Question

Explain why the products of the electrolysis of brine and the electrolysis of molten sodium chloride are different.

Short answer

The products of electrolysis of brine (a solution of sodium chloride in water) and molten sodium chloride differ because of the presence of water in brine and the different redox potentials of water and sodium chloride. In the electrolysis of brine, hydrogen gas, chlorine gas, and aqueous sodium hydroxide are formed due to the reduction of water molecules at the cathode and the oxidation of chloride ions at the anode. Conversely, in the electrolysis of molten sodium chloride, only sodium metal and chlorine gas are formed. The sodium ions are reduced at the cathode, and chloride ions are oxidized at the anode. The additional compound, water, increases the complexity of the brine electrolysis process, resulting in different products compared to the electrolysis of molten sodium chloride.

Step-by-step solution

Understand Electrolysis Process

Electrolysis is a process in which an electric current is passed through a substance to cause a chemical change. In this process, the substance decomposes into its constituent elements or simpler compounds, which are usually the products. The nature of these products depends on the substances involved and the conditions under which electrolysis takes place.

Composition of Brine and Molten Sodium Chloride

Brine is a concentrated solution of sodium chloride (NaCl) in water (H2O). Molten sodium chloride is a product of heating solid sodium chloride to a temperature of 801 °C (1474 °F), causing it to become a liquid.

Electrolysis of Brine

When electrolysis is carried out on brine, the main products are hydrogen gas (H2), chlorine gas (Cl2), and aqueous sodium hydroxide (NaOH): \[ 2H_2O \, \, \,_{(l)} + 2NaCl_{(aq)} \longrightarrow 2NaOH_{(aq)} + H_{2(g)} + Cl_{2(g)} \] In this process, water molecules are reduced at the cathode, forming hydrogen gas and hydroxide ions (OH⁻). At the anode, chloride ions (Cl⁻) are oxidized, producing chlorine gas. The hydroxide ions then combine with the sodium ions (Na⁺) to form sodium hydroxide in the solution.

Electrolysis of Molten Sodium Chloride

The electrolysis of molten sodium chloride, on the other hand, results in the formation of sodium metal (Na) and chlorine gas (Cl2) only, as shown in the following equation: \[ 2NaCl_{(l)} \longrightarrow 2Na_{(s)} + Cl_{2(g)} \] In this process, the chloride ions are oxidized at the anode to form chlorine gas, while the sodium ions are reduced at the cathode to form sodium metal.

Different Products Explanation

The presence of water in the electrolytic decomposition of brine introduces an additional compound and increases the complexity of the process. This results in the production of more products (hydrogen gas and sodium hydroxide), while in the case of electrolysis of molten sodium chloride, which contains only sodium and chloride ions, the products obtained are sodium metal and chlorine gas. The difference in chemical compositions and the redox potentials of water and sodium chloride are responsible for the difference in the products formed during electrolysis of each substance.

Key concepts

Brine

Brine is essentially a highly concentrated solution of sodium chloride (NaCl) dissolved in water. This mixture is commonly found in natural environments, such as in saltwater or the sea.
However, its uses go beyond just being part of larger bodies of water.

• Brine is frequently used in industrial processes, notably in the chlor-alkali industry.
• It's also utilized in the food industry for preserving foods through a salting process.

The presence of water in brine is crucial, especially when it comes to electrolysis. When an electric current is passed through brine, not only are the sodium and chloride components affected, but the water molecules play a significant role in determining the products of the reaction, making it different from molten sodium chloride.

Molten Sodium Chloride

Molten sodium chloride refers to sodium chloride that has been heated to a high temperature until it becomes a liquid. This process occurs at around 801 °C (1474 °F), overcoming the crystal lattice structure of solid NaCl.
This transformation is vital when studying electrolysis without the presence of water.

• In a molten state, sodium chloride is purely composed of Na⁺ and Cl⁻ ions without any impurities like water molecules.
• This lack of water results in a more straightforward electrochemical reaction during electrolysis.

When an electric current is applied, the only products produced are sodium metal and chlorine gas. This contrasts sharply with brine electrolysis, where water also participates in the reaction.

Chemical Reaction

Chemical reactions involve the transformation of substances through the breaking and forming of chemical bonds, resulting in new products. During electrolysis, the reactions depend on the nature of the substance involved and whether other elements, like water, are present.

• In brine electrolysis, a chemical reaction occurs where water, Na⁺, and Cl⁻ ions interact leading to the formation of H₂, Cl₂, and NaOH.
• A different set of chemical changes occurs when molten sodium chloride undergoes electrolysis, yielding Na and Cl₂.

Understanding how these reactions occur helps explain why brine and molten sodium chloride produce different products under electrolysis.

Redox Processes

Redox processes are chemical reactions that involve changes in oxidation states, typically through the transfer of electrons. Electrolysis is inherently a redox process as it requires an external current to induce these changes.

• Reduction occurs at the cathode, where electrons are gained.
• Oxidation happens at the anode, where electrons are lost.

In brine, the redox process involves complex interactions because of water, resulting in the production of hydrogen gas and hydroxide ions from water, alongside chlorine gas from the chloride ions. In molten sodium chloride electrolysis, the redox reactions occur between Na⁺ and Cl⁻ ions, resulting in sodium metal and chlorine gas. Recognizing these processes is key to understanding the differences in outcomes between the two types of electrolysis.