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

Short answer

(a) The ionic compound would be expected to be very soluble in water, as the very negative ΔH_soln suggests strong interactions with water molecules and a large amount of energy is released in the dissolution process. (b) The largest negative number is expected to be ΔH_mix, as it represents the enthalpy change associated with the formation of new solute-solvent interactions, which contributes to the overall very negative ΔH_soln.

Step-by-step solution

Determine the solubility in water

Based on the given information that ΔH_soln is very negative, we can deduce that the overall process of dissolving the ionic compound in water releases a large amount of energy. This suggests that the compound likely forms strong interactions with water molecules, which would lead to a high solubility in water. Answer (a): The ionic compound would be expected to be very soluble in water.

Analysing the enthalpy terms

To answer the second question, we need to understand the three enthalpy terms involved in the dissolution process: 1. ΔH_solvent: This is the enthalpy change associated with breaking solvent (water) molecules' interactions to create space for the solute particles. 2. ΔH_solute: This term represents the enthalpy change involved in breaking the interactions between solute particles. 3. ΔH_mix: This term represents the enthalpy change associated with the formation of new solute-solvent interactions in the mixture. Note that ΔH_soln = ΔH_solvent + ΔH_solute + ΔH_mix. Since ΔH_soln is very negative, this implies that the energy gained through the formation of new solute-solvent interactions is significantly greater than the energy required to break the solute and solvent interactions.

Identify the largest negative term

Considering the relationship between the enthalpy terms and the solubility of the compound in water, we can deduce that: 1. ΔH_solvent and ΔH_solute should be positive or near zero, as energy is required to break the interactions between solvent and solute particles. 2. ΔH_mix must be very negative, as a large amount of energy is released through the formation of strong solute-solvent interactions, making the overall ΔH_soln very negative. Answer (b): We expect the largest negative number to be ΔH_mix.

Key concepts

Enthalpy of solution

When a substance dissolves in a solvent, the enthalpy of solution (\(\Delta H_{ ext{soln}}\)) captures the total energy change in this process.
The enthalpy of solution can be positive or negative, depending on whether the dissolution absorbs or releases heat. For an ionic compound with a very negative \(\Delta H_{ ext{soln}}\), energy is released during the process.
This indicates a strong exothermic reaction, where the system loses energy as new bonds form between the ions and the solvent molecules.
Enthalpy changes depend on several sub-processes:

• Breaking of solute-solute interactions
• Breaking of solvent-solvent interactions
• Formation of new solute-solvent interactions

In the specific case of an ionic compound dissolving in water with a very negative \(\Delta H_{ ext{soln}}\), the energy given off from forming these new interactions is greater than the energy required to break the original solute and solvent interactions.

Solute-solvent interactions

When substances dissolve, solute-solvent interactions become crucial. These interactions determine how well a solute can be integrated into a solvent.
If \(\Delta H_{ ext{mix}}\) is highly negative, this indicates the formation of strong solute-solvent interactions. These interactions often lead to higher solubility in the solvent, as the solute particles are surrounded and stabilized by the solvent molecules.
In the case of ionic compounds dissolving in water, the water molecules, which are polar, can effectively solvate the ions. This happens due to electrostatic attraction between the ionic charges and the dipole of water molecules.
When new interactions form between the solute ions and water molecules, they can release a substantial amount of energy. This released energy further aids in attracting more solute into the solvent, enhancing the overall dissolving process.

Dissolution process

The dissolution process of a solute in a solvent is essentially the pathway where a material transitions from a solid or gaseous state into a uniformly distributed mixture.
This process involves breaking and forming chemical bonds:

• Breaking Solute Bonds: Initial steps involve overcoming intermolecular forces keeping solute molecules together.
• Breaking Solvent Bonds: The solvent must also accommodate the solute, necessitating disruption of some intermolecular bonds in the solvent.
• Forming Solute-Solvent Bonds: New interactions occur between solute and solvent molecules, driving the final steps of dissolution.

In a situation where \(\Delta H_{ ext{soln}}\) is very negative, it suggests the last step, forming solute-solvent interactions, is highly favorable.
The energy released is significant, often more than compensating the energy absorbed to disrupt solute and solvent bonds. This results in net energy release, reinforcing the solubility and efficiency of the dissolution process.