Ch. 12: Water and Aqueous Systems

1. Ch. 12: Water and Aqueous Systems • Liquid Water and Its Properties • Water Vapor and Ice • Aqueous Systems • Heterogeneous Aqueous Systems

2. Liquid Water and Its Properties • Water is a unique compound • Covers 75% of Earth’s surface • A simple triatomic molecule • Highly polar with a bent shape • Water molecules are attracted to one another by intermolecular attractions, mainly hydrogen bonding, which causes: • High surface tension • High specific heat capacity • High heat of vaporization • High boiling point

3. Liquid Water and Its Properties • Surface Properties • The surface of H2O acts like a skin • Surface tension is a result of hydrogen bonding • Water is cohesive, especially at the surface • Water cannot form bonds with the air • Instead, molecules are pulled inward • Explains why drops of H2O are spherical

4. Liquid Water and Its Properties • Surface Properties • All liquids have a surface tension, but water’s is higher than most • It is possible to lower the surface tension of water by adding a surfactant • A wetting agent such as soap or detergent • The detergent molecules interfere with the attraction between the water molecules • Hydrogen bonding also explains water’s unusually low vapor pressure • Limits water’s ability to vaporize or evaporate

5. Liquid Water and Its Properties • Specific heat capacity • It takes 4.18J (1 cal) to raise the temperature of 1 gram of water 10C • This is the specific heat capacity of water • The specific heat capacity of water is nearly constant between 00C and 1000C • Because of hydrogen bonding, the specific heat capacity of H2O is very high • Helps moderate daily air temp around large bodies of H2O • Water absorbs heat from warmer surroundings, which lowers the air temperature • At night, heat is transferred from the warmer water to the surrounding air

6. Heat Capacity and Specific Heat • Specific Heat Capacity(abbreviated “C”) - the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 oC • often called simply “Specific Heat” • Note Table 17.1, page 508(next slide) • Water has a HUGE value, when it is compared to other chemicals

7. Table of Specific Heats Note the tremendous difference in Specific Heat. Water’s value is VERY HIGH.

8. Heat Capacity and Specific Heat • To calculate, use the formula: q = mass (in grams) x T x C • heat is abbreviated as “q” • T = change in temperature • C = Specific Heat • Units are either: J/(g oC) or cal/(g oC)

9. Sample problem: How much energy is required to raise the temperature of 65 mL of water from 20 degrees C to 88 degrees C? (remember C = specific heat, and on the chart on the previous slide, the C for water is 4.18 J/gC

10. Water Vapor and Ice • Evaporation and Condensation • Water absorbs a large amount of heat as it evaporates/vaporizes • Heat of vaporization is the energy needed to convert 1g of substance from a liquid to a gas at the boiling point • Hydrogen bonds must be broken before the liquid changes to the gaseous state

11. Water Vapor and Ice • Evaporation and Condensation • The reverse of vaporization is condensation • The heat of condensation is equal to the heat of vaporization of water • Heat is released during condensation, gained during evaporation • Evaporation and condensation are important to regional temperatures on Earth

12. Water Vapor and Ice • Boiling point • Water has a very high boiling point • Due to hydrogen bonding • Molecular compounds of low molar mass are usually gases or liquids and have low boiling points at normal atmospheric pressure • Water is an exception • It takes a great deal of heat to to disrupt the bonding between the molecules in water • If this were not true, water would be a gas at the usual temperatures found on Earth

13. Water Vapor and Ice • Ice • Liquids usually contract as they cool • Density increases while mass stays constant • Eventually the liquid will solidify • Because the density of the solid is greater than the liquid, the solid will sink

14. Water Vapor and Ice • Ice • As water cools, at first it behaves like a typical liquid • It contacts slightly and it’s density gradually increases (until 40C) • Then the density begins to decrease • Water no longer behaves like a typical liquid • Ice has a 10% lower density than water at 00C • As a result, ice floats • Ice is one of only a few solids that floats in it’s own liquid

15. Water Vapor and Ice • Ice • The fact that ice floats has important consequences for living organisms • Acts as an insulator in bodies of water • Water molecules require a considerable amount of kinetic energy to return to the liquid state • Known as heat of fusion • Very high in water, compared to other low molar mass molecules

