Chapter 13 Properties of liquids

Transcription

1 Chapter 13 Properties of liquids 1

2 over 75% of earth is covered with water water supports and enhance life in chemistry, water provides the medium of numerous reactions 13.1 What is a liquid? liquids lie between the extremes of gases and solids liquids contain particles that are close together liquids are essentially incompressible and have definite volume liquids take the shape of their containers 2

3 13.2 Evaporation when a beaker of liquid is allowed stand uncovered, the volume gradually decreases this process is called evaporation (vaporization) in evaporation, molecules of higher-than-average kinetic energy escape from a liquid, leaving it cooler than it was before they escaped sublimation solids go directly from the solid to the gaseous state, bypassing the liquid state liquid solid evaporation sublimation vapor vapor 3

4 13.3 Vapor pressure the space above the liquid is said to be saturated with vapor an equilibrium or steady state exists between the liquid and the vapor, this equilibrium is dynamic evaporation liquid vapor condensation the pressure exerted by a vapor in equilibrium with its liquid is known as the vapor pressure of the liquid the vapor pressure is independent of the amount of liquid and vapor present, but it increases as the temperature rises 4

5 equal volumes of water, ethyl ether and ethyl alcohol are allowed to evaporate at the same temperature order of evaporation: ether > alcohol > water ether has the highest vapor pressure at any temperature the reason for its high vapor pressure is that the attraction is less between ether molecules than between water and alcohol molecules substances that evaporate readily are said to be volatile most substances that are normally in a solid 5 state are nonvolatile

6 13.4 Surface tension a droplet of liquid that is not under the influence of gravity will form a sphere which minimizes the ratio of surface area to volume the resistance of a liquid to an increase in its surface area is called the surface tension substances with large attractive forces between molecules have high surface tension convex concave capillary action the spontaneous rising of a liquid in a narrow tube cohesive forces within liquid adhesive forces between the liquid and the wall of container adhesive forces > cohesive forces capillary action occurs meniscus the surface of the liquid shows a curve adhesive forces > cohesive forces concave adhesive forces < cohesive forces convex 6

7 13.5 Boiling point the vapor pressure of a liquid increases as temperature increases when the internal vapor pressure becomes equal to the external pressure, the liquid boils the boiling temperature of a pure liquid remains constant as long as the external pressure does not vary boiling point the temperature at which the vapor pressure of a liquid is equal to the external pressure above the liquid the normal boiling point is the boiling point at standard pressure (760 torr) heat of vaporization an amount of energy absorbed by 1 g of a liquid changing to a vapor (gas) at normal boiling point 7

8 vapor pressure-temperature curve normal boiling point 8

9 13.6 Freezing point or melting point freezing or solidifying a liquid changing into a solid melting a solid changing into a liquid melting solid liquid freezing freezing point or melting point the temperature solid-liquid equilibrium is achieved ex. ice-water 9

10 13.7 Changes of state heating curve for a pure substance the absorption of heat by a substance from the solid state to the vapor state boiling melting heat of fusion the energy required to change exactly 1 gram of a solid at its melting point into a liquid heat of vaporization the energy required to change exactly 1 gram of a liquid to vapor at its normal boiling point 10

11 ex how many joules of energy are needed to change 10.0 g of ice at 0 o C to water at 20 o C? 0 o C ice water (10 g)(335 J/g) = 3350 J 0 o C water 20 o C water (10 g)(4.184 J/g oc)(20 o C) = 837 J = 4187 J ex.13.2 how many kj of energy are needed to change 20.0 g of water at 20 o C to steam at 100 o C? 20 o C water 100 o C water (20 g)(4.184 J/g)(80 o C) = 6700 J = 6.7 kj 100 o C water steam (20 g)(2.26 kj/g oc) = 45.2 kj = 51.9 kj 11

12 13.8 The hydrogen bond why does water exhibit the anomalies? strong intermolecular force between water molecules is called hydrogen bond hydrogen bond acts like a weak bond between two polar molecules hydrogen bond is formed between two polar molecules that contain H covalently bonded to a small, highly electronegative atom (F, O, N) hydrogen bond is actually the dipole-dipole attraction between polar molecules containing F H, O H, N H bonds 12

13 in water each molecule is linked to others through hydrogen bonds to form a threedimensional aggregate of water molecules this explains its relatively high melting point, boiling point, heat of fusion, and heat of vaporization fluorine forms the strongest hydrogen bonds this bonding is strong enough to link HF molecules together as dimers (HF) 2 of as larger (HF) n molecular units.. F:.. H H.. F:.. H-bond hydrogen bonding can occur between two different atoms that are capable of forming H-bonds O H N O H N ex would you expect H-bonding to occur between the molecules of ethyl alcohol and dimethyl ether? H-bonding should occur in ethyl alcohol 13

