Solutions

1. Solutions Chapter 13 Pages 395 - 418

2. Types of Mixtures • Solutions • Suspensions • Colloids

3. Solutions Solution: a homogeneous mixture of 2 or more substances in a single phase -atoms, molecules, or ions are thoroughly mixed, resulting in a mixture that has the same composition and properties throughout Soluble: capable of being dissolved

4. Particle Model for a Solution

5. Components of Solutions Solvent – the dissolving medium in a solution Solute – the substance dissolved in a solution (usually the component that is lesser quantity) Types of Solutions -exist as gases, liquids, or solids Alloy – solutions of solids uniformly mixed (brass, sterling silver)

6. Particle Models for Gold and Gold Alloy

7. Properties of Solutions • Particle size: 0.01 – 1 nm; can be atoms, ions, molecules • Do not separate on standing • Cannot be separated by filtration • Do not scatter light • Homogeneous

8. Suspensions Suspension: a mixture in which the particles of a solvent are so large that they settle out unless the mixture is constantly stirred or agitated Examples: muddy water, Italian salad dressing

9. Particle Model for a Suspension

10. Properties of Suspensions • Particle size: over 1000 nm, suspended; can be large particles or aggregates • Particles settle out • Can be separated by filtration • May scatter light, but are not transparent • heterogeneous

11. Colloids Colloids: particles that are intermediate in size between those in solutions and suspensions form mixtures known as colloidal dispersions -particles are small and suspended by constant movement -dispersed phase: colloidal particles -dispersing medium: water Examples: emulsions (mayonnaise), foam (shaving cream), gelatin, aerosols

12. Tyndall Effect -occurs when light is scattered by colloidal particles dispersed in a transparent medium. This property can be used to distinguish between a solution and a colloid.

13. Properties of Colloids • Particle size: 1-1000 n, dispersed; can be aggregates or large molecules • Do not separate on standing • Cannot be separated by filtration • Scatter light (Tyndall effect) • heterogeneous

14. Solutes: Electrolytes vs. Nonelectrolytes • Substances that dissolve in water are classified according to whether they yield molecules or ions in solution. • Ionic compounds dissolve to yield + and – ions surrounded by water molecules. • Ions are free to move and therefore conduct electricity.

15. Particle Models for Electrolytes/Nonelectrolytes in Solution

16. Definitions Electrolyte: a substance that dissolves in water to give a solution that conducts electric current Nonelectrolyte: a substance that dissolves in water to give a solution that does not conduct an electric current Examples: NaCl, HCl (highly polar) Example: sugar solution

17. Factors Affecting the Rate of Dissolution • Increasing the surface area of the solute • Agitating the solution • Heating the solvent

18. Increasing the Surface Area To dissolve: • Molecules or ions of the solute are attracted by the solvent • Dissolution occurs at the surface of the solute • Increasing surface area of solute speeds up the process • The more finely divided a substance is, the greater the surface area per unit mass and the more quickly it dissolves

19. Agitating a Solution • Concentration of dissolved solute is high near surface of a solute • Stirring or shaking disperses solute particles and brings fresh solvent in contact with the solute surface • Stirring is similar to crushing a solid – contact between solvent and solute surface is increased

20. Heating a Solvent • With increase in solvent temperature, solvent molecules move faster and KE increases • At higher temperatures, collisions between solvent and solute molecules are more frequent and of higher energy • This separates solute molecules from one another and disperses them among the solvent molecules

21. Solubility Saturated Unsaturated Supersaturated

22. Solubility -for every combination of solvent and solid solute at a given temperature, there is a limit to the amount of solute that can be dissolved -the point at which this limit is reached depends on the nature of the solute, the nature of the solvent, and the temperature

23. Model -sugar dropped in water -as more solid dissolves, collisions increase -sugar molecules leave solid and move at random in solvent -eventually, molecules are returning to the crystal at the same rate at which they are going into solution -some may collide with solid and remain there Solution equilibrium: the physical state in which the opposing processes of dissolution and crystallization of a solute occur at equal rates

24. Saturated Saturated solution: a solution that contains the maximum amount of dissolved solute How to tell a solution is saturated – when more solute is added it falls to the bottom and does not dissolve

25. Unsaturated Unsaturated solution: a solution that contains less solute than a saturated solution under the existing conditions

