Destroyer: Why does freezing point lower when effective concentration increases?

[rcraven9]

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Ok, so I have a question from the General Chemistry Section of the 2009 version of Destroyer, # 34. It states that Na2SO4 has the highest "Effective Concentration" thus will have the lowest freezing point, lowest vapor pressure, Highest osmotic pressure, and highest boiling point. I understand the vapor pressure, osmotic pressure, and BP changes, but I don't understand why it would have the lowest freezing point if it has the highest concentration of particles in solution. To my understanding, the freezing point of a solution is generally higher than the one of a pure solvent. The more solute particles you have in solution, the more Van der Waals forces, and the better they stick together. In other words, if you have a higher concentration of particles, then it would take less of a drop in temperature to reach the freezing point (as compared to a pure solvent).

Can someone make sense of this for me?

 

[Cool Beans]

Everything you've stated is true and that is precisely how we look at the impact of solute concentration on boiling point. But the more important factor for melting point (same as freezing point) is how well the substance will pack into a crystal lattice. Solute particles serve as irregularities in the crystal lattice favoring the liquid phase over the solid phase moreso than the pure solvent alone. With solute present, the liquid would have to be cooled lower than normal to cause it to freeze forming the crystal lattice due to these irregularities posed by the solute particles.

 

[rcraven9]

Everything you've stated is true and that is precisely how we look at the impact of solute concentration on boiling point. But the more important factor for melting point (same as freezing point) is how well the substance will pack into a crystal lattice. Solute particles serve as irregularities in the crystal lattice favoring the liquid phase over the solid phase moreso than the pure solvent alone. With solute present, the liquid would have to be cooled lower than normal to cause it to freeze forming the crystal lattice due to these irregularities posed by the solute particles.

Awesome, so my logic was completely wrong haha. Thank you for the clarification. It makes a lot more sense now.

 

[lilpucal0v3r]

Everything you've stated is true and that is precisely how we look at the impact of solute concentration on boiling point. But the more important factor for melting point (same as freezing point) is how well the substance will pack into a crystal lattice. Solute particles serve as irregularities in the crystal lattice favoring the liquid phase over the solid phase moreso than the pure solvent alone. With solute present, the liquid would have to be cooled lower than normal to cause it to freeze forming the crystal lattice due to these irregularities posed by the solute particles.

See this is what i thought, with solute, your freezing pt decreases ( MORE negative). However, i can not wrap my head over this problem and i keep getting mixed messages . please see attached


Sorry for bringing up an old thread! super thanks in advance