chapter 13 properties of solutions dr. subhash c. goel south ga statecollege douglas, ga 31533 ©...

Chapter 13

Properties of Solutions

Dr. Subhash C. GoelSouth GA StateCollege

Douglas, GA 31533© 2012 Pearson Education, Inc.

Solutions

Solutions

• Solutions are homogeneous mixtures of two or more pure substances.

• In a solution, the solute is dispersed uniformly throughout the solvent.

© 2012 Pearson Education, Inc.

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Solutions

The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles.


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How Does a Solution Form?

As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them.


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How Does a Solution Form?

If an ionic salt is soluble in water, it is because the ion–dipole interactions are strong enough to overcome the lattice energy of the salt crystal.


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Energy Changes in Solution

• Simply put, three processes affect the energetics of solution:– Separation of solute

particles,– Separation of solvent

particles,– New interactions

between solute and solvent.


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Energy Changes in Solution

The enthalpy change of the overall process depends on H for each of these steps.


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Why Do Endothermic Processes Occur?

Things do not tend to occur spontaneously (i.e., without outside intervention) unless the energy of the system is lowered.


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Why Do Endothermic Processes Occur?

Yet we know that in some processes, like the dissolution of NH4NO3 in water, heat is absorbed, not released.


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Enthalpy Is Only Part of the Picture

The reason is that increasing the disorder or randomness (known as entropy) of a system tends to lower the energy of the system.


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Enthalpy Is Only Part of the Picture

So even though enthalpy may increase, the overall energy of the system can still decrease if the system becomes more disordered.


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Dissolution vs Reaction

Just because a substance disappears when it comes in contact with a solvent, it doesn’t mean the substance dissolved. It may have reacted.


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Dissolution vs Reaction

• Dissolution is a physical change—you can get back the original solute by evaporating the solvent.

• If you can’t get it back, the substance didn’t dissolve, it reacted.


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Types of Solutions

• Saturated– In a saturated solution, the solvent holds as much solute

as is possible at that temperature.– Dissolved solute is in dynamic equilibrium with solid

solute particles.

• Unsaturated– If a solution is unsaturated, less solute than can dissolve

in the solvent at that temperature is dissolved in the solvent.


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Types of Solutions

• Supersaturated– In supersaturated solutions, the solvent holds

more solute than is normally possible at that temperature.

– These solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask.


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Factors Affecting Solubility

• Chemists use the axiom “like dissolves like.”– Polar substances tend to dissolve in polar solvents.– Nonpolar substances tend to dissolve in nonpolar

solvents.


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The more similar the intermolecular attractions, the more likely one substance is to be soluble in another.


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Glucose (which has hydrogen bonding) is very soluble in water, while cyclohexane (which only has dispersion forces) is not.


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• Vitamin A is soluble in nonpolar compounds (like fats).

• Vitamin C is soluble in water.


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Gases in Solution

• In general, the solubility of gases in water increases with increasing mass.

• Larger molecules have stronger dispersion forces.


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Gases in Solution

• The solubility of liquids and solids does not change appreciably with pressure.

• But the solubility of a gas in a liquid is directly proportional to its pressure.


Copyright © Cengage Learning. All rights reserved. 12 | 22

Henry’s law describes the effect of pressure on gas solubility: The solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the solution.

This is expressed mathematically in the equation

S = kHPwhereS = gas solubility

kH = Henry’s law constant for the gasP = partial pressure of the gas over the solution

Exercise 1

A Henry’s Law Calculation

Calculate the concentration of CO2 in a soft drink that is bottled with a partial pressure of CO2 of 4.0 atm over the liquid at 25 C. The Henry’s law constant for CO2 in water at this temperature is3.4 102 mol/L-atm.

Exercise 2

A Henry’s Law Calculation

Calculate the concentration of O2 in water in moles

per liter at 1.00 atm pressure and 20 C. The mole fraction of O2 in air is 0.209 moles. (Remember the sum of the mole fractions of all gases in a mixture is equal to 1) Henry’s law constant for O2 in water at this temperature is1.3 103 mol/L-atm.

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Temperature

Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.


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Temperature

• The opposite is true of gases.– Solubility of gases in

water decreases with increasing temperature.

– Warm lakes have less O2 dissolved in them than cool lakes.


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Ways of Expressing

Concentrations of Solutions


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Mass Percentage

Mass % of A =mass of A in solutiontotal mass of solution

100


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Parts per Million andParts per Billion

ppm =mass of A in solutiontotal mass of solution

106

Parts per million (ppm)

Parts per billion (ppb)

ppb =mass of A in solutiontotal mass of solution

109


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Exercise 3

(a) A solution is made by dissolving 13.5 g of glucose (C6H12O6) in 0.100 kg of water. What is the mass percentage of solute in this solution? (b) A 2.5-g sample of groundwater was found to contain 5.4 g of Zn2+. What is the concentration of Zn2+ in parts per million?

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moles of Atotal moles of all components

XA =

Mole Fraction (X)

• In some applications, one needs the mole fraction of solvent, not solute—make sure you find the quantity you need!


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moles of soluteliters of solution

M =

Molarity (M)

• You will recall this concentration measure from Chapter 4.

• Since volume is temperature-dependent, molarity can change with temperature.


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moles of solutekilograms of solvent

m =

Molality (m)

Since both moles and mass do not change with temperature, molality (unlike molarity) is not temperature-dependent.


