Chapter 11: Water and Aqueous Systems

1. Chapter 11: Water and Aqueous Systems

2. Water and Its Properties • Water in the Liquid State • The atoms in water are joined by covalent bonds • The electrons in the bond between H and O are not shared equally • Instead, the electrons spend more time near O and less time near H

3. This gives the O atoms a partially negative charge and the H atoms a partially positive charge

4. A water molecule has a bent structure • There are 2 lone pairs on the oxygen atom • The entire molecule is polar: having a positive end and a negative end

5. Adjacent water molecules are held together by strong intermolecular forces (IMFs) called hydrogen bonds • This occurs because water is polar so the positive end of 1 water molecule is attracted to the negative end of another

6. This makes it difficult to separate water molecules from each other • Water has many unique properties because of its strong IMFs

7. Properties of water • Surface tension: the attractive force exerted upon the surface molecules of a liquid by the molecules beneath • Molecules on the surface are only attracted to molecules below them and not to molecules above them • This causes the molecules to form spherical drops

8. High surface tension makes it difficult to “break through” the surface of the liquid • Water has high surface tension because of its strong IMFs

9. Surfactant: a substance that lowers the surface tension of a liquid • Surfactants interfere with the IMFs of the liquid • Soaps and detergents act as surfactants when added to water

10. Vapor pressure: the pressure of a gas above the liquid that it evaporated from • Water molecules don’t evaporate easily because of water’s strong IMFs • Therefore water’s vapor pressure is quite low

11. Boiling point: the temperature at which a liquid changes to a gas • In a substance with strong IMFs it is difficult to separate adjacent molecules from each other • Therefore water, with strong IMFs, has a high boiling point

12. Water in the Solid State • Most liquids condense (take up less space) as they cool • Therefore the solid is typically more dense than the liquid • This makes most solids sink in their liquids Solid paraffin is denser than liquid paraffin and sinks

13. However, ice (solid H2O) is less dense than water (liquid H2O) • Thus ice floats in liquid water—very unusual! • Ice forms a rigid, very organized structure with spaces in between molecules

14. If ice wasn’t LESS dense than water…

15. Homogeneous Aqueous Systems • Aqueous solution: water containing dissolved substances • Solute: the dissolved substance • Solvent: the liquid that the solute is dissolved in (water) • A solution cannot be separated by filtration

16. Types of compounds that dissolve in water • Most ionic compounds (metal + nonmetal like NaCl) • All polar molecular compounds • If a molecular compound has lone pairs on the central atom it is polar • If a molecular compound has only 2 atoms and they aren’t the same, it’s polar • Examples

17. Types of compounds that don’t dissolve in water • All nonpolar molecular compounds • A few ionic compounds (ex.—AgCl)

18. “Like dissolves like” • A polar or ionic compound will dissolve in water because they both have positive and negative charges • A nonpolar compound will dissolve in another nonpolar compound because neither of them have charges • BUT—polar won’t dissolve in nonpolar and vice versa

20. Electrolytes and Nonelectrolytes • Electrolyte: a substance that conducts electricity when melted or dissolved in water—occurs because the substance has ions • All ionic compounds conduct when melted • All ionic compounds that are soluble will conduct when dissolved in water • Some molecular compounds conduct when dissolved in water—only ACIDS (start with H like HCl)

21. Nonelectrolyte: a substance that does not conduct electricity either when melted or dissolved in water—because it has no ions • All nonmetal elements are nonelectrolytes • Most molecular compounds are nonelectrolytes (except acids)

22. Not all electrolytes conduct to the same extent • Strong electrolyte: a substance that exists entirely as ions in aqueous solution—so is a very good conductor • Weak electrolyte: a substance that exists mostly as molecules in aqueous solution—so it is a poor conductor

23. Properties of Solutions • Factors which affect the rate at which a solute dissolves • Surface area of the solute—it will dissolve faster in tiny particles than in a big chunk

24. Temperature of the solvent—at higher temps particles collide more often so solute dissolves more quickly • Stirring the solution—causes particles to collide more often so solute dissolves more quickly

25. Types of solutions • Saturated solution: a solution that contains the max amount of solute per given amount of solvent • Unsaturated solution: a solution that contains less than the max amount of solute per given amount of solvent • Supersaturated solution: a solution that contains more than the max amount of solute per given amount of solvent

26. Solubility: the amount of a solute needed to produce a saturated solution • Changes with temperature • Usually given in grams (of solute) per 100 g (of solvent)

27. Effect of Temperature on Solubility • Most ionic compounds are MORE soluble when the solvent is at a higher temperature

28. All gases are LESS soluble when the solvent is at a higher temperature

29. Interpreting Solubility Graphs • Ex: What mass of KNO3 will dissolve at 50°C in 100 g of water? • A solution contains 90 g of Pb(NO3)2in 100 g of water at 40°C. Is the solution sat, supersat or unsaturated?