11 solutions 2a

CHEM 16 GENERAL CHEMISTRY 1

11 SOLUTIONS

Dr. Gil C. Claudio

University of the Philippines, Diliman

First Semester 2014-2015

TABLE OF CONTENTS

TYPES OF SOLUTIONS

THE SOLUTION PROCESS

Enthalpy of SolutionSolution Formation and Equilibrium

QUANTITATIVE WAYS OF EXPRESSING CONCENTRATION

COLLIGATIVE PROPERTIES OF NONVOLATILE NONELECTROLYTE SOLUTI

Vapor Pressures Lowering and Raoults LawBoiling Point Elevation and Freezing Point DepressionOsmotic Pressure

THE VANT HOFF FACTOR

REFERENCES

References of these notes

General Chemistry, 10th ed, by Ralph H. Petrucci, F. GeoffreyHerring, Jeffy D. Madura, and Carey Bisonnette.

Chemistry: The Central Science, 13th ed., by TheodoreL. Brown, H. Eugene LeMay Jr., Bruce E. Bursten, CatherineJ. Murphy, Patrick M. Woodward, and Matthew W. Stoltzfus.

SOLUTIONS

A homogeneous mixture (solution) is a mixture of elementsand/or compounds that has a uniform composition and propertieswithin a given sample. However, the composition and propertiesmay vary from one sample to another.

The solvent is the component that is present in the greatestquantity or that determines the state of matter in which asolution exists.

Other solution components, called solutes, are dissolved inthe solvent.

A concentrated solution has a relatively large quantity ofdissolved solute(s), and a dilute solution has only a smallquantity.

solid (alloys), liquid, and gaseous solutions

SOME COMMON SOLUTIONSPHMB 10E, TABLE 13.1, P 558

Solution Components

Gaseous solutions

Air N2, O2, et.al.Natural gas CH4, C2H6, et.al.

Liquid solutions

Seawater H2O, NaCl, et.al.Vinegar H2O, CH3COOH (acetic acid)Soda pop H2O, CO2, C12H22O11 (sucrose), et.al.

Solid solutions

Yellow brass Cu, ZnPalladium Hydrogen Pd, H2

SOLUTE AND SOLVENT IFAS

Three kinds of intermolecular interactions are involved in solutionformation:

1. Solute-solute interactions between solute particles must beovercome to disperse the solute particles through the solvent.

2. Solvent-solvent interactions between solvent particles mustbe overcome to make room for the solute particles in thesolvent.

3. Solvent-solute interactions between the solvent and soluteparticles occur as the particles mix.

ENTHALPY OF SOLUTION

In the formation of some solutions, heat is given off to thesurroundings (exothermic, Hsoln < 0); in other cases, heat isabsorbed (endothermic,Hsoln > 0 ). We can look at the solutionprocess in three steps.

(a) pure solvent separatedsolvent molecules Ha > 0

(b) pure solute separatedsolute molecules Hb > 0

(c) separated solventand solute molecules solution Hc < 0

pure solvent +pure solute solution Hsoln

Hsoln = Ha + Hb + Hc

SOLUTE-SOLVENT INTERACTIONS 1: IDEAL

Given the solvent (A) and the solute (B)

If A-A, B-B, and A-B IFAs are of the same type and of equalstrength: the solute and solvent molecules mix randomly, andhomogeneous mixture (or solution) results.

Properties of these solutions can generally be predicted fromthe properties of the pure components, thus they are calledideal solutions.

Hsoln 0

e.g., mixtures of liquid hydrocarbons

SOLUTE-SOLVENT INTERACTIONS 2: NONIDEAL


If the attractive forces of A-B > A-A and A-B > B-B: asolution also forms.

Their properties generally cannot be predicted, thus they arecalled nonideal,

A-B interactions release more heat than A-A and B-B, thusthe solution process is exothermic Hsoln < 0

e.g., solutions of acetone and chloroform, H-bonding possiblein the solution, but not in the pure liquids of both

SOLUTE-SOLVENT INTERACTIONS 3: NONIDEAL


If the attractive forces of A-B is slightly less than A-A andB-B, complete mixing may still occur, but the solution formedis nonideal

The solution process is endothermic Hsoln > 0

e.g., mixtures of carbon disulfide (CS2), a nonpolar liquid, andacetone, a polar liquid.

