University:
Swansea University
Course:
CH302 | Organic Chemistry
Academic year:

2023
Views:

328

Pages:

2
Author:

Jeanete Juwi

Boiling Point Elevation (ΔTb ) and Freezing Point Derivation (ΔTf ) The boiling point of a liquid is the temperature at which the vapor pressure of the liquid saturates is the same as the external pressure. If the vapor pressure is the same as the external pressure, then the vapor bubbles that form in the liquid can push themselves to the surface into the gas phase. Therefore, the boiling point of a liquid depends on the external pressure. What is meant by boiling point is the normal boiling point, namely the boiling point at a pressure of 76 cmHg. The normal boiling point of water is 100oC. At an external air pressure of 1 atm, water boils at a temperature of 100 o C (point B). At that time the water vapor pressure was also 1 atm and the saturated vapor pressure of the solution was still below 1 atm (point P). In order for the solution to boil, the temperature needs to be increased so that point P moves to point E. At point E the saturated vapor pressure of the solution has reached 1 atm. So at point E the solution boils and the boiling temperature is point E’. The difference between the boiling point of a solution and the boiling point of the solvent is called the boiling point elevation (ΔTb ). ΔTb = boiling point of solution – boiling point of solvent The freezing point of the solution (point F’) is lower than the freezing point of the solvent (point C). The difference between the freezing point of the solvent and the freezing point of the solution is called the freezing point depression (ΔTf ). ΔTf = solvent freezing point – solution freezing point The increase in boiling point and decrease in freezing point caused by the addition of solute can be formulated as follows. ΔTb = m · Kb or ΔTb = . Kb ΔTf = m · Kf or ΔTf = . Kf with: ΔTb = boiling point elevation ΔTf = freezing point depression Kb = molal boiling point elevation constant (o C/m) Kf = molal freezing point depression constant (o C/m) m = molality g = mass mgof solute (grams) Mr = relative formula mass of solute p = mass of solvent (grams)

Boiling Point Elevation (ΔTb) and Freezing Point Derivation (ΔTf)

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