R3.2.15 Electrolysis of aqueous solutions (Higher Level Only)

Electrolytic Reactions and Applications

Competing Reactions in Electrolysis

Definition

Electrolysis

Electrolysis uses electrical energy to drive non-spontaneous redox reactions.

In aqueous solutions, both water and dissolved ions can participate in these reactions, creating competition at the electrodes.
The products depend on the standard electrode potentials ( $E^{Θ}$ ) of the reacting species.

Reduction at the Cathode

At the cathode, reduction occurs as electrons are supplied by the external circuit. The potential reductions in an aqueous solution include:

Reduction of water: $2 H_{2} O (l) + 2 e^{-} \to H_{2} (g) + 2 O H^{-} (a q) E^{Θ} = - 0.83 V$
Reduction of the cation from the dissolved salt, such as ${Na}^{+}$ or ${Cu}^{2 +}$ .

The species with the less negative (or more positive) reduction potential is preferentially reduced

Example

In $NaCl(aq)$ , the reduction of ${Na}^{+}$ ( $E^{Θ} = - 2.71 V$ ) is less favorable than the reduction of water. Thus, hydrogen gas is formed at the cathode.
In ${CuSO}_{4} (a q)$ , the reduction of ${Cu}^{2 +}$ ( $E^{Θ} = + 0.34 V$ ) is more favorable than the reduction of water. As a result, copper metal is deposited at the cathode.

Tip

Compare the standard electrode potentials to predict which species will be reduced at the cathode.

Oxidation at the Anode

At the anode, oxidation occurs as electrons are removed. The possible oxidations in an aqueous solution are:

Oxidation of water: $2 H_{2} O (l) \to O_{2} (g) + 4 H^{+} (a q) + 4 e^{-} E^{Θ} = - 1.23 V$
Oxidation of the anion from the dissolved salt, such as ${Cl}^{-}$ or ${SO}_{4}^{2 -}$ .

The species with the less negative (or more positive) oxidation potential is preferentially oxidized.

Example

In $NaCl(aq)$ , the oxidation of ${Cl}^{-}$ ( $E^{Θ} = - 1.36 V$ ) is slightly less favorable than the oxidation of water. However, at high concentrations of ${Cl}^{-}$ , chlorine gas ( $C l_{2}$ ) is preferentially produced at the anode.
In ${CuSO}_{4} (a q)$ , sulfate ions ( ${SO}_{4}^{2 -}$ ) cannot be oxidized further, so water is oxidized to oxygen gas ( $O_{2}$ ) at the anode.

Common Mistake

Many students assume that the oxidation of water always occurs at the anode. However, high concentrations of certain ions, like ${Cl}^{-}$ , can shift the reaction to produce other products.

Example

Electrolysis of $NaCl(aq)$

At the cathode: $2 H_{2} O (l) + 2 e^{-} \to H_{2} (g) + 2 O H^{-} (a q)$
Hydrogen gas is formed.
At the anode: $2 C l^{-} (a q) \to C l_{2} (g) + 2 e^{-}$
Chlorine gas is formed.
Overall reaction: $2 N a C l (a q) + 2 H_{2} O (l) \to H_{2} (g) + C l_{2} (g) + 2 N a O H (a q)$
Sodium hydroxide remains in the solution.

Applications of Electrolysis

Electrolysis of Sodium Chloride ( $NaCl(aq)$ )

The chlor-alkali process is a critical industrial application of electrolysis, producing chlorine gas, hydrogen gas, and sodium hydroxide.
These products have diverse uses:
- Chlorine gas: Used in disinfecting water and manufacturing PVC.
- Hydrogen gas: Used in fuel cells and ammonia production.
- Sodium hydroxide: Used in making soap and cleaning agents.

Note

In dilute $NaCl(aq)$ , oxygen gas may form at the anode due to the oxidation of water instead of ${Cl}^{-}$ .

Electrolysis of Copper(II) Sulfate ( ${CuSO}_{4} (a q)$ )

This process demonstrates how electrode material affects the products:

Inert Electrodes (e.g., Carbon or Platinum):
- At the cathode: ${Cu}^{2 +} (a q) + 2 e^{-} \to Cu (s)$ (copper metal is deposited).
- At the anode: $2 H_{2} O (l) \to O_{2} (g) + 4 H^{+} (a q) + 4 e^{-}$ (oxygen gas is released).
Reactive Electrodes (Copper):
- At the cathode: Copper is deposited as before.
- At the anode: Copper metal is oxidized to ${Cu}^{2 +}$ , replenishing the ${Cu}^{2 +}$ ions in solution.

This principle is applied in electroplating and copper purification.

Example

Copper Purification

Impure copper is used as the anode, and pure copper is used as the cathode.
At the anode: $Cu (s) \to {Cu}^{2 +} (a q) + 2 e^{-}$ .
At the cathode: ${Cu}^{2 +} (a q) + 2 e^{-} \to Cu (s)$ .
Impurities either fall off as sludge or remain in solution.

Tip

When purifying copper, ensure that the electrolyte contains ${CuSO}_{4}$ to maintain the concentration of ${Cu}^{2 +}$ ions.

Reflection

Self review

What determines whether water or the salt ions are reduced/oxidized during electrolysis?
Why is chlorine gas produced in concentrated $NaCl(aq)$ but not in dilute solutions?
How does the choice of electrode material affect the products of electrolysis?

Theory of Knowledge

Electrolysis connects science and ethics.

How do we balance industrial progress with sustainability?
Can the adoption of "green" electricity transform the environmental impact of electrolysis?

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Electrolysis of Aqueous Solutions

Electrolysis is a process that uses electrical energy to drive chemical reactions that wouldn't occur spontaneously. In aqueous solutions, both water and dissolved ions can be involved in these reactions, leading to competition at the electrodes.

Electrolysis involves redox reactions, where reduction occurs at the cathode and oxidation occurs at the anode.
In aqueous solutions, both water and the dissolved ions can be reduced or oxidized.
The products formed depend on the standard electrode potentials ( $E^\circ$ ) of the competing reactions.

Analogy

Think of electrolysis as a competition where multiple players (ions and water) are trying to score (get reduced or oxidized). The one with the best 'position' (more favorable electrode potential) usually wins.

Definition

Electrolysis

The process of using electrical energy to drive a non-spontaneous chemical reaction.

Example

In the electrolysis of

\text{NaCl(aq)}

, both water and

\text{Na}^+

ions can be reduced at the cathode, but only one will actually occur.

Note

Always consider both the ions from the salt and water when predicting electrolysis products in aqueous solutions.

R3.2.15 Electrolysis of aqueous solutions (Higher Level Only)

Electrolytic Reactions and Applications

Competing Reactions in Electrolysis

Reduction at the Cathode

Oxidation at the Anode

Electrolysis of NaCl(aq)

Applications of Electrolysis

Electrolysis of Sodium Chloride (NaCl(aq))

Electrolysis of Copper(II) Sulfate (CuSO4(aq))

Copper Purification

Reflection

You've read 2/2 free chapters this week.

Electrolysis of Aqueous Solutions

Electrolysis of $NaCl(aq)$

Electrolysis of Sodium Chloride ( $NaCl(aq)$ )

Electrolysis of Copper(II) Sulfate ( ${CuSO}_{4} (a q)$ )