SOLUTE-SOLVENT INTERACTIONS 4: HETEROGENOUS


If attractive forces of A-B A-A and A-B B-B, thecomponents remain segregated in a heterogeneous mixture

Dissolution does not occur to any significant extent.

e.g., octane and water

PREDICTING SOLUTION FORMATIONPHMB 10E, EXAMPE 13-3, P 565

Predict whether or not a solution will form in each of the followingmixtures and whether the solution is likely to be ideal:

1. ethyl alcohol, CH3CH2OH, and water;

2. the hydrocarbons hexane, CH3(CH2)4CH3, and octane,CH3(CH2)6CH3;

3. octanol, CH3(CH2)6CH2OH, and water.

ANSWERS

1. We expect ethyl alcohol and water to form nonideal solutions.

2. We expect a solution to form, and it should be nearly ideal.

3. We do not expect a solution to form.

FORMATION OF IONIC SOLUTIONS

NaCl(s) Na+(g) + Cl(g) H1 > 0

Na+(g)H2O Na+(aq) H2 < 0

Cl(g)H2O Cl(aq) H3 < 0

NaCl(s)H2O Na+(aq) + Cl(aq) Hsoln

where

H1 = - lattice energy

H2/3 = hydration energy of Na+/Cl

Hsoln = H1 + H2 + H3 for NaCl, Hsoln +5 kJ/mol

DYNAMIC EQUILIBRIUM IN SOLUTIONS

Crystallization, the opposite process of solution, occurs whensolute particles in solution collide with the surface of the solid andreattach.

solute + solventdissolvecrystallize

solution

When the rates of these opposing processes become equal, adynamic equilibrium is established.

SATURATED SOLUTIONS

A saturated solution is one that contains the maximum quantityof solute that is normally possible at the given temperature. Anunsaturated solution contains less solute than the solvent iscapable of dissolving under the given conditions.

A supersaturated solution contains more solute than normallyexpected for a saturated solution, usually prepared from a solutionthat is saturated at one temperature by changing its temperatureto one where supersaturation can occur.

SOLUBILITY

The solubility of a particular solute in a particular solvent is themaximum amount of the solute that can dissolve in a givenamount of the solvent at a specified temperature, assuming thatexcess solute is present.

The extent to which one substance dissolves in anotherdepends on the nature of both substances. It also depends ontemperature and, at least for gases, on pressure.

SOLUBILITY: SOLVENT-SOLUTE INTERACTIONS

In general, the stronger the attractions between solute and solventmolecules, the greater the solubility of the solute in that solvent.

e.g., polar liquids tend to dissolve in polar solvents

Liquids that mix in all proportions (e.g., acetone and water) aremiscible, whereas those that do not dissolve in one another areimmiscible.

SOLUBILITIES OF ALCOHOLS IN H2O AND C6H14

Solubilities of some alcohols in water and in hexane at 20C,expressed in mol alcohol/100 g solvent

solubility solubilityalcohol in H2O in C6H14methanol CH3OH 0.12ethanol CH3CH2OH propanol CH3CH2CH2OH butanol CH3CH2CH2CH2OH 0.11 pentanol CH3CH2CH2CH2CH2OH 0.030 hexanol CH3CH2CH2CH2CH2CH2OH 0.0058

SOLUBILITY OF GASES: PRESSURE EFFECTS

The solubility of a gas in any solvent is increased as the partialpressure of the gas above the solvent increases.

The solubilities of solids and liquids are not appreciablyaffected by pressure.

HENRYS LAW

The relationship between pressure and gas solubility is expressed byHenrys law

C = k Pgas

where

C is the solubility of as gas in a particular solvent at a fixed T

Pgas is the partial pressure of the gas above

k is a proportionality constant

USING HENRYS LAWPHMB 10E, EXAMPLE 13-5, P 572

At 0C and an O2 pressure of 1.00 atm, the aqueous solubility ofO2(g) is 48.9 mL O2 per liter. What is the molarity of O2 in asaturated water solution when the O2 is under its normal partialpressure in air, 0.2095 atm?

