Ch. 12: Water and Aqueous Systems

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

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

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

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

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

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

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

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)

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

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

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

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

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

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

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

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

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

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”

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

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

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

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

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

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

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

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

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

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

Ch. 12: Water and Aqueous Systems