.

Answer: oxygen gas


e) Name the ions that are discharged at the positive electrode to form this gas.

Answer: hydroxide ions (OH)


f) The student then decided to electrolyse a solution of sodium sulfate (Na2SO4 ). Describe the difference in the observation at the negative electrode compared with the electrolysis of copper sulfate solution.

Answer: A colourless gas would be formed at the negative electrode (instead of a red-brown metal). Sodium ions (Na+) are more reactive than hydrogen so harder to discharge (or vice versa). Instead, hydrogen ions (H+) from water are discharged. Hydrogen ions gain electrons to form hydrogen gas / 2H+ + 2e  →  H2


8. The figure below shows the apparatus used for the electrolysis of dilute sulfuric acid.


a) Name the ions that are present in a solution of sulfuric acid.

Answer: hydrogen ions (H+) and sulfate ions (SO42−)


b) The product at the negative electrode is a colourless gas. Identify the colourless gas formed at the negative electrode.

Answer: hydrogen gas (H2 )


c) Give the half equation to show the formation of this gas.

Answer: 2H+ + 2e  →  H2


d) The product at the positive electrode is also a colourless gas. Identify the colourless gas formed at the negative electrode.

Answer: oxygen gas (O2 )


e) Give the half equation to show the formation of this gas.

Answer: 4OH  →  O2 + 2H2O + 4e-


f) 4 cm3 of gas was collected at the negative electrode at room temperature and pressure. Calculate the number of moles of this gas that were produced. The volume of one mole of any gas at room temperature and pressure is 24.0 dm3.

Answer: moles = volume (in dm3) ÷ 24.0 = (4 ÷ 1000) ÷ 24.0 = 0.00016667 mol