Give an example of a salt composed of a cation and an anion,...

Name: Problem 7, Chapter 3: Principles of Descriptive Inorganic Chemistry
Uploaded: 2020-07-24T15:28:47-08:00
Duration: 2 min 7 s
Channel: Arpit Gupta
Description: Give an example of a salt composed of a cation and an anion, each of which is an electrostatic structure maker. Is such a salt likely to be soluble or insoluble? Give the physical reason for this (in terms of the structure of the hydrated ions, the suitability of its lattice, etc.). Is this solubility/insolubility associated with entropy or enthalpy effects?

Give an example of a salt composed of a cation and an anion, each of which is an electrostatic structure maker. Is such a salt likely to be soluble or insoluble? Give the physical reason for this (in terms of the structure of the hydrated ions, the suitability of its lattice, etc.). Is this solubility/insolubility associated with entropy or enthalpy effects?

Key Concepts

Salt Solubility Mechanism

A salt composed of a cation and an anion that are both electrostatic structure makers (for example, a salt like lithium fluoride) is likely to be insoluble. This is because the favorable interactions within the well-organized ionic lattice and the strong hydration interactions together result in a situation where the energy required to break the lattice and disrupt the preformed hydration structure outweighs the thermodynamic benefits. Hence, the lack of solubility is mainly associated with enthalpy effects, as the energy required to overcome lattice cohesion and re-order water molecules is not sufficiently counteracted by the enthalpic gains upon hydration.

Enthalpy vs Entropy in Solvation

The overall thermodynamic favorability of dissolving a salt depends on both the enthalpy changes (such as the release or absorption of energy when forming or breaking bonds) and the entropy changes (the degree of order or disorder). In the case of salts with strong structure maker ions, the large enthalpic stabilization of the lattice and the significant ordering in the hydration shells work against dissolution, often making the process enthalpically unfavorable despite an entropic cost from increased order.

Hydration Shell Formation

When ions dissolve in water, they become surrounded by hydration shells, where water molecules arrange themselves in an ordered fashion around the ion. For structure makers, this ordering is particularly strong, which can lead to a decrease in the overall entropy of the system. The formation of these rigid hydration structures contributes significantly to the enthalpy of the system during solvation.

Electrostatic Structure Maker Ions

Electrostatic structure maker ions are small, highly charged ions that strongly orient the water molecules in their hydration shells, leading to a very ordered and stable structure. This ordering effect influences the behavior of water and is important in determining how ions interact with each other and with the solvent during dissolution processes.

Ionic Lattice and Lattice Energy

In an ionic solid, cations and anions are held together by strong electrostatic forces forming a crystal lattice. The lattice energy, which is the energy required to separate these ions, is a key factor in determining the salt’s solubility. For salts where both ions are effective structure makers, the lattice may be particularly stable, making it energetically expensive to break apart in solution.

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