Electrolysis of solutions

Solutions of ionic compounds in water contain hydrogen and hydroxide ions, as well as the ions of the solute. These H+ and OH- ions compete at the electrodes with the solute ions.

The ions that are successfully released at the electrodes depend on three factors

1. 1 The position of the ion in the electrochemical series
2. 2 The concentration of the ion in the solution
3. 3 The nature of the electrode

1 The position of the ion in the electrochemical series

This is probably better expressed as the position of the redox equilibrium in the electrochemical series. All the redox equilibria are expressed as reductions going from left to right, for example:

Cu2+ + 2e Cu

However, going from right to left this would be an oxidation. Hence it may be called a redox equilibrium.

The Copper ion || Copper equilibrium has an electrode potential of +0.34 V. This is more positive than hydrogen's electrode potential of 0.00 V. This means that copper ions are more easily oxidised than hydrogen ions (remember that as we go down the series the redox equilibria have better oxidising properties, i.e they can remove electrons from other things more easily - including electrodes). Consequently in a competition between the two ions, the copper ions will preferentially pick up the electrons.

As a rule of thumb, if the metal appears below hydrogen in the electrochemical series it will be preferentially deposited.

---

2 The concentration of the ions

When two ions with similar reactivity are in competition then the relative concentration of the two ions becomes an important factor.

A good example of this is the electrolysis of aqueous sodium chloride. When the chloride concentration is large, the chloride ions lose electrons and chlorine gas is released at the electrode, but when it is in low concentration, the hydroxide ions from the water are preferentially released.

---

3 The nature of the electrode

Usually, inert electrodes such as graphite or platinum are used for electrolysis. These electrodes do not interfere with the reactions occuring at the surface of the electrode, they simply act as a point of contact between the electrical circuit and the solution.

However, if metal electrodes are used in metal ion solutions they can get involved in the reactions by dissolving as ions, leaving their electrons behind (this can only happen when the metal takes the place of the anode, the positive electrode) - this is called electrode participation, it is discussed in greater detail in section 9.73.

---

top

Electrolysis of sodium chloride solution

The ions present in the solution are: sodium ions, chloride ions, hydrogen ions, hydroxide ions

At the cathode

The positive ions are attracted to the negative cathode. There is competition between the sodium ions, Na⁺(aq), and the hydrogen ions, H⁺(aq). As the hydrogen ion || hydrogen redox equilibrium is lower in the electrochemical series than the sodium ion || sodium redox equilibrium, the hydrogen ions are preferentially reduced (hydrogen ions are a better oxidising agent than sodium ions) and hydrogen gas is produced at the electrode (bubbles are seen)

2H+ + 2e → H2

One way to remember this is to consider what would happen if sodium were to be liberated in water. Sodium is far too reactive and would react immediately with the water producing hydrogen gas. The end result would be the same - hydrogen gas is produced

At the anode

There is competition between the negative ions at the positive anode. The chloride ions compete with the hydroxide ions to release their electrons to the anode. When the solution is fairly concentrated the chloride ions preferentially lose electrons to become chlorine atoms (and then molecules)

2Cl- - 2e → Cl2

However, in dilute solutions of sodium chloride the hydroxide ions are preferentially oxidised. This is a concentration effect.

Ions remaining in solution

The ions that are removed from the solution, then, are the hydrogen ions and the chloride ions. This means that the sodium ions and the hydroxide ions remain in the solution - i.e sodium hydroxide is also produced.

Note: When the solution of chloride ions is dilute then OH^-. ions are preferentially released at the anode.

---

top

Electrolysis of copper(II) sulfate solution

The ions present in the solution are: copper ions, Cu²⁺; sulfate ions,SO₄^2-; hydrogen ions, H+; and hydroxide ions, OH-.

At the cathode

The positive ions are attracted to the negative cathode. There is competition between the copper ions and the hydrogen ions. As the hydrogen ion | hydrogen redox equilibrium appears higher in the electrochemical series than the copper ion | copper equilibrium, then the copper ions are preferentially reduced and copper metal is deposited at the electrode (a pink layer is observed)

Cu2+ + 2e → Cu

At the anode

There is competition between the negative ions at the positive anode. The sulfate ions compete with the hydroxide ions to release their electrons to the anode. The hydroxide ions are much better reducing agents and are preferentially released AS OXYGEN GAS and water

4OH- - 4e → 2H2O + O2

Ions remaining in solution

The ions that are removed from the solution, then, are the copper ions and the hydroxide ions. This means that the hydrogen ions and the sulfate ions remain in the solution - i.e sulfuric acid is also produced.

---

top

Electrolysis of dilute sulfuric acid

Ions present: hydrogen ions, H⁺; sulfate ions, SO₄^2-; hydroxide ions, OH^-.

In this case there is no choice at the cathode; hydrogen ions must be released as hydrogen gas.

2H+ + 2e → H2

At the anode

There is competition between the sulfate ions and the hydroxide ions. sulfate ions are too big and bulky to ever be released, therefore it must be the hydroxide ions:

4OH- - 4e → 2H2O + O2

Ions remaining in solution

The only ions removed are hydrogen ions (not all of them) and hydroxide ions. Therefore the ions remaining are hydrogen ions and sulfate ions.

These are the ions of sulfuric acid. So the net result of electrolysing sulfuric acid is that the sulfuric acid gets more concentrated and the water solvent is electrolysed producing hydrogen oxygen gas at the electrodes.

For this reason the process is often known as the electrolysis of water - it is only the elements of water that are released.

---

top

Summary

Reactive metals (more reactive than hydrogen) are never deposited during electrolysis of aqueous solutions. If the metal ion comes from a metal more reactive than hydrogen, then hydrogen gas is liberated at the cathode.

Halide ions (chloride, bromide, iodide) are released preferentially and if these are not present, the hydroxide ions from the water are released at the anode.

---

---

top