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Properties of Solutions

Properties of Solutions. 15.2. Solvents and Solutes. An aqueous solution is water that contains dissolved substances. In a solution, the dissolving medium is the solvent . In a solution, the dissolved particles are the solute . Solutions.

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Properties of Solutions

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  1. Properties of Solutions

  2. 15.2 Solvents and Solutes • An aqueous solution is water that contains dissolved substances. • In a solution, the dissolving medium is the solvent. • In a solution, the dissolved particles are the solute.

  3. Solutions • Solutions are homogeneous mixtures of two or more pure substances. • In a solution, the solute is dispersed uniformly throughout the solvent.

  4. dry Dissolution vs reaction NiCl2(s) Ni(s) + HCl(aq) NiCl2(aq) + H2(g) • Dissolution is a physical change—you can get back the original solute by evaporating the solvent. • If you can’t, the substance didn’t dissolve, it reacted.

  5. 15.2 Why does a solution form? • LIKE DISSOLVES LIKE • Polar solvents such as water dissolve ionic compounds and polar compounds. • Nonpolar solvents such as gasoline dissolve nonpolar compounds.

  6. Vitamin A is soluble in nonpolar compounds (like fats). • Vitamin C is soluble in water.

  7. Which vitamin is water-soluble and which is fat-soluble?

  8. Why does a solution form?Energy Changes in Solution To determine the enthalpy change, we divide the process into 3 steps. • Separation of solute particles. • Separation of solvent particles to make ‘holes’. • Formation of new interactions between solute and solvent.

  9. Enthalpy Is Only Part of the Picture Entropy is a measure of: • Dispersal of energy in the system. • Number of microstates (arrangements) in the system. b. has greater entropy,  is the favored state

  10. Assessing Entropy Change In the process illustrated below, water vapor reacts with excess solid sodium sulfate to form the hydrated form of the salt. The chemical reaction is Does the entropy of the system increase or decrease?

  11. Solution formation How does a solid dissolve into a liquid? What ‘drives’ the dissolution process? What are the energetics of dissolution?

  12. How Does a Solution Form? • Solvent molecules attracted to surface ions. • Each ion is surrounded by solvent molecules. • Enthalpy (DH) changes with each interaction broken or formed. Ionic solid dissolving in water

  13. How Does a Solution Form The ions are solvated (surrounded by solvent). If the solvent is water, the ions are hydrated. The intermolecular force here is ion-dipole.

  14. How Does a Solution Form? • Solvent molecules attracted to surface ions. • Each ion is surrounded by solvent molecules. • Enthalpy (DH) changes with each interaction broken or formed.

  15. 16.1 Solution Formation • The compositions of the solvent and the solute determineIF a substance will dissolve. • The factors that determine theRATE which asubstance dissolves are: • stirring (agitation) • temperature • the surface area of the dissolving particles

  16. 16.1 Solubility • The solubility of a substance is the amount of solute that dissolves in a given quantity of a solvent at a specified temperature and pressure to produce a saturated solution. • Solubility is often expressed in grams of solute per 100 g of solvent.

  17. 16.1 Solubility • Some liquids combine in all proportions, while others don’t mix at all. • Two liquids are miscible if they dissolve in each other in all proportions. • Two liquids are immiscible if they are insoluble in each other.

  18. 16.1 Solubility • A saturated solution contains the maximum amount of solute for a given quantity of solvent at a given temperature and pressure. • An unsaturated solution contains less solute than a saturated solution at a given temperature and pressure.

  19. 16.1 Solubility • In a saturated solution, the rate of dissolving equals the rate of crystallization, so the total amount of dissolved solute remains constant.

  20. Factors Affecting Solubility The stronger the intermolecular attractions between solute and solvent, the more likely the solute will dissolve. Example: ethanol in water Ethanol = CH3CH2OH Intermolecular forces = H-bonds; dipole-dipole; dispersion Ions in water also have ion-dipole forces.

  21. Degree of saturation • Unsaturated Solution • Less than the maximum amount of solute for that temperature is dissolved in the solvent. • No solid remains in flask.

  22. Degree of saturation • Supersaturated • Solvent holds more solute than is normally possible at that temperature. • These solutions are unstable; crystallization can often be stimulated by adding a “seed crystal” or scratching the side of the flask.

  23. 16.1 Factors Affecting Rate of Solubility • Temperature • The solubility of most solid substances increases as the temperature of the solvent increases. • The solubilities of most gases are greater in cold water than in hot.

  24. Temperature Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

  25. 16.1 Factors Affecting Solubility • Pressure • Changes in pressure have little effect on the solubility of solids and liquids, but pressure strongly influences the solubility of gases. • Gas solubility increases as the partial pressure of the gas above the solution increases.

  26. Factors Affecting the Solubility of Gases • Solubility of most gases decreases as temperature increases • Gases tend to have weak intermolecular forces • Ex: N2 and O2 form weak dipole-induced dipole forces and weak dispersion forces with water • As the kinetic energy of particles within a solution increases the gas particles break free from weak attractions and re-enter the gas phase

  27. Factors Affecting Solubility of Gases • Real life: • Warm soda goes flat faster than cold soda • Fishing • Thermal pollution • The Bends

  28. Gases in Solution • In general, the solubility of gases in water increases with increasing mass. Why? • Larger molecules have stronger dispersion forces.

  29. Gases in Solution

  30. Gases in Solution • The solubility of liquids and solids does not change appreciably with pressure. • But, the solubility of a gas in a liquid is directly proportional to its pressure. Increasing pressure above solution forces more gas to dissolve.

  31. 16.1 Factors Affecting Solubility of Gases • Henry’s law states that at a given temperature, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid.

  32. Temperature • Carbonated soft drinks are more “bubbly” if stored in the refrigerator. • Warm lakes have less O2 dissolved in them than cool lakes.

  33. 16.1 Factors Affecting Solubility

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