Question

Which one of the following steps in the dissolving process must have a negative value for \(\Delta E\) ? Explain your answer. (a) The physical separation of solute particles (b) The formation of solvent-solute interactions (c) The physical separation of solvent particles (d) None of the above

Short answer

The correct answer is (b) The formation of solvent-solute interactions. This step has a negative value for $\Delta E$ because energy is released when new attractive forces between solvent and solute particles are formed. In contrast, the separation of solute and solvent particles requires energy, corresponding to positive energy changes.

Step-by-step solution

Understanding the energy changes in the dissolving process

The dissolving process involves energy changes in three main steps: the separation of solute particles, the formation of solvent-solute interactions, and the separation of solvent particles. 1. Separation of solute particles: Energy is required to break the attractive forces between solute particles. This corresponds to a positive energy change. 2. Formation of solvent-solute interactions: Energy is released when new attractive forces between solvent and solute particles are formed. This corresponds to a negative energy change. 3. Separation of solvent particles: Energy is needed to break the attractive forces between the solvent particles to make space for the solute particles. This corresponds to a positive energy change.

Determining the correct answer

According to the energy changes explained in Step 1, only the formation of solvent-solute interactions has a negative value for the energy change as more energy is released when new attractive forces between solvent and solute particles are formed. Answer: (b) The formation of solvent-solute interactions.

Key concepts

Solute-Solvent Interactions

When a solute dissolves in a solvent, there is a complex dance of attractions and movements at the molecular level. Solute-solvent interactions are the attractive forces that occur between the particles of the solute and the particles of the solvent. These can range from ionic bonds in the case of salts in water to hydrogen bonds and van der Waals forces for different types of substances and solvents.

During the dissolution process, the solvent molecules surround the solute particles, which can be ions or molecules, and work to separate them from their larger bulk structure. The key to these interactions is to lower the overall energy of the system, leading to a stable solution. This stability is achieved because energy is released when the solute and solvent particles interact, which is often referred to as the 'heat of solution'. The interesting part here is that even though energy is used to separate the initial solute-solute and solvent-solvent interactions, if the new solute-solvent attractions are stronger, the overall process can release energy.

Enthalpy Change of Solution

Enthalpy change of solution, denoted as \( \Delta H_{solution} \), is a term in chemistry that refers to the energy change that occurs when a solute dissolves in a solvent to form a solution. It can be thought of as the heat effect associated with the dissolving process at constant pressure. The three key steps involved in dissolving—solute separation, solvent separation, and solute-solvent interaction—all contribute to the total enthalpy change.

The sign of \( \Delta H_{solution} \) tells us whether the dissolving process is endothermic (positive \( \Delta H \)) or exothermic (negative \( \Delta H \)). An interesting feature of this change is that it doesn't only reflect the strength of the interactions within the solution, but also offers insight into whether the dissolution process is favored energetically. A negative \( \Delta H_{solution} \) indicates that the process releases heat and the solute-solvent interactions are particularly strong, leading to a spontaneous dissolution under certain conditions.

Physical Separation of Particles

The physical separation of particles in the dissolving process refers to the initial stages where the attractive forces holding the solute particles together, and those keeping the solvent molecules together, must be overcome. This separation is crucial for making space for the solute and solvent particles to interact and for the solution to form.

In the case of a solute like table salt (NaCl) dissolving in water, energy is required to break the ionic bonds between the Na+ and Cl- ions (solute particles), as well as to disrupt the hydrogen bonds between water molecules (solvent particles). While solute particle separation and solvent particle separation often require an input of energy (positive enthalpy change), resulting in an endothermic process, the overall dissolving process can still be exothermic. This is due to the strength of the new solute-solvent interactions that can release more energy than what was consumed to separate the particles, thereby making the dissolution process energetically favorable overall.