IB Chemistry Topic R3.2.15 Electrolysis of Aqueous Solutions Revision Notes (HL)

IB Chemistry Topic R3.2.15 Electrolysis of Aqueous Solutions Revision Notes (HL) | RevisionDojo

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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^\Theta $) 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:
1. Reduction of water:$$2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq) \quad E^\Theta = -0.83 \, \text{V}$$
2. Reduction of the cation from the dissolved salt, such as $ \text{Na}^+ $ or $ \text{Cu}^{2+} $.

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

Example

In $ \text{NaCl(aq)} $, the reduction of $ \text{Na}^+ $ ($ E^\Theta = -2.71 \, \text{V} $) is less favorable than the reduction of water.
- Thus, hydrogen gas is formed at the cathode.
In $ \text{CuSO}_4(aq) $, the reduction of $ \text{Cu}^{2+} $ ($ E^\Theta = +0.34 \, \text{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:
1. Oxidation of water:$$2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^- \quad E^\Theta = -1.23 \, \text{V}$$

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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^\Theta $) 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:
1. Reduction of water:$$2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq) \quad E^\Theta = -0.83 \, \text{V}$$
2. Reduction of the cation from the dissolved salt, such as $ \text{Na}^+ $ or $ \text{Cu}^{2+} $.

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

Example

In $ \text{NaCl(aq)} $, the reduction of $ \text{Na}^+ $ ($ E^\Theta = -2.71 \, \text{V} $) is less favorable than the reduction of water.
- Thus, hydrogen gas is formed at the cathode.
In $ \text{CuSO}_4(aq) $, the reduction of $ \text{Cu}^{2+} $ ($ E^\Theta = +0.34 \, \text{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:
1. Oxidation of water:$$2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^- \quad E^\Theta = -1.23 \, \text{V}$$

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry R3.2.15 Electrolysis of Aqueous Solutions Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Competing Reactions in Electrolysis

Reduction at the Cathode

Oxidation at the Anode

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Competing Reactions in Electrolysis

Reduction at the Cathode

Oxidation at the Anode

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