Problem 17 - Chemistry: The Central Science in SI Units, Global Edition

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 \(\Delta H_{\text {mix }} ?\)

Step-by-Step Solution

Answer

(a) Considering the very negative ΔH_soln, we would expect the ionic compound to be very soluble in water. (b) Among ΔH_solvent, ΔH_solute, and ΔH_mix, we would expect the largest negative number to be ΔH_mix, as it accounts for the exothermic interactions between the solute ions and water molecules during the dissolution process.

Step 1: In order to determine whether the ionic compound is very soluble or nearly insoluble, we have to consider the relationship between the enthalpy of solution (ΔH_soln) and solubility. Generally, a negative value of ΔH_soln indicates that the dissolution process is exothermic, meaning it releases heat. This typically results in an increase in solubility because as the process releases heat, it becomes more energetically favorable for the solute to dissolve in the solvent. (a) #Step 2: Assess the solubility of the ionic compound#

Since the ionic compound has a very negative ΔH_soln in water, we would expect it to be very soluble in water. (b) #Step 3: Understand the enthalpy changes for solute, solvent, and mixing process#

Step 2: The overall ΔH_soln can be written as the sum of three enthalpy changes: ΔH_soln = ΔH_solute + ΔH_solvent + ΔH_mix Where: - ΔH_solute is the enthalpy change when the solute (ionic compound) breaks apart into its individual ions in a hypothetical process. - ΔH_solvent is the enthalpy change when the solvent (water) molecules rearrange to accommodate the dissolved solute. - ΔH_mix is the enthalpy change when the solute and solvent combine and interact. #Step 4: Determine the largest negative term among ΔH_solvent, ΔH_solute, or ΔH_mix#

Since ΔH_soln is very negative and it is the sum of ΔH_solute, ΔH_solvent, and ΔH_mix, one or more of these terms must also be very negative. - ΔH_solute: The process of breaking apart the solute into its ions is typically endothermic, meaning it absorbs heat. So ΔH_solute is usually positive. - ΔH_solvent: The process of rearranging water molecules around the dissolved ions is also usually endothermic, absorbing heat. So ΔH_solvent is generally positive. - ΔH_mix: The process of combining the solute and solvent is usually an exothermic process, resulting in a negative enthalpy change as ions from the solute form strong interactions and attractions with the water molecules, thereby releasing heat. Considering the above points, we would expect the largest negative number to be ΔH_mix as it is the major exothermic phase of the dissolution process.

Key Concepts

Solubility of Ionic Compounds

When we talk about the solubility of ionic compounds, it's all about how well these compounds can dissolve in a solvent like water. An ionic compound consists of positively and negatively charged ions. When these compounds dissolve, the water molecules work their magic. They pull apart the individual ions, allowing them to spread throughout the liquid.
Here's the interesting part: the solubility of these compounds often varies. Some ionic compounds dissolve easily, while others don't. A major player in this behavior is the enthalpy of solution, denoted by \(\Delta H_{\text{soln}}\). If this value is negative, it suggests the process releases heat (exothermic), making dissolution more likely! So, if \(\Delta H_{\text{soln}}\) is highly negative, as in the exercise above, the compound is likely very soluble.
Factors influencing solubility include:

• The strength of attraction between solvent molecules and the ions.
• The lattice energy of the compound that needs to be overcome.
• Temperature and pressure conditions.

Understanding these factors provides insight into why some ionic compounds readily dissolve in water while others remain stubbornly solid.

Exothermic Processes

Exothermic processes are fascinating because they release energy into their surroundings, usually in the form of heat. When a process is exothermic, it is energetically favorable, meaning it's more likely to happen without the need for additional energy input.
In the context of dissolving ionic compounds, the interaction between the ions and water molecules can be exothermic. This part of the process, called \(\Delta H_{\text{mix}}\), often results in a negative enthalpy change. It suggests that the combination of solute and solvent releases energy, typically warming the solution slightly.
Here are some key aspects of exothermic processes:

• They lower the system's potential energy.
• They generally increase the surroundings' temperature.
• The process becomes more favorable, increasing solubility as energy is released.

Essentially, if exothermic interactions like these are significant, they drive the dissolution of ionic compounds, key to understanding why some solutions form readily.

Enthalpy Changes in Solution

Enthalpy changes when a solution forms involve several critical steps that determine whether the solution process will be energetically favorable. The overall change in enthalpy for a dissolving process is represented by \(\Delta H_{\text{soln}}\), which is the sum of three specific energy changes.

• \(\Delta H_{\text{solute}}\): The energy it takes to break apart the ionic bonds in a solute. This step typically requires energy (is endothermic), so \(\Delta H_{\text{solute}}\) is positive.
• \(\Delta H_{\text{solvent}}\): The energy needed to rearrange the solvent molecules, often water, so that they can interact with the solute. This too is usually endothermic and positive.
• \(\Delta H_{\text{mix}}\): When the solvent and solute interact, forming new attractions which release energy, making this step exothermic and negative.

To determine if a solution process is favorable, we look at which of these values dominates. In the original exercise, \(\Delta H_{\text{mix}}\) is likely the most substantial negative value, leading to the overall process being exothermic, with a negative \(\Delta H_{\text{soln}}\).
Understanding these changes helps predict whether a compound will dissolve well in a solvent, emphasizing the intimate balance of energetic exchanges.