Chapter 13: Problem 17

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 $\Delta H_{\text {mix }} ?$

Short Answer

Expert verified

(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 by step solution

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#

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.