Question

Explain why we need to consider a van't Hoff factor for ionic solutes but not for molecular solutes.

Short answer

The van't Hoff factor is needed for ionic solutes as they dissociate into multiple ions, affecting particle count and colligative properties, unlike molecular solutes that remain as single particles.

Step-by-step solution

Understanding Ionic Solutes

Ionic solutes are compounds that dissociate into ions when dissolved in a solvent (usually water). For example, when table salt (NaCl) is dissolved in water, it separates into Na⁺ and Cl⁻ ions. This dissociation means that the number of particles in the solution increases, which impacts colligative properties such as boiling point elevation and freezing point depression.

Understanding Molecular Solutes

Molecular solutes consist of molecules that do not dissociate into ions in solution. They remain intact, and thus, the number of dissolved particles equals the number of molecules dissolved. For example, when sugar is dissolved in water, it stays as complete sugar molecules.

Effect of Dissociation on Colligative Properties

Colligative properties depend on the number of particles in solution rather than their identity. Since ionic solutes dissociate and increase the number of particles, their effect on colligative properties is greater compared to molecular solutes that do not dissociate.

Introduction to the van't Hoff Factor

The van't Hoff factor (\(i\)) is used to quantify the effect of solute dissociation on colligative properties. It represents the number of particles a solute forms in solution. For example, the van't Hoff factor for NaCl is approximately 2 because it dissociates into 2 ions.

Application of the van't Hoff Factor

While calculating changes in colligative properties, the van't Hoff factor is multiplied by the molality in calculations for ionic solutions, reflecting the actual number of particles. This adjustment is unnecessary for molecular solutes, where \(i=1\), since they do not dissociate.

Key concepts

Ionic Solutes

Ionic solutes are fascinating substances that transform when they meet a solvent, typically water. Imagine placing table salt (NaCl) into water. Here, a magical transformation occurs—this salt doesn't stay together like a rigid cube. Instead, it dissociates into sodium ions (Na⁺) and chloride ions (Cl⁻). This dissociation is crucial because it leads to an increased number of particles in the solution. More particles mean modifications in specific solution properties, known as colligative properties. Colligative properties change depending on the number of particles, not the nature of those particles. Hence, when ionic solutes like NaCl dissolve, creating more ions, they significantly impact these properties.

Molecular Solutes

Molecular solutes behave quite differently from ionic solutes. Picture dissolving sugar in water. The sugar molecules spread throughout the water but remain as whole entities. They do not break into ions but simply continue as complete molecules. This molecular behavior results in the total number of dissolved particles equaling the number of molecules you originally had. As a result, molecular solutes like sugar don’t drastically alter the colligative properties because there’s no increase in the number of particles post-dissolution.

Colligative Properties

Colligative properties are unique traits of solutions that depend purely on the concentration of solute particles. Whether dealing with ionic solutes or molecular solutes, these properties are indifferent to what type of particle is in the solution. Such properties include

• boiling point elevation
• freezing point depression
• vapor pressure lowering
• osmotic pressure

The more particles that a solute contributes to the solution, the more substantial its effect on these properties will be. Since ionic solutes dissociate into more particles, they have a more profound influence compared to non-dissociative molecular solutes.

Dissociation in Solution

Dissociation in solution is a key concept for understanding why the van't Hoff factor is essential for ionic but not molecular solutes. When an ionic compound dissolves, it separates into ions, which increases the total number of particles in the solution. This process is dissociation. For example, dissolving MgCl₂ in water yields one magnesium ion and two chloride ions, totaling three particles. This affected particle count demands the use of the van't Hoff factor, which corrects for the increase in particle number due to dissociation.
In contrast, molecular solutes don’t dissociate. They stay intact when dissolved, thus retaining their particle number without expansion. This is why the van't Hoff factor for molecular solutes is always 1, as there is no multiplier effect from dissociation.