chemistry

Ch 12. Properties of Solutions; Mixtures of Substances at the Molecular LevelBrady & Senese, 5th Ed.

Index12.1. Substances mix spontaneously when there is no energy barrier to mixing12.2. Heats of solution come from unbalanced intermolecular attractions12.3. A substance's solubility changes with temperature12.4. Gases become more soluble at higher pressures12.5. Molarity changes with temperature; molality, weight percentages, and mole fractions do not12.6. Solutes lower the vapor pressure of a solvent12.7. Solutions have lower freezing points and higher boiling points than pure solvents12.8. Osmosis is flow of solvent through a semipermeable membrane due to unequal concentrations12.9. Ionic solutes affect colligative properties differently than nonionic solutes

Mixing ProcessesMixing occurs due to interaction between molecules like dissolves likeAs partition is removed, molecules are able to move freely and interactMixed state is statistically more probable

12.1. Substances mix spontaneously when there is no energy barrier to mixing

The Process Of Dissolution Polar solutes dissolve in polar solvents Non-polar solutes dissolve in non-polar solventsDipoles of solvent may induce dipoles in solute, effecting dissolution


Miscibility of LiquidsLiquids that can dissolve in one another are miscible, while insoluble liquids are immiscibleEthanol and water are miscible, while benzene and water are not


Learning CheckWhich of the following are miscible in water?

wateracetic acidcarbon disulfideammonia


Your Turn!Which of the following are likely to be miscible with water?CH3CH2CH2CH3C6H6CH3CO2HAll are expected to be miscible


Dissolution Of An Ionic Compound In WaterPositive end of the dipole of the water surrounds the anions of the ionic solid, extracting them from the latticeNegative end of the dipole orients toward the cations, surrounding and extracting them from the lattice


Dissolution Of A Polar Compound In WaterDipole of the water interacts with the oppositely charged dipoles of the solid, extracting them from the crystal


Enthalpy (Heat) Of SolutionHeat of solution (soln ) is the energy exchanged when a solute dissolves in a solvent at constant pressureEnthalpy is a state function: the pathway can be written in any way and the result will be the sameWhen soln=0, solution is called an ideal solution

12.2. Enthalpy of solution comes from unbalanced intermolecular attractions

Dissolution Of An Ionic SolidVisualized in steps:step1: ionic solid breaks apart into vapor phase lattice energy (U) step 2: vapor phase interacts with solvent solvation energy (Hsolv); if solvent is water, (hydration) soln (ion in water)= U + solvation

12.2. Heats of solution come from unbalanced intermolecular attractions

Dissolution: Liquid In LiquidStep1: solute expandsStep2: solvent expandsStep 3 solute & solvent mixIf the soln=0, we have an ideal solutionsoln = 1 + 2 + 3


Dissolution: Liquid in Liquid (Ideal)


Dissolution: Gas In Liquidstep1: expansion of solventstep2: mixing soln = 1 + 2


Your Turn!What factor does not affect the value of Hsoln ?The polarities of solute and solventThe size of the soluteThe charge on the soluteThe temperature of the solutionAll affect the value


Saturated SolutionsSolute is at equilibrium with the dissolved soluteAddition of more dissolved solute results in supersaturation and precipitation of excess solidThe presence of less solute than the solubility results in an unsaturated solution

12.3. A substance's solubility changes with temperature

Solubility Varies With TemperatureSolubility may increase or decrease with increasing temperatureThe extent to which temperature has an effect is specific to the solute and solventMost gases are less soluble in water at high temperature, while most solids are more soluble


Case Study: Dead ZonesDuring the industrial revolution, factories were built on rivers so that the river water could be used as a coolant for the machinery. The hot water was dumped back into the river and cool water recirculated. After some time, the rivers began to darken and many fish died. The water was not found to be contaminated by the machinery. What was the cause of the mysterious fish kills?increased temperature lowered amounts of dissolved oxygen


Effects Of Temperature On SolubilitySolubility varies with temperature according to the enthalpy of solvationThe efficiency of a solvation process (K) depends on the enthalpy (H) in Joules, the ideal gas constant (R), and the temperature (T) in KelvinIf the dissolution process is endothermic ( is +), increasing temperature results in greater efficiency


Your Turn!The solubility of a substances increases with increased temperature if:Hsolution >0Hsolution

Pressure Effects On Solubility Of GasesCg=kHPgC = concentration of dissolved gas (M)kH = Henrys ConstantP = pressure applied to system (mm Hg)kH (M/mm Hg)N28.4210 -7O21.6610-4CO24.4810-5Gases are all more soluble at higher pressures (the cause of the bends)

