Electrolysis
Francine Taylor-Campbell, Contributor

It is important to remember that when substances are electrolysed they decompose to form new products. So when molten aluminium oxide is electrolysed, it decomposes or forms aluminium and oxygen. Most molten substances break down to form their respective components.

The problems arise when we consider the electrolysis of substances that are in an aqueous phase, since the ions from water, which is a weak electrolyte, will participate in the reaction. The type of ions discharged at the anode or at the cathode will now depend on their position in the electrochemical series, their concentration and the nature of the electrode.

This means that if chloride ions and hydroxide ions are present in the same dilute solution, hydroxide ions will be discharged in preference to chloride ions because they are lower in the series. However, if the solution is concentrated, chloride ions will be the ones that are discharged at the anode.

Q1. The following equations represent changes that occur during electrolysis. Use only these equations to answer the questions.

(i) 2Cl^- = Cl₂ + 2e

(ii) Cu²⁺ + 2e = Cu

(iii) Cu = Cu²⁺ + 2e

(iv) 2H⁺ + 2e = H₂

(v) 4OH^- = 2H₂O + O₂ + 4e

(a) Write one equation which represents a reduction.

(b) Write the equation that shows the process which must occur when an object is electroplated with copper.

(c) Write two equations which show the processes that occur when dilute sulphuric acid is electrolysed.

ANSWERS

1. Cu²⁺ + 2e = Cu and 2H⁺ + 2e = H₂ represent a reduction since the ions are both gaining electrons.

(b) For an object to be electroplated copper ions must be made available with which to coat the object. Cu = Cu²⁺ + 2e shows the formation of Cu²⁺ ions.

(c) When dilute sulphuric acid is electrolysed, the H+ and OH- ions from water are discharged at the cathode and anode respectively. The equations are 2H⁺ + 2e = H₂ and 4OH^- = 2H₂O + O₂ + 4e.

The diagram represents the electrolysis of aqueous copper sulphate using copper electrodes.

(i) Give the formula of the four ions present in aqueous copper II sulphate.

(ii) Explain why the concentration of the electrolyte does not change as the electrolysis proceeds.

(iii) Explain why solid copper II sulphate does not conduct an electric current.

(b) Electrolysis is used to manufacture aluminium, chlorine and sodium hydroxide. For one of these processes give (i) the electrolyte used (ii) the materials used for the electrodes (iii) the commercial use of the product.

ANSWERS

2.(a) (i) Cu²⁺, H⁺, OH^-, SO₄²⁺

(ii) Copper dissolves from the anode based on this equation Cu = Cu²⁺ + 2e and copper is deposited on the cathode based on the equation Cu²⁺ + 2e = Cu. As copper ions are removed from solution, more copper dissolves which means that the amount of copper remains constant and the concentration does not change.

(iii) The ions are held in fixed positions and cannot move to carry an electric current.

(b) For aluminium, the electrolyte is bauxite Al₂O3 in cryolite using graphite electrodes. It is used to make aircraft parts. For chlorine, aqueous NaCl is used with graphite electrodes. It is used to sterilise drinking water.

NOTE: For sodium hydroxide, aqueous NaCi is the electrolyte using graphite anode and mercury (flowing) cathode. It's an important industrial chemical.

These William Knibb High School students share an English language past paper during the Youthlink's CSEC Exams Technique Seminar at the Montego Bay Civic Centre, St James, recently. - Photo by Sheena Gayle

Francine Taylor-Campbell is an independent contributor.