Elevation in Boiling Point
Elevation in Boiling Point
Elevation in boiling point is a colligative property of solutions that describes how the boiling point of a solvent increases when a solute is dissolved in it. This phenomenon occurs because the presence of solute particles in the solvent reduces the number of solvent molecules at the surface, which in turn decreases the rate of evaporation. As a result, more heat (energy) is required to bring the solvent to a boil.
Colligative Properties
Colligative properties are properties that depend on the number of solute particles in a solution, rather than the identity of the solute. Elevation in boiling point is one of these properties, along with freezing point depression, osmotic pressure, and vapor pressure lowering.
Boiling Point Elevation Formula
The change in boiling point ((\Delta T_b)) can be calculated using the formula:
[ \Delta T_b = i \cdot K_b \cdot m ]
where:
- ( \Delta T_b ) is the elevation in boiling point.
- ( i ) is the van't Hoff factor, which represents the number of particles the solute splits into or forms in the solution.
- ( K_b ) is the ebullioscopic constant, which is a property of the solvent and measures how much the boiling point increases per molal concentration of the solute.
- ( m ) is the molality of the solution, which is the number of moles of solute per kilogram of solvent.
Examples
Let's consider a solution of sodium chloride (NaCl) in water. NaCl dissociates into two ions: Na(^+) and Cl(^-). Therefore, the van't Hoff factor (i) for NaCl is 2.
If the ebullioscopic constant ((K_b)) of water is (0.512 \, \text{°C kg/mol}) and we dissolve (1 \, \text{mol}) of NaCl in (1 \, \text{kg}) of water, the elevation in boiling point would be:
[ \Delta T_b = 2 \cdot 0.512 \, \text{°C kg/mol} \cdot 1 \, \text{mol/kg} = 1.024 \, \text{°C} ]
This means the boiling point of water, which is normally (100 \, \text{°C}), will increase to (101.024 \, \text{°C}) when (1 \, \text{mol}) of NaCl is dissolved in (1 \, \text{kg}) of water.
Table of Differences and Important Points
| Property | Pure Solvent | Solution |
|---|---|---|
| Boiling Point | Lower | Higher due to solute presence |
| Vapor Pressure | Higher | Lower due to solute particles |
| Evaporation Rate | Higher | Lower due to fewer solvent molecules at the surface |
| Heat Required | Less | More to achieve boiling |
Factors Affecting Boiling Point Elevation
- Nature of the Solvent: Different solvents have different ebullioscopic constants ((K_b)).
- Quantity of Solute: The more solute particles present, the greater the elevation in boiling point.
- Van't Hoff Factor ((i)): The degree of ionization or association of the solute affects the boiling point elevation.
Applications
- Cooking: Adding salt to water increases its boiling point, which can affect cooking times.
- Chemical Synthesis: Controlling the boiling point of solvents can be crucial in chemical reactions and separations.
- Antifreeze: Solutions like ethylene glycol in water are used in car radiators to raise the boiling point of the coolant, preventing it from boiling over in hot conditions.
Conclusion
Elevation in boiling point is a crucial concept in chemistry that has practical applications in various fields. Understanding this colligative property allows chemists and engineers to manipulate the boiling points of solutions for desired outcomes in industrial processes, culinary arts, and more.