ANSWER: 4.57 104 M O2

SOLUBILITY OF GASES: TEMPERATURE EFFECTS

We cannot make an all-inclusive generalization about the effect oftemperature on the solubilities of gases in solvents.

The solubilities of most gases in water decrease with anincrease in temperature.

For solutions of gases in organic solvents, the situation isoften the reverse.

The solubility behavior of the noble gases in water is morecomplex.

SOLUBILITY: TEMPERATURE EFFECTS

The solubility of most solid solutes in water increases as thesolution temperature increases.

with some exceptions

MOLARITY

The molarity M of a solute in a solution is defined as

molarity =moles of solute

L of solution

E.g., if you dissolve 0.500 mol Na2CO3 in enough water to form0.250 L solution, the molarity of Na2CO3 in the solution is 2.00 M.

MOLALITY

Molality m is a solution concentration expressed as the amount ofsolute, in moles, divided by the mass of solvent, in kg.

molality =moles of solute

kg of solvent

E.g., if you mix 0.200 mol NaOH and 0.500 kg of water, theconcentration of the solution is 0.400 m in NaOH.

MOLALITY AND MOLARITY

Molarity M depends on the volume of solution, whereas molality mdepends on the mass of solvent.

When water is the solvent, the m M for dilute solutionsbecause 1 kg of solvent 1 kg of solution 1 L.

The molality of a given solution does not vary withtemperature because masses do not vary with temperature.Not true for molarity due to the temperature dependentvolume expansion of the solvent. Thus, molality is often usedwhen a solution is to be used over a range of temperatures.

MOLE FRACTION AND MOLE PERCENT

Mole fraction xi describes a mixture in terms of the fraction of allthe molecules that are of a particular type. It is the amount of onecomponent, in moles, divided by the total amount of all thesubstances in the mixture.

xi =moles of component i

total amt of all solution components in moles

The sum of the mole fractions of all the solution components is 1.

xi + xj + xk + = 1

A mole percent is a mole fraction expressed on a percentagebasis, that is, mole fraction 100%.

MASS AND VOLUME PERCENT

Mass percentage of a component in a solution is

mass % of a component =mass of component in soln

total mass of soln 100

Volume percentage of a component of a solution is

volume % of a component =volume of component in soln

total volume of soln 100

Another possibility is to express the mass of a solute and thevolume of the solution.

mass/volume % of a component =mass of component in soln

total volume of soln 100

PARTS PER MILLION/BILLION/TRILLION

We often express the concentration of very dilute solutions in partsper million (ppm), parts per billion (ppb), or parts per trillion(ppt).

Similar to mass % but use 106 (a million), 109 (a billion), or1012 (trillion) in place of 100 as a multiplier for the ratio ofthe mass of solute to the mass of solution.

ppm of a component =mass of component in soln

total mass of soln 106

1 ppm is 1 g of a solute in 106 g of solution or 1 mg solute per 1kg solution ( 1 L water solution).

1 ppm = 1 mg/L

1 ppb = 1 g/L

1 ppt = 1 ng/L

CONCENTRATION IN VARIOUS UNITSPHMB 10E, EXAMPLE 13-1, PP 560-561

An ethanol-water solution is prepared by dissolving 10.00 mL ofethanol, CH3CH2OH (d =0.789 g/mL), in a sufficient volume ofwater to produce 100.0 mL of a solution with a density of 0.982g/mL. What is the concentration of ethanol in this solutionexpressed as (a) volume percent; (b) mass percent; (c)mass/volume percent; (d) mole fraction; (e) mole percent; (f)molarity; (g) molality?

ANSWERS: (a) 10.0%; (b) 8.03%; (c) 7.89%; (d) 0.0330; (e)3.30%; (f) 1.71 M CH3CH2OH; (g) 1.89 m CH3CH2OH

CONVERTING MOLARITY TO MOLE FRACTIONPHMB 10E, EXAMPLE 13-2, P 561

Laboratory ammonia is 14.8 M NH3(aq) with a density of 0.8980g/mL. What is xNH3 in this solution?

ANSWER: 0.292

COLLIGATIVE PROPERTIES

Colligative propertiesvapor pressure lowering, freezing pointdepression, boiling point elevation, osmotic pressurehave valuesthat depend on the number of solute particles in a solution but noton their identity.

