Boiling Point Elevation
The RAISING of the normal boiling point of a liquid by adding a non-volatile substance to it.
- When you add a non-volatile solute to a pure solvent, the boiling point of the liquid solvent is elevated due to the lowered vapor pressure. This means that the solution will boil at a higher temperature.
- The reason is because the non-volatile substance solute decreases the number of water molecules at the surface.
- The solvent particles will need a lot more energy to bounce upwards and push the solute particles out of the way and eventually escape into the atmosphere.
FORMULA: BOILING POINT ELEVATION
The equation enables you to calculate how much change in boiling temperature of a solution after adding solutes
ΔTb = change in temperature
Kb = boiling point constant (solvent)
M = molality of solutes
i = van’t Hoff factor of solute
- You can also predict the boiling point of a solution if you know the constant(kb) and molality(M) of the solution
Calculate the boiling point elevation of a solution containing
30.0 g of glucose (C₆H₁₂O₆) dissolved in 600 g of water.
Given Kb (constant) for water is 0.52 °C/m (per mole)
CALCULATE THE MOLAR MASS OF GLUCOSE
C6H12O6 = (12 x 6) + (1 x 12) + (16 x 6) = 180
|C||Carbon||12.01078||6||12 x 6|
|O||Oxygen||15.99943||6||16 x 6|
Step1 – Calculate the moles of glucose
Moles of glucose = Mass / Molar Mass
= 30.0 / 180 = 0.167 moles
Step2 – Calculate the molality of glucose
Molality of glucose = moles of glucose / mass of water
= 0.167 / 0.600 = 0.28 molal
Step2 – Calculate the boiling point elevation
ΔTb = Kb • M • i
For glucose (C6H12O6), the van’t Hoff factor = 1
= 0.52 x 0.28 x 1 = 0.146°C
So, the water with glucose will boil at:
(100 + 0.146 = 100.146°C)
HOW ATMOSPHERIC PRESSURE AFFECTS BOILING POING
Further reading on how atmospheric pressure affects boiling point.