Colligative Properties

Can a solute change the properties of a solvent when added together to create a solution?

Consider aqueous solutions solvent = water How do properties of aqueous solution compare to properties of pure water?

Conductivity pure water does not conduct electricity some aqueous solutions do to conduct electricity a solution must contain ions

Conductivity depends on nature of the solute particles molecular (covalent) solutes DO NOT conduct in any form C6H12O6(s) C6H12O6(aq) ionic solutes DO conduct in aqueous and liquid forms but NOT in solid form NaCl(s) NaCl(l) √ NaCl(aq) √

Colligative Properties Depend on number particles in solution not type particles doesn’t matter what kind of particles (ions or molecules) Concentration of particles does matter

What are some colligative properties? Vapor Pressure decreases (↓) Freezing Point Boiling Point increases (↑) change in Osmotic Pressure

higher the concentration of solute particles, the greater (larger) the effect on colligative properties

Does it matter if solute is ionic or covalent? YES number of particles will vary effected by concentration: greater concen = greater effect lower concen = lesser effect

Ionic solutes dissolve in H2O dissociate into (+) & (-) ions [ more particles/mole of solute]

1 mole salt 3 moles ions NaCl(s) + H2O(l)  Na+1(aq) + Cl-1(aq) MgCl2(s) + H2O(l)  Mg2+(aq) + 2Cl(aq) 1 mole salt 3 moles ions NaCl(s) + H2O(l)  Na+1(aq) + Cl-1(aq) 1 mole salt  2 moles ions

Covalent solutes dissolve as molecules  1-to-1 relationship between moles solute & moles dissolved particles

Covalent substances C6H12O6(s) + H2O(l)  C6H12O6(aq) 1 mole sugar → 1 mole dissolved molecules

As solute is added to a solvent, what happens to freezing point & boiling point of solution? The freezing point ↓ & the boiling point ↓ The freezing point ↓ & the boiling point ↑ The freezing point ↑ & the boiling point ↓ The freezing point ↑ & the boiling point ↑ Answer: 2

- do you want greatest or least change? Which solution containing 1 mole of solute dissolved in 1000 g of water has the lowest freezing point? C2H5OH(aq) NaCl(aq) KOH CaCl2 Be careful! - do you want greatest or least change? What if question asked which solution has highest freezing point? Answer: 4

Of the following solutions, the one that will freeze at the lowest temperature contains 1 mole of nonvolatile solute dissolved in 1) 250 g of solvent 2) 500 g of solvent 3) 750 g of solvent 4) 1000 g of solvent Answer: 1

Which solute, when added to 1000 g of water, will produce a solution with the highest boiling point? 1) 29 g of NaCl 2) 58 g of NaCl 3) 31 g of C2H6O2 4) 62 g of C2H6O2 1 mole of NaCl → 2 moles of ions 1 mole of C2H6O2 → 1 mole of molecules

Which solution will freeze at the lowest temperature? 1) 1 g of NaCl dissolved per 100 g of H2O 2) 1 g of NaCl dissolved per 1000 g of H2O 1 g of C12H22O11 dissolved per 100 g of H2O 4) 1 g of C12H22O11 dissolved per 1000 g of H2O

Which solution will freeze at the lowest temperature? 1) 1 g of NaCl dissolved per 100 g of H2O __1 g____ = 0.017 mol → 0.034 mol of ions 58.8 g/mol (NaCl) (Na+1 and Cl-1) 2) 1 g of C12H22O11 dissolved per 100 g of H2O __1 g___ = 0.0043 mol of molecules 232 g mol

Effect of Non-volatile Solute on Colligative properties Boiling Point Elevation (↑) Freezing Point Depression (↓) the more particles, the bigger the effect!

Applications of colligative properties Salting roads in the winter time ? NaCl or ? MgCl2 Using salted ice to make ice cream keeps ice cream from becoming like concrete