VAPOR PRESSURE LOWERING

Given a homogeneous solution of a volatile liquid solvent and anonvolatile solute, the solvent molecules are stabilized in theirliquid state by the mixing and thus have a lower tendency toescape into the vapor state. Therefore, the vapor pressure of thesolvent is lower than the vapor pressure of the pure solvent.

RAOULTS LAW

Raoults law states that the vapor pressure of a solutioncomponent PA is equal to the product of the vapor pressure of thepure liquid PA and its mole fraction in solution xA

PA = xAP

A

Strictly speaking, Raoults law applies only to ideal solutions andto all volatile components of the solutions.

However, even in nonideal solutions, the law often worksreasonably well for the solvent in dilute solutions (e.g.,xsolv > 0.98).

VAPOR PRESSURE OF A SOLUTIONBLBMWS 13E, EXERCISE 13.7, PP 549-550

Glycerin C3H8O32 is a nonvolatile nonelectrolyte with a density of1.26 g/mL at 25C. Calculate the vapor pressure at 25C of asolution made by adding 50.0 mL of glycerin to 500.0 mL of water.The vapor pressure of pure water at 25C is 23.8 torr, and itsdensity is 1.00 g/mL.

ANSWER: 23.2 torr

IDEAL SOLUTION OF VOLATILE LIQUIDS

If two volatile liquids (A and B) are mixed and form an idealsolution, both liquids contribute to the total vapor pressure of thesolution. The partial pressures contributed by each follows Raoultslaw, and the total pressure is the sum of the partial pressures

PA = xAP

A

PB = xBP

B

Ptotal = PA + PB

VAPOR PRESSURES OF IDEAL SOLUTIONSPHMB 10E, EXAMPLE 13-6, P 574

The vapor pressures of pure benzene and pure toluene at 25C are95.1 and 28.4 mmHg, respectively. A solution is prepared in whichthe mole fractions of benzene and toluene are both 0.500. Whatare the partial pressures of the benzene and toluene above thissolution? What is the total vapor pressure?

ANSWER: Pbenzene = 47.6 mmHg, Ptoluene = 14.2 mmHg, andPtotal = 61.8 mmHg.

BOILING POINT ELEVATION

The normal boiling point of a liquid is the temperature at which itsvapor pressure equals 1 atm.

Because the solution has a lower vapor pressure than the puresolvent, a higher temperature is required for the solution toachieve a vapor pressure of 1 atm.

As a result, the boiling point of the solution is higher thanthat of the pure solvent.

BOILING POINT ELEVATION

The increase in the boiling point of a solution, relative to the puresolvent, depends on the molality of the solute.

Tb = Tb(solution) Tb(solvent) = iKbm

where

m is the molality of the solute

Kb is the molal boiling point elevation constant for thesolvent

and i is the vant Hoff factor, i = 1 for nonelectrolytes

FREEZING POINT DEPRESSION

The freezing point of a solution is the temperature at which thefirst crystals of pure solvent form in equilibrium with the solution.

the freezing point of the solution is lower than that of thepure liquid.

FREEZING POINT DEPRESSION

The decrease in the freezing point of a solution, relative to thepure solvent, depends on the molality of the solute.

Tf = Tf (solution) Tf (solvent) = iKfm

where

m is the molality of the solute

Kf is the molal freezing point depression constant for thesolvent


SOME CONSTANTS

Freezing-point depression and boiling-point elevation constants

normal FP Kf normal BP Kbsolvent C C/m C C/m

Acetic acid 16.6 3.90 118 3.07Benzene 5.53 5.12 80.10 2.53Nitrobenzene 5.7 8.1 210.8 5.24Phenol 41 7.27 182 3.56Water 0.00 1.86 100.0 0.512Ethanol -114.6 1.99 78.4 1.22carbon tetrachloride -22.3 29.8 76.8 5.02chloroform -63.5 4.68 61.2 3.63

CALCULATION OF BP AND FPBLBMWS 13E, EXERCISE 13.8, PP 553-554

Automotive antifreeze contains ethylene glycol, CH2(OH)CH2(OH)a nonvolatile nonelectrolyte, in water. Calculate the boiling pointand freezing point of a 25.0% by mass solution of ethylene glycolin water.

