How do ionic solutes affect the boiling point?

Question

Natalie Anton · Accepted Answer

undefined Ionic solutes affect the boiling point of a solvent by increasing it. When an ionic solute, such as a salt, is dissolved in a solvent, such as water, the resulting solution experiences boiling point elevation. This phenomenon occurs due to the disruption of the solvent's vapor pressure by the presence of the solute particles.

In a pure solvent, molecules escape from the liquid phase into the vapor phase, exerting pressure known as vapor pressure. When a solute is added, especially an ionic solute, it forms ions in the solution. These ions attract water molecules, reducing their ability to escape into the vapor phase. As a result, the vapor pressure of the solution is lower than that of the pure solvent.

According to Raoult's Law, the vapor pressure of a solution is directly proportional to the mole fraction of the solvent. Since the presence of the solute decreases the mole fraction of the solvent, the vapor pressure of the solution is lower than that of the pure solvent. Consequently, the boiling point of the solution is elevated to a temperature at which its vapor pressure matches the external pressure, usually atmospheric pressure.

The extent of boiling point elevation depends on the concentration of the solute particles in the solution. Higher concentrations of ionic solutes result in greater boiling point elevation. Additionally, the number of ions formed by the solute also influences the boiling point elevation. For example, a salt like sodium chloride (NaCl) dissociates into two ions (Na⁺ and Cl⁻) in solution, resulting in a more significant effect on the boiling point compared to a non-ionic solute that does not dissociate.

In summary, ionic solutes increase the boiling point of a solvent by reducing its vapor pressure, leading to boiling point elevation in the solution.

Natalie Anton · Answer

Ionic solutes raise the boiling point more than nonionic solutes at the same concentration do.

Boiling point elevation is a colligative property. It depends only on the number of particles in the solution.

Solute particles are distributed throughout the solution. They "get in the way" of the solvent particles when the solvent wants to evaporate, so we must heat the solution to a higher temperature to make it boil.

The formula for boiling point elevation is

#ΔT_b = iK_b m#

where #m# is the molality of the solution, #K_b# is the molal boiling point elevation constant for the solvent, and #i# is a number related to the number of particles the solute contributes to the solution (the van’t Hoff #i# factor).

The van't Hoff #i# factor is a number that tells you how many moles of solute particles you get from 1 mol of solute.

Non-electrolytes don't dissociate when they dissolve. Thus, one mole of glucose will have one mole of particles in solution, and #i# = 1.

#ΔT_b = K_b m#

NaCl dissociates into Na⁺ and Cl⁻ in water. So if you have 1 mol of NaCl, you'll have 2 mol of particles and i= 2

#ΔT_b = 2K_b m#

For CaCl₂, i = 3, for FeCl₃, I = 4, etc.

Thus, a 1 mol/kg solution of FeCl₃ will raise the boiling point of water 4 times as much as a 1 mol/kg solution of glucose.

EXAMPLE

Calculate the boiling point of a 0.15 mol/kg aqueous solution of sodium chloride. #K_b# for water is 0.512 °C·kg·mol⁻¹.

Solution

#ΔT_b = iK_b m# = 2 × 0.512 °C·kg·mol⁻¹× 0.15 mol·kg⁻¹ = 0.15 °C

#T_b = T_b^o + ΔT_b# = 100.00 °C + 0.15 °C = 100.15 °C

Wyatt Atkins · Answer

undefined Ionic solutes, when dissolved in a solvent, such as water, increase the boiling point of the solution compared to the pure solvent. This is due to the presence of ions in the solution, which disrupt the intermolecular forces between solvent molecules, making it more difficult for them to escape into the vapor phase. As a result, a higher temperature is required to reach the vapor pressure necessary for boiling. The extent of the boiling point elevation depends on the concentration of the ionic solute and the number of ions it dissociates into in the solution, according to the formula:

$$
\Delta T_b = i \cdot K_b \cdot m
$$

Where:
- $\Delta T_b$ is the boiling point elevation,
- $i$ is the van't Hoff factor (the number of particles the solute dissociates into in solution),
- $K_b$ is the ebullioscopic constant of the solvent,
- $m$ is the molality of the solution (moles of solute per kilogram of solvent).