26. Supersaturated -if solubility increases w/temperature, the excess solute usually comes out of solution, leaving the solution saturated at the lower temp -sometimes, if the solution cools undisturbed, the excess solute does not separate Supersaturated solution: a solution that contains more dissolved solute than a saturated solution contains under the same conditions

27. -dropping a small crystal of the solute into a supersaturated solution (seeding) or disturbing the solution causes a rapid formation of crystals by the excess solute -once crystals begin to form, the process continues until equilibrium is established at the lower temp Solubility: of a substance is the amount of that substance required to form a saturated solution with a specific amount of solvent at a specified temperature

28. -temperature must be specified because solubility varies with temperature (for gases, pressure must also be specified) -the maximum amount of solute that dissolves and reaches equilibrium is always the same under the same conditions

29. Solute-Solvent Interactions Rule of Thumb: “like dissolves like” -substances are alike when similar in type of bonding, the polarity or nonpolarity of molecules, and the intermolecular forces between solute and solvent

30. Dissolving Ionic Compounds in Aqueous Solution -polarity of water plays important role -charged ends attract ions in ionic compounds and surround them to keep them separated from other ions in the solution -attraction is strong enough to draw ions away from crystal surface and into solution Hydrated: the solution process with water as the solvent -ions are said to be “hydrated”

31. -entire crystal dissolves and hydrated ions are uniformly distributed -as crystals reform they retain specific ratios of water molecules and are known as hydrates (CuSO4 . 5H2O) -a hydrate’s behavior is just like that of the anhydrous form; dissolving results in hydrated ions and water -ions and polar molecules are linked to water with dipole-dipole attractions

32. Nonpolar Solvents -generally, ionic compounds are not soluble in nonpolar solvents Examples: CCl4 C6H5CH3 (toluene) gasoline -nonpolar solvent molecules do not attract ions strongly enough to overcome the forces holding the crystal together -nonpolar molecules are linked to nonpolar solvents with London dispersion forces

33. Liquid Solutes and Solvents Immiscible: liquid solutes and solvents that are not soluble in each other Miscible: liquids that dissolve freely in one another in any proportion -the nonpolar molecules of these substances exert no strong forces of attraction or repulsion, and the molecules mix freely Examples: nonpolar with polar, toluene with water, gasoline with water, oil and water Examples: nonpolar w/nonpolar, gas with fats, oils, and greases

34. Effects of Pressure on Solubility -pressure changes have very little effect on the solubilities of liquids -increases in pressure increase gas solubilities in liquids -equilibrium eventually reached between rates at which gas molecules enter and leave the gas phase gas + solvent solution

35. -increasing the pressure of the solute gas above the solution puts stress on the equilibrium -an increase in gas pressure causes the equilibrium to shift so that fewer molecules are in the gas phase Example: gas escapes immediately when pressure is reduced by opening a soda bottle. The soda effervesces when the bottle is opened and the pressure is reduced.

36. Henry’s Law Henry’s Law: the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas on the surface of the liquid -named after English chemist, William Henry -in a mixture of gases, each gas exerts a pressure. Assuming they don’t react, each gas will dissolve to the same extent it would if no other gases were present.

37. Application of Henry’s Law In sodas, the solubility of CO2 is increased by increasing the pressure. It’s raised to 5-10 atm. When the cap is removed, the pressure is reduced to 1 atm and some of the CO2 escapes. Effervescence – the rapid escape of a gas from a liquid in which it is dissolved

38. Effects of Temperature on Solubility On Gases On Solids -increasing temperature usually decreases gas solubility -more difficult to predict -often, increase in temp increases solubility of solids -increase in temperature; increase in KE -sometimes the increase in solubility is large and sometimes only slight (see solubility table ) -therefore, more molecules overcome attraction and return to gas phase -at higher temps, equilibrium is reached with fewer gas molecules in solution -in some cases, solubility decreases with an increase in temp

39. Temperature – Solubility

40. Heats ofSolution -formation of a solution means an energy change -dissolving some solids results in heat formation – exothermic (hot) -dissolving others results in the absorption of heat – endothermic (cold) -both solvent and solute must undergo changes in the forces holding them together -energy is required to separate solute molecules and solvent molecules

41. -solvated: a solute particle that is surrounded by solvent molecules Heat of Solution: net amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent -since heating decreases solubility of a gas, dissolution of gases is exothermic (negative values)

42. Energy Changes in the Solution Process