Problem 57 - General Chemistry: Principles and Modern Applications

Question

The melting point of \(\mathrm{NaCl}(\mathrm{s})\) is \(801^{\circ} \mathrm{C},\) much higher than that of \(\mathrm{NaOH}\left(322^{\circ} \mathrm{C}\right) .\) More energy is consumed to melt and maintain molten NaCl than NaOH. Yet the preferred commercial process for the production of sodium is electrolysis of \(\mathrm{NaCl}(\mathrm{l})\) rather than \(\mathrm{NaOH}(1)\) Give a reason or reasons for this discrepancy.

Step-by-Step Solution

Answer

Electrolysis of NaCl is preferred over NaOH due to its higher stability, reflected in its higher melting point, which makes for more efficient sodium production. Its widespread abundance and lower cost, being easily sourced from seawater, also make it more commercially viable despite the high energy consumption required to melt it.

Step 1: Understand the Properties of NaCl and NaOH

The first step is to understand the properties of NaCl and NaOH. NaCl is a strong electrolyte and it exists abundantly in nature. It has a high melting point which simply means it's rather stable. NaOH, on the other hand, while it is also a strong electrolyte, it's less stable and also less abundant in nature.

Step 2: Analyze the Electrolysis Process

The electrolysis of these substances involves their decomposition into constituent elements. So for NaCl and NaOH, this would result in production of sodium (Na). However, considering NaCl's stability (reflected by its higher melting point), it means the production of sodium through its electrolysis would be more efficient and reliable.

Step 3: Evaluate Commercial Reasoning

From a commercial perspective, electrolysis of NaCl may be the preferred process because of its costs and availability. Molten NaCl can be easily sourced from seawater, making it a cheaper and more readily available resource than NaOH. Despite the increased energy consumption required to reach its melting point, the overall cost and the efficiency of this process make it preferable.

Key Concepts

Chemical Properties of NaCl

Sodium chloride (NaCl), commonly known as salt, is an essential compound with a myriad of applications, including use in the culinary world and as a de-icing agent. Chemically, NaCl is an ionic compound formed by the electrostatic attraction between positively charged sodium ions (Na⁺) and negatively charged chloride ions (Cl^–). Being an ionic compound contributes to NaCl's considerable stability and its ability to conduct electricity when dissolved in water or molten state.

Due to its strong ionic bonds, NaCl tends not to react with other substances under normal conditions, hence its notable chemical inertness. Its presence in vast amounts in the earth's oceans as dissolved salts makes it not only abundant but also a cornerstone of numerous industrial processes, including the electrolysis to produce elemental sodium.

Melting Point of Ionic Compounds

The melting point of an ionic compound is a direct reflection of the strength of the bonds between its ions. For NaCl, this is observed at a robust temperature of 801°C, showcasing the substantial attraction between the sodium and chloride ions within its crystal lattice structure. Ionic compounds typically possess high melting points due to these energetic ion-ion interactions. In contrast, covalent compounds, which are bonded by shared electrons, usually melt at lower temperatures.

The melting point is not solely a measure of bond strength but also can indicate the thermal stability of a compound, which has implications for its usage. High melting points in industrial processes typically require more energy expenditure to transition the substance into a liquid state, affecting the cost-efficiency and likewise the method selection for chemical production.

Commercial Production of Sodium

The commercial production of elemental sodium predominantly occurs through the electrolysis of molten NaCl, commonly known as the Downs process. While energy-intensive due to the high melting point of NaCl, the abundance and availability of the salt make it a preferable starting input over other compounds like NaOH.

During the electrolytic process, molten NaCl decomposes into sodium (Na) and chlorine gas (Cl₂). The sodium produced is a highly reactive metal, which finds uses in various applications such as in the creation of organic compounds and as a reducing agent in metallurgy. The efficiency and reliability of producing sodium via electrolysis of NaCl, despite higher energy demands, are offset by the scalability and environmental aspects of sourcing NaCl, especially from seawater. Consequently, the overall cost of production remains commercially viable for the massive market that relies on pure sodium.