Ch. 17: 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 PropertiesWater is a unique compoundCovers 75% of Earths surfaceA simple triatomic moleculeHighly polar with a bent shapeWater molecules are attracted to one another by intermolecular attractions, mainly hydrogen bonding, which causes:High surface tensionHigh specific heat capacityHigh heat of vaporizationHigh boiling point

Liquid Water and Its PropertiesSurface PropertiesThe surface of H2O acts like a skinSurface tension is a result of hydrogen bondingWater is cohesive, especially at the surfaceWater cannot form bonds with the airInstead, molecules are pulled inwardExplains why drops of H2O are spherical Liquid Water and Its PropertiesSurface PropertiesAll liquids have a surface tension, but waters is higher than mostIt is possible to lower the surface tension of water by adding a surfactantA wetting agent such as soap or detergentThe detergent molecules interfere with the attraction between the water moleculesHydrogen bonding also explains waters unusually low vapor pressureLimits waters ability to vaporize or evaporate

Liquid Water and Its PropertiesSpecific heat capacityIt takes 4.18J (1 cal) to raise the temperature of 1 gram of water 10CThis is the specific heat capacity of waterThe specific heat capacity of water is nearly constant between 00C and 1000CBecause of hydrogen bonding, the specific heat capacity of H2O is very highHelps moderate daily air temp around large bodies of H2OWater absorbs heat from warmer surroundings, which lowers the air temperatureAt night, heat is transferred from the warmer water to the surrounding air Heat Capacity and Specific HeatSpecific Heat Capacity (abbreviated C) - the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 oCoften called simply Specific HeatNote Table 17.1, page 508 (next slide)Water has a HUGE value, when it is compared to other chemicals6Table of Specific Heats

Note the tremendous difference in Specific Heat.Waters value is VERY HIGH.Heat Capacity and Specific HeatTo calculate, use the formula: q = mass (in grams) x T x Cheat is abbreviated as qT = change in temperatureC = Specific HeatUnits are either: J/(g oC) or cal/(g oC)

8Sample 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 IceEvaporation and CondensationWater absorbs a large amount of heat as it evaporates/vaporizesHeat of vaporization is the energy needed to convert 1g of substance from a liquid to a gas at the boiling pointHydrogen bonds must be broken before the liquid changes to the gaseous state Water Vapor and IceEvaporation and CondensationThe reverse of vaporization is condensationThe heat of condensation is equal to the heat of vaporization of waterHeat is released during condensation, gained during evaporationEvaporation and condensation are important to regional temperatures on Earth Water Vapor and IceBoiling pointWater has a very high boiling pointDue to hydrogen bondingMolecular compounds of low molar mass are usually gases or liquids and have low boiling points at normal atmospheric pressureWater is an exceptionIt takes a great deal of heat to to disrupt the bonding between the molecules in waterIf this were not true, water would be a gas at the usual temperatures found on Earth Water Vapor and IceIceLiquids usually contract as they coolDensity increases while mass stays constantEventually the liquid will solidifyBecause the density of the solid is greater than the liquid, the solid will sink

Water Vapor and IceIceAs water cools, at first it behaves like a typical liquidIt contacts slightly and its density gradually increases (until 40C)Then the density begins to decreaseWater no longer behaves like a typical liquidIce has a 10% lower density than water at 00CAs a result, ice floats Ice is one of only a few solids that floats in its own liquid Water Vapor and IceIceThe fact that ice floats has important consequences for living organismsActs as an insulator in bodies of waterWater molecules require a considerable amount of kinetic energy to return to the liquid stateKnown as heat of fusionVery high in water, compared to other low molar mass molecules

Aqueous Solutions Solvents and solutesWater samples containing dissolved substances are called aqueous solutionsThe dissolving medium is the solventThe dissolved particles are the soluteSolutes and solvents may be solids, liquids or gasesSolutions are homogeneous mixturesThey are stable mixtures

Aqueous SolutionsSolvents and solutesSubstances that dissolve most readily in water include ionic cmpds and polar covalent moleculesNon-polar molecules like grease do no dissolve in waterNon-polar molecules will dissolve in other non-polar molecules Aqueous SolutionsThe Solution ProcessSolvation is the process that occurs when a solute dissolvesThe negatively and positively charged particles are surrounded by solvent moleculesIn some ionic cmpds, internal attractions are stronger than external attractions these cmpds cannot be solvated and are said to be insolubleThe rule is like dissolves like Aqueous SolutionsElectrolytes and nonelectrolytesCmpds that conduct an electric current in aqueous solution or the molten state are called electrolytesAll ionic cmpds are electrolytesSome are insoluble in waterCmpds that do not conduct an electric current are called nonelectrolytes They are not composed of ionsMost carbon cmpds are nonelectrolytesSome very polar molecular cmpds are nonelectrolytes in the pure state, but become electrolytes when they dissolve

Aqueous SolutionsElectrolytes and nonelectrolytesNot all electrolytes conduct an electric current to the same degreeSome electrolytes are strongWhen dissolved, almost all of the solute exists as separate ionsEx: NaClSome electrolytes are weakWhen dissolved, only a fraction of the solute exists as separate ionsEx: HgCl2

Aqueous SolutionsWater of hydrationThe water in a crystal is called the water of hydration or water of crystallizationA cmpd that contains water is called a hydrateWhen writing the formula, a dot is used to connect the formula of the cmpd and the number of water molecules per formula unitHydrates appear dry and are unchanged in normally moist airWhen heated above 1000C, hydrates lose their water of hydration

Aqueous SolutionsHydratesThe forces holding the H2O in hydrates is not very strongHeld by weak forcesResults in a higher that normal vapor pressureIf the vapor pressure is higher than the vapor pressure in the air, the hydrate will effloresce by losing the water of hydration

Aqueous SolutionsHygroscopic substancesSome hydrated salts that have a low vapor pressure remove water from air to form higher hydratesSalts and other substances that remove water from air are hygroscopicMany are used as dessicantsSome cmpds are so hygroscopic that they become wet when exposed to air these are called deliquescent cmpdsRemove enough H2O to dissolve completely and form solutionsOccurs when the soln formed has a lower vapor pressure than that of air

Heterogeneous Aqueous SystemsSuspensionsMixtures from which particles settle out upon standingColloidsMixtures containing particles that are intermediate in size between suspensions and true solutionsThe particles are in the dispersed phaseThey are spread through the dispersion medium, which can be a solid, liquid or gas

Heterogeneous Aqueous SystemsColloidsProperties differ from suspensions and solutionsMay be cloudy when concentrated, clear when diluteIntermediated sized particles cannot be filtered and do not settle outExhibit the Tyndall effect scattering of visible light in all directionsColloids scintillate (flash light) when studied under a microscopeDue to the erratic movement of the particles that reflect light This chaotic movement is known as Brownian motion

Heterogeneous Aqueous SystemsColloidsProperties differ from suspensions and solutionsColloids scintillate (flash light) when studied under a microscopeDue to the erratic movement of the particles that reflect light This chaotic movement is known as Brownian motionCaused by collisions of molecules, which prevent the colloidal properties from settling

Heterogeneous Aqueous SystemsColloidsColloids may also absorb ions onto their surfaceAll the particles in a particular system will have the same chargeRepulsion of like charges keep the colloids from forming aggregatesAdding an opposite charge will cause separation of the colloid

Heterogeneous Aqueous SystemsEmulsionsColloidal dispersions of liquids in liquidsRequires an emulsifying agentEx: soap and detergentsAllow formation of colloidal dispersions between liquids that do not normally mix by forming bonds with the water molecules