Question

Industrially, sodium metal is obtained by electrolyzing molten sodium chloride. The reaction at the cathode is \(\mathrm{Na}^{+}+e^{-} \longrightarrow \mathrm{Na}\). We might expect that potassium metal would also be prepared by electrolyzing molten potassium chloride. However, potassium metal is soluble in molten potassium chloride and therefore is hard to recover. Furthermore, potassium vaporizes readily at the operating temperature, creating hazardous conditions. Instead, potassium is prepared by the distillation of molten potassium chloride in the presence of sodium vapor at \(892^{\circ} \mathrm{C}\) : $$ \mathrm{Na}(g)+\mathrm{KCl}(l) \rightleftharpoons \mathrm{NaCl}(l)+\mathrm{K}(g) $$ In view of the fact that potassium is a stronger reducing agent than sodium, explain why this approach works. (The boiling points of sodium and potassium are \(892^{\circ} \mathrm{C}\) and \(770^{\circ} \mathrm{C}\), respectively.)

Short answer

The approach works because at \(892^{\circ} \mathrm{C}\), sodium is in gas form and can readily react with molten KCl. This reaction displaces the K, which then turns into gaseous form due to the high temperature, and can be collected separately. The displacement occurs despite K being a stronger reducing agent than Na as K is initially in the less reactive state, being bonded with Cl.

Step-by-step solution

Understand the reaction

The reaction in this situation is: \( \mathrm{Na}(g)+\mathrm{KCl}(l) \rightarrow\mathrm{NaCl}(l)+\mathrm{K}(g) \). The main thing to consider is that both sodium and potassium can be considered reducing agents, with potassium being a stronger reducing agent than sodium.

Consider the boiling points

The boiling points of both elements are mentioned, specifically that of sodium and potassium are \(892^{\circ} \mathrm{C}\) and \(770^{\circ} \mathrm{C}\) respectively. These temperatures are vital when it comes to determining what state the elements will be in during the reaction, and thus their capacity to react.

Combine the information creating the solution

At \(892^{\circ} \mathrm{C}\), sodium is in gas form (this is its boiling point) and KCl is molten. In this state, Sodium can react with KCl readily, displacing the K and forming NaCl. Now, the K is in gaseous form (its boiling point is lower than the reaction temperature). Therefore, K (now in gaseous) form can be collected separately. This occurs successfully because although Potassium (K) is a stronger reducing agent than Sodium (Na), in this specific reaction, Na is in a more reactive state (gas), while Potassium is initially bonded with chlorine.

Conclude the solution based on the reaction process

The reaction is successful due to the fact the reaction happens at a temperature above both the boiling points of sodium and potassium, but with sodium being in an advantageous state to react with KCl, it does so readily, displacing potassium which in turn is then freed in a gaseous form that can be separately collected. This process is thus an efficient way to collect Potassium metal while minimizing the risk of hazardous conditions due to potassium vaporizing, as stated in the problem.

Key concepts

Sodium Metal Production

Sodium metal is a highly reactive alkali metal which is produced industrially through the process of electrolysis. Electrolysis is an electrochemical process which involves passing an electric current through a molten or dissolved salt to cause a chemical reaction at each electrode.

When it comes to the production of sodium metal specifically, molten sodium chloride, commonly known as table salt, is electrolyzed. In this electrochemical reaction, the molten salt is decomposed to produce sodium metal and chlorine gas. The reaction at the cathode can be represented as \( \mathrm{Na}^{+} + e^{-} \longrightarrow \mathrm{Na} \). This process requires a significant amount of electrical energy to initiate and maintain the reaction.

Sodium produced by this method is essential for various applications including the synthesis of organic compounds and the manufacture of synthetic rubber. It's also used in sodium vapor lamps which emit very efficient light.Electrolysis of molten salts is a foundational industrial process and understanding its specifics can clarify why certain elements, like sodium, can be successfully produced in this manner.

Potassium Metal Preparation

Potassium, another alkali metal similar to sodium, is more challenging to prepare due to its specific properties. In theory, one might attempt to prepare potassium metal through electrolysis of molten potassium chloride, analogous to the sodium production process. However, the method for potassium differs due to its solubility in molten potassium chloride and its low boiling point compared to the operational temperature.

Instead, an alternate method known as the sodium-potassium exchange reaction is used. The process takes place as follows: \[\mathrm{Na}(g) + \mathrm{KCl}(l) \rightleftharpoons \mathrm{NaCl}(l) + \mathrm{K}(g)\]. This reaction occurs at the boiling point of sodium, 892°C, where sodium is a gas that reacts with liquid potassium chloride to produce potassium gas and liquid sodium chloride. Since potassium's boiling point is lower than the reaction temperature, it can be distilled off and collected as a metal. Using sodium, which is less aggressive as a reducing agent, to facilitate the distillation of potassium may seem counterintuitive, but it is efficient because it takes advantage of the physical state of substances at high temperatures and their reactivity.

Reducing Agents in Chemistry

Reducing agents play a crucial role in chemical reactions, especially in processes such as electrolysis and metal extraction. A reducing agent, or reductant, is a substance that donates electrons to another substance, reducing its oxidation state. In the context of the sodium-potassium exchange reaction, both sodium and potassium can be classified as reducing agents, yet potassium is actually a stronger reducing agent than sodium.

In a chemical reaction, a stronger reducing agent will more readily lose electrons and reduce another substance. The fact that sodium, the weaker reducing agent in comparison to potassium, is able to facilitate the recovery of potassium as a metal is a result of the specific conditions of the reaction. At the high temperatures of the process, sodium's physical state allows it to interact with molten KCl and displace potassium, despite not being the more powerful reductant on paper.

This distinction is an important concept in chemistry because it shows that the effectiveness of a reducing agent can be influenced not only by its inherent properties but also by the reaction conditions. It's fundamental to consider both the chemical and physical properties of the elements involved to understand why certain reactions proceed the way they do.