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Properties of Solutions

Properties of Solutions. Chapter 11. Composition of Solutions. Solutions = homogeneous mixtures, any state of matter. Solid. Gas. Liquid. Gas. Liquid. Solid. How to Calculate Concentration. Mole Fraction. Molarity. Molality. Mass Percent. Normality.

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Properties of Solutions

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  1. Properties of Solutions Chapter 11

  2. Composition of Solutions • Solutions = homogeneous mixtures, any state of matter Solid Gas Liquid Gas Liquid Solid

  3. How to Calculate Concentration Mole Fraction Molarity Molality Mass Percent Normality (mol solute*cation charge) L solution

  4. “Like Dissolves Like” Polar solvents dissolve polar solutes Non-Polar solvents dissolve non-polar solutes WHY? ENTHALPY OF SOLUTION 3 part process Solute components separate (endo) Overcome IMF’s in solvent (endo) Solvent and Solute interact (exo) Energies of Solutions

  5. Enthalpy of Solution • DHsoln = DH1 + DH2 + DH3 • Heat of solution can either be + or – • DH is expected to be small for non-polar molecules except for when they are very large molecules. DHsoln is large and + This rarely occurs DHsoln is large and - This usually occurs DHsoln is close to zero Exceptions, because Spontaneity does not Depend solely on enthalpy

  6. Try Me! • Decide whether liquid hexane (C6H14) or liquid methanol (CH3OH) is more appropriate solvent for substances, grease (C20H42) and potassium iodide (KI).

  7. Factors Affecting Solubility • Structure---Polarity • Non polar are hydrophobic • Polar are hydrophillic • Some are a combo with different ends. • Vitamin C and scurvy • Pressure---works only on gases • C = kP • [gas] a Pvapor • Temperature---not always a direct relationship • Gas in water; as T increases, solubility decreases • See solubility curves for individual compound reactions to temperature.

  8. Colligative Properties • A property of a solution that depends only on the number, and not on the identity of the solute particles present. • Vapor Pressure • Boiling Point Elevation • Freezing Point Depression • Osmotic Pressure

  9. Vapor Pressure • Non-volatile Solute added to solvent • Reduces the tendency of the solvent molecules to escape (evaporate) • Equilibrium pressure of the solvent is higher than equilibrium pressure of solution. More of the solvent (pure) has to evaporate to be at equilibrium than is the case in a beaker full of solution.

  10. François Raoult and his Law • Vapor pressure of a solution is directly proportional to the mole fraction of the solvent present. Psoln = XsolventPosolvent Just what I was thinking!! Me Too! I agree!

  11. Try Me! • Calculate the expected vapor pressure at 25oC for a solution prepared by dissolving 85.58 g of table sugar (sucrose molar mass = 342.3 g/mol) in 320.5 cm3 of water. The vapor pressure of water is 23.76 torr at this temperature.

  12. Answer • 23.431 torr

  13. More solute, lower vapor pressure • Predict the vapor pressure of a solution prepared by mixing 35.0 g solid Na2SO4 with 175 g water at 25oC. The vapor pressure of water is still 23.76 torr.

  14. Volatile solutes • Raoult’s Law is modified to account for the vapor pressure of both solute. • Ptotal = Pa + Pb = XaPao + XbPbo • Always some degree of deviation from ideal…more similar the solute is to the solvent the more ideal it behaves.

  15. Boiling Point Elevation • When a solute is added, vapor pressure is lowered. • Must be heated to higher temperature to give reach normal boiling point. • Boiling point is higher than usual. • DT = Kbmsolute

  16. Freezing Point Depression • When a solute is added, the vapor pressure is lowered. • Water in the solution has a lower vapor pressure than that of pure ice. • Ice will not form. • Freezing point has been lowered. • DT = Kfmsolute

  17. Try Me! • A chemist is trying to identify a human hormone, which controls metabolism by determining its molar mass. A sample weighing 0.546 g was dissolved in 15.0 g benzene, and the freezing point depression was determined was determined to be 0.240oC. Calculate the molar mass of the hormone.

  18. Osmotic Pressure • Solvent, but not solute can pass through a semi-permeable membrane via osmosis. • When pure solvent and solution volumes are stabilized (the system is at equilibrium) there is a greater pressure on the solution side. • The difference in pressure is the Osmotic Pressure.

  19. P = MRT • Used to determine osmotic pressure. • P is osmotic pressure • M is molarity of solution • R is the gas law constant (0.0821 Latm/Kmol) • T is the temperature in Kelvin

  20. Colligative Properties of Electrolyte Solutions • The type of solute DOES matter sometimes • Only in ability to dissociate • Compare: 0.1 m glucose to 0.1m NaCl

  21. Van’t Hoff Factor • Van’t Hoff Factor expresses the relationship between moles of solute and solution: • This is a multiplier number that describes the ratio of elevation or depression. i = moles particles moles solute DT = iKbm DT = iKfm P = iMRT

  22. Ion Pairing • Van’t Hoff factor is fallible. • Ion Pairing: at a certain instant, some ions may not be dissociated, and thus count as a single particle. • Still use equations, but keep in mind that there can be some deviation.

  23. Colloids • Suspension of tiny particles in a medium caused by electrostatic repulsion. • Tyndall Effect: Scattering of light effect by suspended particles.

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