Question

An ionic compound has a very negative \(\Delta H_{\text {soln }}\) in water. Would you expect it to be very soluble or nearly insoluble in water? Explain in terms of the enthalpy and entropy changes that accompany the process.

Short answer

An ionic compound with a very negative \(\Delta H_{\text{soln}}\) in water is likely to be very soluble in water. This is because the dissolution process is exothermic, releasing energy which favors solubility. Also, the dissolution leads to an increase in entropy (disorder), which is typically associated with spontaneous processes. Since both enthalpy and entropy changes favor solubility, we can expect the ionic compound to be highly soluble in water.

Step-by-step solution

Understand the given information

We know that the ionic compound has a very negative ΔHsoln in water, which means that the dissolution process is exothermic (releases heat).

Understand the concept of solubility

The solubility of a substance is defined as the maximum amount of it that can dissolve in a certain amount of solvent at a specific temperature. In general, the solubility of a substance increases with increasing temperature, and an increase of entropy (ΔS) usually favors solubility.

Analyze the enthalpy change (ΔH)

A negative ΔHsoln indicates that the dissolution process is exothermic. As a result, the attractive forces between the ions in the ionic compound and the water molecules are stronger than the attractive forces holding the ions together in the ionic solid. The dissolution process releases energy, which is favorable for solubility.

Analyze the entropy change (ΔS)

When an ionic compound dissolves in water, the ions in the solid separate into free ions in the aqueous solution. This process leads to an increase in the disorder or randomness (i.e., an increase in entropy, ΔS) of the system. Since an increase in entropy is typically associated with spontaneous processes, the dissolution of the ionic compound in water will likely be favored.

Combine enthalpy and entropy effects

The dissolution of the ionic compound in water is accompanied by both a negative ΔH (favorable) and an increase in ΔS (favorable). Therefore, we can expect the ionic compound to be very soluble in water.

Key concepts

Enthalpy Change

When we talk about enthalpy change (\( \Delta H \)) in chemistry, we're referring to the heat absorbed or released during a chemical process. For ionic compounds dissolving in water, this change can be either positive (endothermic) or negative (exothermic).

A very negative enthalpy change indicates that the process is exothermic, meaning heat is released when the compound dissolves. This happens because the new interactions between the ions and the water molecules release more energy than is required to break the ionic bonds in the solid. As a result, such reactions often happen spontaneously and can lead to high solubility of the compounds in water.

Keep in mind:

• Exothermic processes, where heat is released, are generally favorable for solubility.
• The more negative the \( \Delta H \), the more energetically favorable the process is.

Entropy Change

Entropy, denoted by \( \Delta S \), is a measure of disorder or randomness in a system. During the dissolution of an ionic compound in water, entropy plays a crucial role.

When an ionic compound dissolves, the structured, orderly solid breaks apart into ions that are free to move in the solution. This process increases the system's disorder, resulting in a positive change in entropy. High entropy values often signify that the process is more likely to be spontaneous and favorable.

Key points about entropy in dissolving ionic compounds:

• Increased disorder usually promotes solubility.
• The separation of ions from the solid to free ions increases the system's entropy.
• A positive \( \Delta S \) enhances the likelihood of dissolution.

Ionic Compounds

Ionic compounds are composed of positive and negative ions held together by strong electrostatic forces called ionic bonds. These compounds have unique properties when it comes to solubility.

In water, an ionic compound usually dissolves if the attraction between the water molecules and the ions overcomes the ionic bonds holding the compound together. Water, being a polar solvent, is particularly good at interacting with charged ions.

For ionic compounds:

• If water can more effectively stabilize the ions than the compound itself, it dissolves.
• The interaction between water molecules and the ions must release enough energy to favor dissolution.
• Solubility also depends on temperature and pressure, but strong water-ion interactions are key.

Understanding how both enthalpy and entropy changes affect ionic compounds can help explain why some are very soluble while others are not.