Question

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

Short answer

(a) Electrolysis is a process where an electrical current is passed through a substance, causing a non-spontaneous redox reaction. The substance must be in a liquid state to allow for ion movement. (b) Electrolysis reactions are not thermodynamically spontaneous as they require an external electrical energy source to drive them. (c) In the electrolysis of molten NaCl, the anode process involves the oxidation of chloride ions (Cl⁻) to form chlorine gas (Cl₂) with the half-reaction: \( 2Cl^- \rightarrow Cl_{2} + 2e^- \). (d) Sodium metal is not obtained during electrolysis of an aqueous solution of NaCl because the reduction of water molecules to form hydrogen gas and hydroxide ions is energetically more favorable than the reduction of sodium ions to form sodium metal. Thus, hydrogen gas is produced at the cathode, not sodium metal.

Step-by-step solution

(a) Definition of electrolysis)

Electrolysis is a process by which an electrical current is passed through a substance, resulting in a non-spontaneous redox reaction. The substance being electrolyzed must be in a liquid state, either molten or dissolved in a solvent, that allows for the movement of ions. The electrical current promotes the transfer of chemical species (ions) in the solution.

(b) Spontaneity of electrolysis reactions)

Electrolysis reactions are not thermodynamically spontaneous. These reactions require an external electrical energy source to drive them. In essence, this energy converts the electrode potentials associated with the redox reaction into an external electrical current. The energy required for electrolysis can come from various sources, such as batteries or electrolytic cells.

(c) Anode process in the electrolysis of molten NaCl)

In the electrolysis of molten sodium chloride (NaCl), the process occurring at the anode (the positive electrode) involves the oxidation of chloride ions (Cl⁻) to form chlorine gas (Cl₂). The overall half-reaction taking place at the anode is: \[ 2Cl^- \rightarrow Cl_{2} + 2e^- \]

(d) Electrolysis of aqueous NaCl and sodium metal production)

When an aqueous solution of NaCl undergoes electrolysis, sodium metal is not obtained. This is because, in an aqueous solution, the reduction of water molecules (H₂O) to form hydrogen gas (H₂) and hydroxide ions (OH⁻) is energetically more favorable than the reduction of sodium ions (Na⁺) to form sodium metal (Na). The half-reaction occurring at the cathode (the negative electrode) can be represented as: \[ 2H_2O + 2e^- \rightarrow H_2 + 2OH^- \] As a result, hydrogen gas is produced at the cathode, and sodium metal is not formed during the electrolysis of an aqueous solution of NaCl.

Key concepts

Thermodynamics of Electrolysis

Understanding the thermodynamics of electrolysis is crucial in figuring out why electrical energy is needed to drive certain chemical reactions. Under normal conditions, spontaneous reactions occur due to a favorable balance of enthalpy and entropy contributing to a negative Gibbs free energy. However, in electrolysis, we're dealing with non-spontaneous reactions, where the Gibbs free energy is positive.

This is where external electrical energy comes into play. It provides the necessary energy to force the reaction to occur in the desired direction. The source of this electrical energy can be a battery or any other source of direct current. Essentially, you can think of electrolysis as a way to 'charge up' a chemical reaction so that electrons move against the natural 'gradient' of spontaneity.

Measuring the electrical energy input gives insight into the thermodynamic requirements of the process. By applying the required potential, called the electrolysis cell potential, persistent and directed movement of ions is achieved, leading to the desired chemical changes.

Anode Reaction in Electrolysis

In electrolysis, the anode is the positive electrode where oxidation takes place. During the electrolysis of molten NaCl, the chloride ions (Cl⁻) are attracted towards the anode. Once there, they release their excess electrons and are oxidized. This produces chlorine gas which bubbles away from the anode.

The chemical reaction can be represented by the equation:
\[ 2Cl^- \rightarrow Cl_{2} + 2e^- \]
This type of reaction, where a substance loses electrons, is known as an oxidation reaction. It's part of the redox processes occurring in the cell, where the loss of electrons at the anode is balanced by their gain at the cathode.

Electrolysis of Molten NaCl

The electrolysis of molten NaCl, or sodium chloride, is a simplified system that allows us to observe the basic principles of electrolysis without the complicating presence of water.

When molten, NaCl dissociates into its constituent ions, Na⁺ and Cl⁻. Upon applying an electrical current, sodium ions move towards the cathode, where they gain electrons and are reduced to sodium metal:\[ Na^{+} + e^- \rightarrow Na \].Simultaneously, as mentioned earlier, the chloride ions move to the anode to release electrons and form chlorine gas.

This process is critical for industries such as the production of aluminum and diverse chlorinated compounds by providing a method to separate elemental sodium and chlorine from their ionic compound.

Electrolysis of Aqueous NaCl

The electrolysis of aqueous NaCl presents a more complex scenario because of the involvement of water. Instead of producing sodium metal at the cathode, we witness the preferential discharge of water to form hydrogen gas and hydroxide ions. This surprising twist occurs because the reduction of water molecules requires less energy than the reduction of sodium ions.

Consequently, the actual half-reaction at the cathode is: \[ 2H_2O + 2e^- \rightarrow H_2 + 2OH^- \]
Sodium ions remain in solution and bond with the produced hydroxide ions to form sodium hydroxide (NaOH), which can still be a useful product. This phenomenon is central to the chlor-alkali industry, where chlorine, hydrogen gas, and sodium hydroxide are produced simultaneously through the electrolysis of brine, a concentrated solution of NaCl.