Question

Elemental \(\mathrm{Li}\) and \(\mathrm{Na}\) are prepared by electrolysis of a molten salt, whereas \(\mathrm{K}, \mathrm{Rb},\) and \(\mathrm{Cs}\) are prepared by chemical reduction. (a) In general terms, explain why the alkali metals cannot be prepared by electrolysis of their aqueous salt solutions. (b) Use ionization energies (see the Family Portraits for Group \(1 \mathrm{~A}(1)\) in Section 14.3 and for Group \(2 \mathrm{~A}(2)\) in \(\operatorname{Section} 14.4)\) to explain why calcium should not be able to isolate \(\mathrm{Rb}\) from molten \(\operatorname{Rb} \mathrm{X}(\mathrm{X}=\) halide \()\) (c) Use physical properties to explain why calcium is used to isolate Rb from molten \(\mathrm{RbX}\). (d) Can Ca be used to isolate Cs from molten CsX? Explain.

Short answer

Alkali metals can't be prepared by electrolysis of aqueous solutions because they react with water. Calcium can't isolate Rb due to its insufficient ionization energy but can do so chemically. Calcium can also isolate Cs from CsX.

Step-by-step solution

- Problem Statement

The exercise involves understanding the preparation of alkali metals through electrolysis and chemical reduction, focusing on their ionization energies and physical properties.

- Part (a) Explanation

Alkali metals cannot be prepared by electrolysis of their aqueous salt solutions because they are highly reactive and will readily react with water. During electrolysis, the metal ion would be reduced at the cathode. However, in an aqueous solution, the water would be reduced instead, forming hydrogen gas and hydroxide ions, rather than the metal.

- Part (b) Ionization Energies

The ionization energy is the energy required to remove an electron from an atom. For calcium, the first ionization energy is lower than for alkali metals like Rb. This means that calcium atoms lose their outermost electrons more readily compared to Rb atoms. Therefore, calcium cannot effectively reduce Rb ions in molten RbX because its ability to ionize doesn't surpass that of Rb.

- Part (c) Physical Properties of Calcium and Rubidium

Calcium is used to isolate Rubidium from molten RbX because calcium has a higher affinity to form stable compounds with halides. When calcium is introduced to molten RbX, it reacts and forms CaX2 while releasing metallic rubidium due to calcium's stronger reducing properties in this context.

- Part (d) Isolation of Cs from CsX

Calcium can also be used to isolate Cs from molten CsX. Since cesium and rubidium have similar chemical properties (both being in Group 1A), the reactivity of calcium with the halide and the subsequent release of metallic cesium will mimic that process seen with rubidium.

Key concepts

Electrolysis

Electrolysis is a process where electrical energy is used to drive a chemical reaction that would not normally occur on its own. For alkali metals, electrolysis is often used to obtain pure metals from their compounds. When these metals are in an aqueous solution, the presence of water complicates the process. During electrolysis, we want to reduce the metal ions at the cathode. But water is more easily reduced, forming hydrogen gas and hydroxide ions. This means, instead of getting pure alkali metals, we would get hydrogen gas. So, preparing alkali metals via electrolysis requires a molten salt, where water is absent, ensuring the metal ions can be directly reduced to their elemental form.

Ionization Energy

Ionization energy is the energy needed to remove an electron from an atom. It plays a crucial role in the reactivity of elements. For example, calcium's first ionization energy is lower than that of rubidium. This means calcium loses its outermost electron more easily compared to rubidium. When trying to use calcium to isolate rubidium from a compound like RbX, calcium’s lower ionization energy makes it a less effective reducer according to initial expectations. Actually, it’s calcium’s affinity for forming stable compounds with halides that objects its reducing capability.

Chemical Reduction

Chemical reduction involves gaining electrons. In the context of preparing alkali metals, chemical reduction is used when electrolysis isn't practical due to water reactivity. For metals like potassium, rubidium, and cesium, their salts are reduced using a chemical method. This normally involves another element like calcium, which reacts with the halides (compounds involving halogens) in the molten form. The calcium replaces the alkali metal, which is then freed in its pure metal form. The higher tendency of calcium to form stable compounds with halides makes it an efficient reducer for these alkali metal preparations.

Alkali Metals

Alkali metals are found in Group 1 of the periodic table, characterized by having a single electron in their outermost shell. This single electron configuration makes them highly reactive, especially with water, which is why they are not typically prepared from aqueous solutions via electrolysis. Instead, methods such as the electrolysis of molten salts and chemical reduction are used. The high reactivity of alkali metals increases from lithium to cesium down the group due to decreasing ionization energy and increasing atomic size. Their high reactivity necessitates careful handling and specific preparation methods.

Reactivity

Reactivity describes how readily a substance engages in chemical reactions. For alkali metals, reactivity increases as you move down the group in the periodic table. This is due to the decrease in ionization energy. For example, lithium is less reactive than potassium, rubidium, or cesium. Rubidium and cesium, being more reactive, are prepared using chemical reduction instead of electrolysis. In a reactive scenario, calcium can help isolate these metals from their compounds by reacting more readily with the halides, allowing the metallic alkali metal to be released.