12.4. Gases become more soluble at higher pressures

Learning CheckWhat is the concentration of dissolved nitrogen in a solution that is saturated in N2 at 2.0 atm kH= 8.4210 -7 (M / mm Hg)

Cg=kHPgCg= 8.4210 -7 (M / mm Hg) 2.0 atm 760 mmHg/atmCg=1.3 10-3 M


Your Turn! When you open a bottle of seltzer, it fizzes. How should you store it to increase the time before it goes flat? Heat it and pressurize itCool it and pressurize itHeat it and reduce the pressureCool it and reduce the pressure


Units Of ConcentrationMolarity (M) = moles solute / L solution changes with TemperatureMolality (m) = moles solute/kg solventmole fraction (X) X = moles component/ total molesPercent by mass (%)(mass solute / mass solution)*100

12.5. Molarity changes with temperature; molality, weight percentages, and mole fractions do not

Units Of Very Low ConcentrationsParts per million (ppm)g solute/mL soln Parts per billion (ppb)ng solute/ mL solnIn extremely dilute solutions mostly solvent is presentWhen the solvent is water (d1g/mL) thus for ppm g solute/g soln1/106 magnitude difference leads to the name 1 part per 1 billion


Organize Your Thoughts!All concentration units are a ratio of informationDevelop a sense of the data that you have available


Learning Check: What Does Molarity Tell Us?M=moles solute/L solution. What are the m, X, % and ppm concentration of a 1.0M solution of KCl with a density of 0.99 g/mL74.5599050.815915.4451.815X = 0.019m = 1.1% =7.5ppm=7.5(104)


Learning Check: What Does Molality Tell Us?m=moles solute/kg solvent.What are the M, X, % and ppm concentration of 1.0 m KCl with a density of 0.98 g/mL74.551074.5555.5156.511096 mL=1.096 L% = 6.9ppm =6.9104M = 0.91X = 0.018


Learning Check: What Does Mole Fraction Mean?Xsolute = moles solute/moles total. What are the M, m, % and ppm concentration of a solution that has XKCl = 0.060 with a density of 0.87 g/mL4.4730.9416.9321.40324.601 mL=.024601 L% = 21ppm =1.8105M =2.4m = 3.5


Learning Check: What Does % Mass Tell Us%=(mass solute/mass solution) x 100. What are the M, m, X and ppm concentration of a 1.05 % KCl solution with a density of 1.15 g/mL.014084398.9554.925555.066486.957 mL=.086957 LX = 2.2610-4ppm =1.21104M =0.162m = 0.142


Your turn!Which of the following corresponds to a 3.5M solution of NaCl with a density of 0.997 g/mL?MM H2O: 18.0153; NaCl: 58.443


Raoults LawVapor pressure of a liquid varies as a function of purity X= mole fraction of solventP0= vapor pressure of pure solventPsolution=XsolventP0solventPsolution=XAP0A+XBPB0Where A and B are both volatile components.

12.6. Solutes lower the vapor pressure of a solvent

Learning CheckThe vapor pressure of 2-methylheptane is 233.95 torr at 55C. 3-ethylpentane has a vapor pressure of 207.68 at the same temperature. What would be the pressure of the mixture of 78.0g 2-methylheptane and 15 g 3-ethylpentane? Psolution=XAP0A+XBP0Bmole 2-methylheptane : 78.0g/114.23 g/mol = 0.68283 molmole 3-ethylpentane: 15g/100.2 g/mol = 0.1497 molX2-methylheptane=0.8202 ; X3-ethylpentane =1-0.8202 = 0.1798

P = 230 torr


Learning CheckThe vapor pressure of 2-methyl hexane is 37.986 torr at 15C. What would be the pressure of the mixture of 78.0g 2-methylhexane and 15 g naphthalene which is nearly non-volatile at this temperature?Psolution=XsolventP0solventmol 2-methylhexane: 78.0g/100.2 g/mol = 0.778443 molmol naphthalene: 15 g/128.17 g/mol = 0.11703X2-methylhexane = 0.869309Psolution = 0.869309 37.986 torrP=33.02 torr


Your Turn!n-hexane and n-heptane are miscible in a large degree and both volatile. If the vapor pressure of pure hexane is 151.28 mm Hg and heptane is 45.67 at 25, which equation can be used to determine the mole fraction of hexane in the mixture if the mixtures vapor pressure is 145.5 mm Hg?X(151.28 mmHg)=145.5 mmHgX(151.28 mmHg) + (X)(45.67 mm Hg) = 145.5 mmHgX(151.28 mmHg)+(1-X)(45.67 mm Hg)=145.5 mm HgNone of these


