User:Michaelmok1010/Boiling-point elevation

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Boiling-point elevation is the event in which the boiling point of a liquid (a solvent) increases when a non-volatile solute is added into the solvent. This results in a pure solvent having a lower temperature than its solution. The boiling point can be measured accurately using an ebullioscope.

*Broke up the lead sentence as it was too long

Article body

- The boiling point elevation is a colligative property, which means that boiling point elevation is dependent on the number of dissolved particles, but not their identity. **When integrated in the original text, will be 5**. Boiling point elevation occurs when a solute is mixed with a solvent, and the solute dilutes the solvent. This event happens for all solutes in solutions, including the Ideal solution, and is independent of specific solute–solvent interactions. Boiling point elevation occurs when a solution is both an electrolyte, like salts, and a non-electrolyte, which can be explained by the Van't Hoff Factor. Due to this factor, the boiling point increases as electrolytes dissociate and contribute more particles while non-electrolytes do not dissociate but increase solute concentration.

In terms of thermodynamic, boiling point elevation has an entropic origin and can be explained by using the vapor pressure or chemical potential. The vapor pressure affects the solute shown by Raoult's Law while the free energy change and chemical potential are shown by Gibbs free energy. Most solutes remain in the liquid phase and do not enter the gas phase, except at very high temperatures.

In terms of vapor pressure, a liquid boils when its vapor pressure equals the surrounding pressure. A nonvolatile solute lowers the solvent’s vapor pressure, meaning a higher temperature is needed for the vapor pressure to equalize the surrounding pressure, causing the boiling point to elevate.

In terms of chemical potential, at the boiling point, the liquid and gas phases have the same chemical potential. Adding a nonvolatile solute lowers the solvent’s chemical potential in the liquid phase, but the gas phase remains unaffected. This shifts the equilibrium between phases to a higher temperature, elevating the boiling point.**Original text [1] Citation**

Relationship between Freezing-point Depression (new section)

Freezing-point depression is analogous to boiling point elevation, though the magnitude of freezing-point depression is higher for the same solvent and solute concentration. These phenomena extend the liquid range of a solvent in the presence of a solute.

*Made changes with wording and changing some passive sentences to their active form to give a more concise text. (Major wording changes are in bold)

*Added relationship to Van't Hoff Factor and Raoult's Law

- will be adding diagrams that visualize how the solute increases boiling point (a microscopic view of molecule movements / space for movements)

Related Equations for Calculating Boiling Point (previously "The equation for calculations at dilute concentration")

The extent of boiling-point elevation can be calculated by applying Clausius–Clapeyron relation and Raoult's law together assuming the non-volatility of the solute. The result is that in dilute ideal solutions, the extent of boiling-point elevation is directly proportional to the molal concentration (amount of substance per mass) of the solution according to the equation:

ΔT_b = K_b · b_c

where the boiling point elevation, is defined as T_{b (solution)} − T_{b (pure solvent)}.

• K_b, the ebullioscopic constant, which depends on the properties of the solvent. K_b can be calculated as K_b = RT_b²M/ΔH_v, where R is the gas constant; T_b is the boiling temperature of the pure solvent [in K]; M is the molar mass of the solvent; ΔH_v is the heat of vaporization per mole of the solvent.
• b_c is the colligative molality. b_c can be calculated by taking dissociation into account because the boiling point elevation is a colligative property, which depends on the number of particles in solution. The calculation is mostly done by using the van 't Hoff factor i as b_c = b_solute · i, where b_solute is the molality of the solution. The factor i accounts for the number of individual particles (typically ions) formed by a compound in solution. Examples:
  • i = 1 for sugar in water
  • i = 1.9 for sodium chloride in water, due to the near full dissociation of NaCl into Na⁺ and Cl⁻ (often simplified as 2)
  • i = 2.3 for calcium chloride in water, due to nearly full dissociation of CaCl₂ into Ca²⁺ and 2Cl⁻ (often simplified as 3)

Non integer i factors result from ion pairs in solution, which lower the effective number of particles in the solution.

Equation after including the van 't Hoff factor

ΔT_b = K_b · b_solute · i

The above formula reduces precision at high concentrations, due to nonideality of the solution. If the solute is volatile, one of the key assumptions used in deriving the formula is not true because the equation derived is for solutions of non-volatile solutes in a volatile solvent. In the case of volatile solutes, the equation can represent a mixture of volatile compounds more accurately, and the effect of the solute on the boiling point must be determined from the phase diagram of the mixture. In such cases, the mixture can sometimes have a lower boiling point than either of the pure components; a mixture with a minimum boiling point is a type of azeotrope.

Uses

Together with the formula above, the boiling-point elevation can be used to measure the degree of dissociation or the molar mass of the solute. This kind of measurement is called ebullioscopy (Latin-Greek "boiling-viewing"). However, superheating is a factor that can affect the precision of the measurement and would be challenging to avoid because of the decrease in molecular mobility. Therefore, ΔT_b would be hard to measure precisely even though superheating can be partially overcome by the invention of the Beckmann thermometer. In reality, cryoscopy is used more often because the freezing point is often easier to measure with precision.

- Added a few changes based on peer review

References

- Akhter, M., Alam, M.M. (2023). Colligative Properties. In: Physical Pharmacy and Instrumental Methods of Analysis. Springer, Cham. https://doi.org/10.1007/978-3-031-36777-9_3