ANSWERS: Tb = 2.7C and Tf = 10.0

C

MOLAR MASS FROM FP DEPRESSIONBLBMWS 13E, EXERCISE 13.10, P 557

A solution of an unknown nonvolatile nonelectrolyte was preparedby dissolving 0.250 g of the substance in 40.0 g of CCl4. Theboiling point of the resultant solution was 0.357C higher thanthat of the pure solvent. For CCl4, Kb = 5.02

C/m. Calculate themolar mass of the solute.

ANSWER: 88.0 g/mol

OSMOSIS AND OSMOTIC PRESSURE

Osmosis is the net flow of solvent molecules through asemipermeable membrane, from a more dilute solution (or from thepure solvent) into a more concentrated solution.

Osmotic pressure is the pressure that would have to be applied toa solution to stop the passage through a semipermeable membraneof solvent molecules from the pure solvent.

OSMOTIC PRESSURE: DIAGRAM

H

P1a P2a P1b P2b

Inital State

water

dissolved salt

Final State

semi-permeable membrane

commons.wikimedia.org/wiki/File:Osmose en.svg

OSMOTIC PRESSURE: EQUATION

The osmotic pressure is given by

= iMRT

where

M is the concentration in molarity

R is the ideal gas constant

T is the absolute temperature


RELATIVE PRESSURES

Given two solutions separated by a semipermeable membrane

If two solutions of identical osmotic pressure are separated bya semipermeable membrane, no osmosis will occur. The twosolutions are isotonic with respect to each other.

If one solution is of lower osmotic pressure, it is hypotonicwith respect to the more concentrated solution.

The more concentrated solution is hypertonic with respect tothe dilute solution.

OSMOSIS IN BLOOD CELLS

commons.wikimedia.org/wiki/File:Osmotic pressure on blood cells diagram.svg

OSMOSIS IN PLANTS

commons.wikimedia.org/wiki/File:Turgor pressure on plant cells diagram.svg

OSMOTIC PRESSURE CALCULATIONSBLBMWS 13E, EXERCISE 13.9, P 556

The average osmotic pressure of blood is 7.7 atm at 25C. Whatmolarity of glucose C6H12O6 will be isotonic with blood?

ANSWER: 0.31 M

MOLAR MASS FROM OSMOTIC PRESSUREBLBMWS 13E, EXERCISE 13.11, P 557

The osmotic pressure of an aqueous solution of a certain proteinwas measured to determine the proteins molar mass. The solutioncontained 3.50 mg of protein dissolved in sufficient water to form5.00 mL of solution. The osmotic pressure of the solution at 25Cwas found to be 1.54 torr. Treating the protein as a nonelectrolyte,calculate its molar mass.

ANSWER: 8.45 103 g/mol

ION PAIRS IN ELECTROLYTES

The colligative properties of solutions depend on the totalconcentration of solute particles, regardless of whether the particlesare ions or molecules.

since NaCl Na+(aq) + Cl(aq), then i = 2

expected Tf for a 0.100 m NaCl solution is 0.372C, but it

is only 0.348C.

The difference is due to electrostatic attractions between ions.

As the ions move about in solution, ions of opposite chargecollide and stick together for brief moments. While they aretogether, they behave as a single particle called an ion pair.

THE VANT HOFF FACTOR

Instead of using i = number of separated ions, we can calculatethe vant Hoff Factor from freezing-point depression using

i =Tf (measured)

Tf (calculated from nonelectrolyte)

SOME VANT HOFF FACTORS

Measured and expected vant Hoff factors for several substances at25C

compound 0.100 m 0.0100 m 0.00100 m expected

sucrose 1.00 1.00 1.00 1.00NaCl 1.87 1.94 1.97 2.00MgSO4 1.21 1.53 1.82 2.00K2SO4 2.32 2.70 2.84 3.00Pb(NO3)2 2.13 2.63 2.89 3.00

Types of SolutionsThe Solution ProcessEnthalpy of SolutionSolution Formation and Equilibrium

Quantitative Ways of Expressing ConcentrationColligative Properties of Nonvolatile Nonelectrolyte SolutionsVapor Pressures Lowering and Raoult's LawBoiling Point Elevation and Freezing Point DepressionOsmotic Pressure

The van't Hoff Factor

11 solutions 2a

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