Solute Effects On Phase Changes:Regardless of the identity of the dissolved particles, the presence of an impurity will result in a change in the boiling point and freezing point. The effect is solely dependent on the nature of the solvent, a factor labeled K, and the concentration of particles present (m)T=mKboiling point elevation T=Tmix-Tpurefreezing Point Depression T=Tpure-Tmix

12.7. Solutions have lower freezing points and higher boiling points than pure solvents

Effects Of Impurities On Phase Changes


Some BP/FP Constants


Learning CheckAccording to the Sierra Antifreeze literature, the freezing point of a 40/60 solution of sierra antifreeze and water is -4 F. What is the molality of the solution?11=m-4F = 1.8 (C) + 32-20. C


Learning Check:In the previous sample of a Sierra antifreeze mixture, 100 mL is known to contain 42 g of the antifreeze and 60. g of water, what is the molar mass of the compound found in this antifreeze if it has a freezing point of -4F? 650 g/mol solute0.6452 mol solute


Learning Check:In the previous sample of a Sierra antifreeze mixture, the freezing point is -4F? What will be its boiling point?from before:-4F = 1.8 (C) + 32 =-20. CT=105 C


Your Turn!Beer is known to be around a 5% ethanol (C2H5OH) solution with a density of 1.05 g/mL. What is its expected boiling point?( Kf=0.51/m)100C101C102C103CNot enough information givenMM: H2O=18.0153; C2H5OH=46.069


OsmosisWhen two solutions are separated by a semi-permeable membrane, solvent molecules flow from areas of low concentration to areas of high concentrationAs this occurs, the height of liquid rises in the higher concentration solution, building up Osmotic pressure ()

12.8. Osmosis is flow of solvent through a semipermeable membrane due to unequal concentrations

Relative Concentration Terms In OsmosisHypotonic solutions have lower ion concentrations than the cells.Hypertonic solutions have higher ion concentrations than the cellsIsotonic solutions have the same ion concentration as the cells


Osmosis=MRTthe concentration, is in molarity, MT=Temperature, in KelvinR=Ideal Gas Constant, 0.082057 Latm/molKThe basis for kidney function, rising sap, and dialysis


Learning Check: OsmosisA solution of D5W, 5% dextrose (C6H1206) in water is placed into the osmometer shown at right. It has a density of 1.0 g/mL. The surroundings are filled with distilled water. What is the expected osmotic pressure at 25C?


Learning CheckFor a typical blood plasma, the osmotic pressure at body temperature (37C) is 5409 mm Hg. If the dominant solute is serum protein, what is the concentration of serum protein?


DialysisPores on the semi-permeable membrane may be of varied sizeIn dialysis, the pores are fairly large, allowing transfer of solvent, ions, and small proteinsLarger cells, such as red blood cells are prevented from passing through the poresThe dialysis bath may be enriched in substances lacking in the blood, and is hypotonic in waste products in the bloodExchange of vital components may be made


Your Turn!Suppose that your tap water has 250 ppb of dissolved H2S , and that its density is about 1.0 g/mL. What is its osmotic pressure at 25C?0.00058 atm0.064 atm0.059 atmNone of these

MM: H2S =34.0760.21 atm


Ionic Solutes Affect Colligative Properties Differently Than Non-ionic Solutessubstances that ionize make more particles in a solution than their own concentration suggests i is a factor that demonstrates how many ions are formed per formula unit or molecule the apparent molality of particles is then im.

12.9. Ionic solutes affect colligative properties differently than nonionic solutes

Learning CheckIn preparing pasta, 2 L of water at 25C are combined with about 15 g salt (NaCl, MM= 58.44g/mol) and the solution brought to a boil. What is the expected boiling point of the water?T=imKbpmass of water =volume density =2000 mL 1.0 g/mL=2000g water = 2 kgmol NaCl = 15g / 58.44 g/molmol NaCl = 0.25667m=0.25667 mol / 2kg=0.123T=100.1 C