16. Aqueous Solutions • Solvents and solutes • Water samples containing dissolved substances are called aqueous solutions • The dissolving medium is the solvent • The dissolved particles are the solute • Solutes and solvents may be solids, liquids or gases • Solutions are homogeneous mixtures • They are stable mixtures

17. Aqueous Solutions • Solvents and solutes • Substances that dissolve most readily in water include ionic cmpds and polar covalent molecules • Non-polar molecules like grease do no dissolve in water • Non-polar molecules will dissolve in other non-polar molecules

18. Aqueous Solutions • The Solution Process • Solvation is the process that occurs when a solute dissolves • The negatively and positively charged particles are surrounded by solvent molecules • In some ionic cmpds, internal attractions are stronger than external attractions – these cmpds cannot be solvated and are said to be insoluble • The rule is “like dissolves like”

19. Aqueous Solutions • Electrolytes and nonelectrolytes • Cmpds that conduct an electric current in aqueous solution or the molten state are called electrolytes • All ionic cmpds are electrolytes • Some are insoluble in water • Cmpds that do not conduct an electric current are called nonelectrolytes • They are not composed of ions • Most carbon cmpds are nonelectrolytes • Some very polar molecular cmpds are nonelectrolytes in the pure state, but become electrolytes when they dissolve

20. Aqueous Solutions • Electrolytes and nonelectrolytes • Not all electrolytes conduct an electric current to the same degree • Some electrolytes are strong • When dissolved, almost all of the solute exists as separate ions • Ex: NaCl • Some electrolytes are weak • When dissolved, only a fraction of the solute exists as separate ions • Ex: HgCl2

21. Aqueous Solutions • Water of hydration • The water in a crystal is called the water of hydration or water of crystallization • A cmpd that contains water is called a hydrate • When writing the formula, a dot is used to connect the formula of the cmpd and the number of water molecules per formula unit • Hydrates appear dry and are unchanged in normally moist air • When heated above 1000C, hydrates lose their water of hydration

22. Aqueous Solutions • Hydrates • The forces holding the H2O in hydrates is not very strong • Held by weak forces • Results in a higher that normal vapor pressure • If the vapor pressure is higher than the vapor pressure in the air, the hydrate will effloresce by losing the water of hydration

23. Aqueous Solutions • Hygroscopic substances • Some hydrated salts that have a low vapor pressure remove water from air to form higher hydrates • Salts and other substances that remove water from air are hygroscopic • Many are used as dessicants • Some cmpds are so hygroscopic that they become wet when exposed to air – these are called deliquescent cmpds • Remove enough H2O to dissolve completely and form solutions • Occurs when the soln formed has a lower vapor pressure than that of air

24. Heterogeneous Aqueous Systems • Suspensions • Mixtures from which particles settle out upon standing • Colloids • Mixtures containing particles that are intermediate in size between suspensions and true solutions • The particles are in the dispersed phase • They are spread through the dispersion medium, which can be a solid, liquid or gas

25. Heterogeneous Aqueous Systems • Colloids • Properties differ from suspensions and solutions • May be cloudy when concentrated, clear when dilute • Intermediated sized particles cannot be filtered and do not settle out • Exhibit the Tyndall effect – scattering of visible light in all directions • Colloids scintillate (flash light) when studied under a microscope • Due to the erratic movement of the particles that reflect light • This chaotic movement is known as Brownian motion

26. Heterogeneous Aqueous Systems • Colloids • Properties differ from suspensions and solutions • Colloids scintillate (flash light) when studied under a microscope • Due to the erratic movement of the particles that reflect light • This chaotic movement is known as Brownian motion • Caused by collisions of molecules, which prevent the colloidal properties from settling

27. Heterogeneous Aqueous Systems • Colloids • Colloids may also absorb ions onto their surface • All the particles in a particular system will have the same charge • Repulsion of like charges keep the colloids from forming aggregates • Adding an opposite charge will cause separation of the colloid

28. Heterogeneous Aqueous Systems • Emulsions • Colloidal dispersions of liquids in liquids • Requires an emulsifying agent • Ex: soap and detergents • Allow formation of colloidal dispersions between liquids that do not normally mix by forming bonds with the water molecules