Question

(a) What is electrolysis? (b) Are electrolysis reactions thermodynamically spontaneous? (c) What process occurs at the anode in the electrolysis of molten \(\mathrm{NaCl}\) (d) Why is sodium metal not obtained when an aqueous solution of NaCl undergoes electrolysis?

Short answer

(a) Electrolysis is a non-spontaneous process in which electrical energy is used to drive chemical reactions, leading to the decomposition of a compound. (b) Electrolysis reactions are not thermodynamically spontaneous, as they require an external source of electrical energy. (c) In the electrolysis of molten NaCl, chloride ions are oxidized at the anode to form chlorine gas, with the half-reaction: \(2\mathrm{Cl}^-(l) \rightarrow \mathrm{Cl}_2(g) + 2e^-\). (d) Sodium metal is not obtained during electrolysis of aqueous NaCl solution because water has a lower reduction potential than sodium, and its reduction at the cathode is favored over sodium ions, with the half-reaction: \(2\mathrm{H}_2\mathrm{O}(l) + 2e^- \rightarrow \mathrm{H}_2(g) + 2\mathrm{OH}^-(aq)\).

Step-by-step solution

(a) Definition of Electrolysis)

Electrolysis is a process in which electrical energy is used to drive non-spontaneous chemical reactions, leading to the decomposition of a compound. In this process, an electric current is passed through an electrolyte (a liquid or solution that conducts electricity), causing the transfer of electrons between the electrodes and the electrolyte, and the subsequent formation of new substances at the electrodes.

(b) Spontaneity of Electrolysis Reactions)

Electrolysis reactions are not thermodynamically spontaneous. They require an external source of electrical energy to drive the reaction and overcome the activation energy barrier. The process is non-spontaneous as the standard reduction potential of the cathode is less than the standard reduction potential of the anode.

(c) Process at the Anode during Electrolysis of Molten NaCl)

In the electrolysis of molten NaCl, the process that occurs at the anode (positive electrode) involves the oxidation of chloride ions to form chlorine gas. The overall half-reaction that takes place at the anode is: \[2\mathrm{Cl}^-(l) \rightarrow \mathrm{Cl}_2(g) + 2e^-\]

(d) Reason for the absence of Sodium Metal during Electrolysis of Aqueous NaCl Solution)

When an aqueous solution of NaCl undergoes electrolysis, sodium metal is not obtained due to the presence of water molecules. Water has a lower reduction potential than sodium, and thus it gets reduced in preference to sodium ions. At the cathode (negative electrode), water molecules are reduced to form hydrogen gas and hydroxide ions, rather than the reduction of sodium ions to sodium metal. The overall half-reaction at the cathode is: \[2\mathrm{H}_2\mathrm{O}(l) + 2e^- \rightarrow \mathrm{H}_2(g) + 2\mathrm{OH}^-(aq)\]

Key concepts

Thermodynamics of Electrolysis

Electrolysis is a fascinating process because it lets us drive chemical reactions that wouldn't happen on their own. This is because the reactions involved are not thermodynamically spontaneous.

For a chemical reaction to occur without external help, it needs to be spontaneous, meaning it naturally tends to happen under the given conditions. In simple terms, a reaction like a spontaneous tumble down a hill doesn’t need extra energy.

However, in electrolysis, we deal with reactions that are like pushing a ball uphill! Here, electrical energy is necessary to force these reactions to happen, as they have a positive Gibbs free energy change (ΔG > 0).

Since the standard reduction potential at the cathode is less than at the anode, we're going against the natural flow of electrons, so the process requires a push — and that's given by an electrical current.

Anode Reactions

An important part of electrolysis is the reaction that occurs at the anode, or the positive electrode. This is where oxidation happens — remember, oxidation is when a substance loses electrons.

In the specific case of molten sodium chloride (\( \mathrm{NaCl} \)), chloride ions (\( \mathrm{Cl}^- \)) are oxidized at the anode. This means they lose electrons to form chlorine gas, which is released. Let’s break down the reaction at the anode:

• The chloride ions contribute two electrons to the circuit.
• Ordinarily invisible chloride ions transform into visible chlorine gas as a product.

This can be represented by the half-reaction: \(2 \mathrm{Cl}^-(l) \rightarrow \mathrm{Cl}_2(g) + 2e^- \)

What's cool here is thinking about how a simple solution of table salt eventually gives off chlorine gas, used in many industrial processes, due to the magic of electrons hopping off!

Aqueous Sodium Chloride Electrolysis

Electrolyzing an aqueous sodium chloride solution is quite different from molten electrolysis. Due to the presence of water, electrons are more "tempted" to reduce water molecules instead of sodium ions.

This is because water has a lower reduction potential than sodium, making it a "more attractive" option for gaining electrons. Let's see what happens:

• At the cathode (negative electrode), water molecules get reduced instead of sodium, forming hydrogen gas and hydroxide ions (\( \mathrm{OH}^- \)).
• As a result, we see bubbling hydrogen gas escaping at the cathode.

This process can be expressed through the following half-reaction:\(2 \mathrm{H}_2\mathrm{O}(l) + 2e^- \rightarrow \mathrm{H}_2(g) + 2\mathrm{OH}^-(aq)\)

Thus, instead of sodium metal, we get hydrogen gas - another surprising twist, where the medium (water) directly influences the course of the reaction!