Case StudySuppose you run out of salt. What mass of sugar (C12H22O11, MM=342.30 g/mol) added to 2 L of water would raise the temperature of water by 0.10 C?T=imKbpmass of water =volume density =2000 mL 1.0 g/mL=2000g water = 2 kg0.39215 mol = ?g / 342.30 g/molmass sucrose =130 g0.196 m=? mol / 2kg0.39215molm=.196


click on the link to advance to that sectionFigure 12.1 Mixing of gases. When two gases, initially in separate compartments (a), suddenly find themselves in the same container (b), they mix spontaneously.ChemFAQ: Why do gases mix spontaneously?Figure 12.2 Hydrogen bonds in aqueous ethanol. Ethanol molecules form hydrogen bonds (...) to water molecules.Figure 12.3 Hydration of ions. Hydration involves a complex redirection of forces of attraction and repulsion. Before this solution forms, water molecules are attracted only to each other, and Na+ and Cl- ions have only each other in the crystal to be attracted to. In the solution, the ions have water molecules to take the places of their oppositely charged counterparts; in addition, water molecules are attracted to ions even more than they are to other water molecules.Figure 12.4 Hydration of a polar molecule. A polar molecule of a molecular compound (such as the sugar glucose) can trade the forces of attraction it experiences for other molecules of its own kind for forces of attraction to molecules of water in an aqueous solution. Red areas indicate high electron density; blue areas have low electron density. Chem FAQs: What is a heat of solution? What is an ideal solution?Figure 12.5 Enthalpy diagram for the heat of solution one mole of potassium iodide. Adding the lattice energy to the hydration energy gives a positive value for Hsoln, indicating the solution process is endothermic.Chem FAQs: What is a heat of solution? What is an ideal solution?Chem FAQ: How can the solution of one liquid into another be modeled?Figure 12.6 Enthalpy of solution for the mixing of two liquids. To analyze the enthalpy change for the formation of a solution of two liquids, we can imagine the hypothetical steps shown here. Step 1. The molecules of the liquid designated as the solvent move apart slightly to make room for the solute molecules, an endothermic process. Step 2. The molecules of the solute are made to take up a larger volume to make room for the solvent molecules, which is also an endothermic change. Step 3. The expanded samples of solute and solvent spontaneously intermingle, their molecules also attracting each other making the step exothermic.Figure 12.7 Enthalpy changes in the formation of an ideal solution. The three-step and the direct-formation paths both start and end at the same place with the same enthalpy outcome. The sum of the positive H values for the two endothermic steps, 1 and 2, numerically equals the negative H value for the exothermic step, 3. The net H for the formation of an ideal solution is therefore zero.Figure 12.8 A molecular model of gas solubility. (a) A gas dissolves in an organic solvent. Energy is absorbed to open pockets in the solvent that can hold the gas molecules. In the second step, energy is released when the gas molecules enter the pockets where they are attracted to the solvent molecules. Here the solution process is shown to be endothermic. (b) At room temperature, waters loose network of hydrogen bonds already contain pockets that can accommodate gas molecules, so little energy is needed to prepare the solvent to accept the gas. In the second step, energy is released as the gas molecules take their places in the pockets where they experience attractions to the water molecules. In this case, the solution process is exothermic.Figure 12.9 Solubility in water versus temperature for several substances. Most substances become more soluble when the temperature of the solution is increased, but the amount of this increased solubility varies considerably.Chem FAQ: How does pressure affect the solubility of gases?

Chem FAQ: Convert %(w/w) to molarity. Prepare solutions of a specified concentration. Prepare solutions of a specified molality.subscripts are not significant but retained to prevent roundingChem FAQ: Convert mass percent to molality.Chem FAQ: How do I estimate vapor pressures with Raoult's Law?I=vant Hoff factor: For our purposes, number of ions/mol dissolved substance. Chem FAQs: Why does adding a solute lower the freezing point of a solution? (Why does salt melt ice?) Estimate the freezing point of a solution. Estimate the molar mass of a solute from experimental freezing point depression data. Estimate the freezing point of a salt solution.Figure 12.14 Phase diagrams for water and an aqueous solution. Phase diagram for pure water and an aqueous solution of a nonvolatile solute. http://www.sierraantifreeze.com/tech.htmlhttp://www.sierraantifreeze.com/tech.htmlhttp://www.sierraantifreeze.com/tech.htmlChem FAQs: Estimate osmotic pressure using the van't Hoff equation. Estimate a molar mass of a solute from experimental osmotic pressure measurements.Figure 12.16 Osmosis and osmotic pressure. (a) Initial conditions. A solution, B, is separated from pure water, A, by an osmotic membrane; no osmosis has yet occurred. (b) After a while, the volume of fluid in the tube has increased visibly. Osmosis has taken place. (c) A back pressure is needed to prevent osmosis. The amount of back pressure is the osmotic pressure of the solution.Monitoring serum Colloid osmotic pressure (COP) after heart surgery allows prediction of pulmonary edema formation.

Figure 12.18 Simple osmometer. When solvent moves into the solution by osmosis, the level of the solution in the capillary rises. The height reached can be related to the osmotic pressure of the solution.Chem FAQ: What is percent ionization?

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