Problem 17 - Chemistry The Central Science In Si Units

Question

An ionic compound has a very negative \(\Delta H_{\text {soln }}\) in water. (a) Would you expect it to be very soluble or nearly insoluble in water? (b) Which term would you expect to be the largest negative number: \(\Delta H_{\text {solvent }}, \Delta H_{\text {solute }}\), or \(\Delta H_{\operatorname{mix}} ?\)

Step-by-Step Solution

Answer

(a) The compound is very soluble in water. (b) \( \Delta H_{\text{mix}} \) is the most negative term.

Step 1: Understanding ΔHsoln

The enthalpy change of solution \( \Delta H_{\text{soln}} \) is defined as the sum of the enthalpies of the solvent, solute, and mixing processes. Mathematically, \( \Delta H_{\text{soln}} = \Delta H_{\text{solvent}} + \Delta H_{\text{solute}} + \Delta H_{\text{mix}} \). A very negative \( \Delta H_{\text{soln}} \) implies that the solution process releases a significant amount of energy, indicating that the solute likely dissolves exothermically.

Step 2: Analyzing Solubility

For a compound to be very soluble in a solvent, the energy released (exothermic process) when new interactions form between solute and solvent particles must outweigh the energy required to break the solute-solute and solvent-solvent interactions. Given that \( \Delta H_{\text{soln}} \) is very negative, it is likely that this compound is very soluble in water as energy released is higher than energy consumed.

Step 3: Evaluating the Components of ΔHsoln

Since \( \Delta H_{\text{soln}} \) is very negative, the most negative contribution typically comes from \( \Delta H_{\text{mix}} \), which is the energy associated with forming new solute-solvent interactions. This value needs to be large and negative to outweigh any positive values from \( \Delta H_{\text{solvent}} \) and \( \Delta H_{\text{solute}} \), indicating strong interactions between solute and solvent molecules.

Key Concepts

Enthalpy Change of Solution

The enthalpy change of solution, denoted as \( \Delta H_{\text{soln}} \), refers to the total energy change when a solute dissolves in a solvent. This process involves three main components: breaking the solute particles apart (\( \Delta H_{\text{solute}} \)), separating the solvent molecules (\( \Delta H_{\text{solvent}} \)), and forming new interactions between the solute and solvent (\( \Delta H_{\text{mix}} \)). Together, these determine whether the dissolution process is endothermic (absorbing energy) or exothermic (releasing energy).
When \( \Delta H_{\text{soln}} \) is very negative, it means that the energy released during the mixing process is much greater than the energy required to break the initial solute and solvent interactions.
This is an indicator of a strong exothermic process, where the overall energy balance favors dissolution. Essentially, more energy is released during the creation of new bonds compared to the energy needed to break the initial ones.

Exothermic Dissolution

In an exothermic dissolution, the process of dissolving a solute in a solvent releases energy. This occurs when the new interactions formed between the solute and solvent are stronger than the original interactions within the solute and solvent separately.
There are a few key characteristics of exothermic dissolution:

• The temperature of the solution may increase due to the release of energy.
• It often results in a spontaneous dissolution process.
• The solute tends to be very soluble if the temperature rise significantly helps in overcoming the solution's energy barriers.

In simpler terms, if dissolving something in water feels hot or warm to the touch, it's likely undergoing exothermic dissolution.
This type of dissolution not only facilitates the solubility of many compounds but can also aid in understanding why certain substances dissolve more completely and quickly than others.

Solute-Solvent Interactions

Solute-solvent interactions are the cornerstone of the dissolution process. These interactions occur when the solute particles get surrounded and stabilized by the solvent molecules. Strong interactions between the solute and solvent mean that the solute will likely be very soluble in the solvent.
The component \( \Delta H_{\text{mix}} \), representing the change in enthalpy due to solute-solvent interactions, needs to be significantly negative to ensure that a solute dissolves well.
Solute-solvent interactions can include:

• Hydrogen bonding
• Dipole-dipole interactions
• Ionic interactions, especially in water due to its polarity

These interactions lower the energy of the system. This is why the formation of these strong new bonds is crucial. They help offset the energy required to disrupt the original solute and solvent interactions, thereby favoring dissolution and creating a stable solution.