14 13.9 Hydrates hydrates solids that contain water molecules as part of their crystalline structure water in hydrate is known as water of hydration or water of crystallization formula for hydrate ex. BaCl 2 2H 2 O barium chloride dihydrate each formula unit contains 1 Ba 2+, 2 Cl -, and 2 H 2 O molecules water molecules in hydrates are bonded by electrostatic forces between polar water molecules and the positive or negative ions of the compound water of crystallization can be removed by moderate heating of the compound ex. 100 o C BaCl 2 2H 2 O(s) CuSO 4 5H 2 O(s) 250 o C BaCl 2 (s) + 2H 2 O(g) CuSO 4 (s) + 5H 2 O(g) 14

15 (CaSO 4 ) 2 H 2 O(s) + 3H 2 O(l) plaster of Paris 2 CaSO 4 2H 2 O(s) 15

16 13.10 Water, a unique liquid Water, a unique liquid water covers ~75% of Earth s surface ~ 97% of Earth s water is in the ocean saline water contains vast amount of dissolving mineral more than 70 elements have been detected in the mineral of seawater only Cl, Na, Mg, Br commercially obtained from the sea 3% fresh water 2/3 is locked up in polar ice caps and glaciers water is the most abundant compound in human body ~70% of total body mass ~92% of blood plasma is water ~80% of muscle tissue is water ~60% of a red blood cell is water 16

17 1. physical properties of water both heat of fusion (335 J/g) and heat of vaporization (2.26 kj/g) are high this indicates strong attractive forces between the molecules maximum density of water is g/ml at 4 o C the density of ice at 0 o C is g/ml 17

18 2. structure of the water molecule water molecule has a bent structure O H bond length: nm bond angle H-O-H: 105 o highly polar molecule 3. formation of water water is very stable to heat pure water is a nonconductor of electricity addition of small amount of H 2 SO 4 or NaOH, water is readily decomposed into H 2 and O 2 by electric current electrical energy 2 H 2 O(l) 2 H 2 (g) + O 2 (g) H 2 SO 4 or NaOH formation of water H 2 burns in air strongly exothermic reaction 2 H 2 (g) + O 2 (g) 2 H 2 O(g) kj acid-base neutralization HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) 18

19 combustion of H-containing materials 2C 2 H 2 (g) + 5O 2 (g) 4CO 2 (g) + 2H 2 O(g) kj metabolic oxidation in living cells enzyme C 6 H 12 O 6 (aq) + 6O 2 (g) 6CO 2 (g) + 6H 2 O(g) kj reverse of photosynthesis 4. reactions of water with metals and nonmetals Na, K, Ca react with cold water 2Na + 2H 2 O H 2 + 2NaOH 2K + 2H 2 O H 2 + 2KOH Ca + 2H 2 O H 2 + Ca(OH) 2 Zn, Al, Fe react with steam at high temperature Zn + H 2 O H 2 + ZnO 2Al + 3H 2 O 3H 2 + Al 2 O 3 3Fe + 4H 2 O 4H 2 + Fe 3 O 4 19

20 Cu, Ag, Hg do not react with cold water or steam to produce H 2 F 2, Cl 2, Br 2 react with water 2F 2 + 2H 2 O 4HF + O 2 2Cl 2 + 2H 2 O HCl + HOCl chlorine water 2Br 2 + 2H 2 O HBr + HOBr bromine water steam passed over hot coke water gas 1000 o C C(s) + H 2 O(g) CO(g) + H 2 (g) 5. reactions of water with metals and nonmetal oxides metal oxides are known as basic anhydrides CaO + H 2 O Ca(OH) 2 Na 2 O + H 2 O 2 NaOH nonmetal oxides are known as acid anhydrides CO 2 + H 2 O H 2 CO 3 SO 2 + H 2 O H 2 SO 3 N 2 O 5 + H 2 O 2 HNO 3 20

21 13.11 Water purification natural fresh waters are not pure to make such water safe to drink, it is treated by the following processes: 1. screening removal of large objects 2. flocculation and sedimentation lime CaO and alum Al 2 (SO 4 ) 3 are added to form precipitate of Al(OH) 3 which traps most of the fine suspended matter 3. sand filtration removing all the remaining suspended matter and bacteria 4. aeration removal of objectionable odors and tastes 5. disinfection addition of Cl 2 gas to kill harmful bacteria ozone is also used 21

22 water that contains dissolved Ca and Mg salts is called hard water there are four techniques used to soften hard water: 1. distillation 2. calcium and magnesium precipitation adding Na 2 CO 3 and lime CaO, CaCO 3 and Mg(OH) 2 are precipitated 3. ion exchange hard water is passed through a tank of zeolite (a complex sodium aluminum silicate) Na 2 (zeolite) + Ca 2+ Ca(zeolite) + 2 Na + 4. demineralization two-stage ion-exchange system metal ions are replaced by H + anions are replaced by OH - water that contains < 2000 ppm of salts is reasonably good for drinking 22 seawater contains ppm of dissolved salts

23 water pollution disposal of hazardous waste products causes water pollution problem 23