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Exercise 4

Calculation of Molality

A solution is made by dissolving 4.35 g glucose (C6H12O6) in 25.0 mL of water at 25 C. Calculate the molality of glucose in the solution. Water has a density of 1.00 g/mL.

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Exercise 5

Calculation of Mole Fraction and Molality

An aqueous solution of hydrochloricacid contains 36% HCl by mass. (a)Calculate the mole fraction of HClin the solution. (b) Calculate the molality of HCl in the solution.

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Changing Molarity to Molality

If we know the density of the solution, we can calculate the molality from the molarity, and vice versa.


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Exercise 6Calculation of Molarity using density of solution

A solution with a density of 0.876 g/mL contains 5.0 g of toluene (C7H8) and 225 g of benzene. Calculate the molarity of the solution.


Colligative properties of solutions are properties that depend on the concentration of the solute molecules or ions in solution but not on the chemical identity of the solute.

1. Vapor-pressure lowering

2. Boiling-point elevation

3. Freezing-point lowering

4. Osmotic pressure

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Vapor Pressure

Because of solute–solvent intermolecular attraction, higher concentrations of nonvolatile solutes make it harder for solvent to escape to the vapor phase.



Therefore, vapor pressure of a solution, P, is less than the vapor pressure of the pure solvent, P°.

Raoult’s LawWhen the solute is nonvolatile, the vapor pressure of a solution is the mole fraction of the solvent times the vapor pressure of pure solvent.

osolventsolventsolution PXP

oPP solventsolution

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Exercise 7 Calculation of Vapor Pressure of a Solution

Glycerin (C3H8O3) is a nonvolatile nonelectrolyte with a density of 1.26 g/mL at 25 C. Calculate the vapor pressure at 25 C of a solution made by adding 50.0 mL of glycerin to 500.0 mL of water. The vapor pressure ofpure water at 25 C is 23.8 torr (Appendix B), and its density is 1.00 g/mL.

12 | 41Copyright © Cengage Learning. All rights reserved.

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Boiling-Point ElevationSince vapor pressures are lowered for solutions, it requires a higher temperature to reach atmospheric pressure. Hence, boiling point is raised.

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Freezing-Point DepressionThe construction of the phase diagram for a solution demonstrates that the freezing point is lowered while the boiling point is raised.

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Boiling-Point Elevation and Freezing-Point Depression

• The change in temperature is directly proportional to molality (using the van’t Hoff factor).

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The van’t Hoff Factor (i)• What is the van’t Hoff factor?• It takes into account dissociation in solution!• Theoretically, we get 2 particles when NaCl

dissociates. So, i = 2.• In fact, the amount that particles remain together

is dependent on the concentration of the solution.

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Note• Kb is the molal boiling-point

elevation constant, a property of the solvent.

Tb is added to the normal boiling point of the solvent.

• Kf is the molal freezing-point depression constant of the solvent.

Tf is subtracted from the normal freezing point of the solvent.

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Boiling-Point Elevation and Freezing-Point Depression

Note that in both equations, T does not depend on what the solute is, but only on how many particles are dissolved.

Tb = i.Kb m

Tf = i.Kf m


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Exercise 8 Calculating Of Boiling-Point Elevation and Freezing-Point Depression

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

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Colligative Properties of Electrolytes

Since the colligative properties of electrolytes depend on the number of particles dissolved, solutions of electrolytes (which dissociate in solution) should show greater changes than those of nonelectrolytes.


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Exercise 9 Freezing-Point Depression in Aqueous Solutions

List the following aqueous solutions in order of their expected freezing point: 0.050 m CaCl2, 0.15 m NaCl, 0.10 m HCl, 0.050 m CH3COOH, 0.10 m C12H22O11.

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Osmosis

• Some substances form semipermeable membranes, allowing some smaller particles to pass through, but blocking other larger particles.

• In biological systems, most semipermeable membranes allow water to pass through, but solutes are not free to do so.


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Osmosis

In osmosis, there is net movement of solvent from the area of higher solvent concentration (lower solute concentration) to the area of lower solvent concentration (higher solute concentration).


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Osmotic Pressure

The pressure required to stop osmosis, known as osmotic pressure, , is

= ( )RT = MRTnV

where M is the molarity of the solution.

If the osmotic pressure is the same on both sides of a membrane (i.e., the concentrations are the same), the solutions are isotonic.


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Osmosis in Blood Cells

• If the solute concentration outside the cell is greater than that inside the cell, the solution is hypertonic.

• Water will flow out of the cell, and crenation results.


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Osmosis in Cells• If the solute concentration

outside the cell is less than that inside the cell, the solution is hypotonic.

• Water will flow into the cell, and hemolysis results.

• To prevent crenation or hemolysis of red blood cell, the IV solution must be isotonic with the intracellular fluids of the blood cells. © 2012 Pearson Education, Inc.

Solutions

Exercise 10 Calculating Involving Osmotic Pressure

The average osmotic pressure ofblood is 7.7 atm at 25 C. What molarity of glucose (C6H12O6) will be isotonic with blood?

© 2012 Pearson Education, Inc.Chemistry, The Central Science, 12th EditionTheodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward

Determination of Molar Mass

• Colligative properties of the solutions can be used to determine molar mass of the solute.


Exercise11 Molar Mass from Boiling Point Elevation

A solution of an unknown nonvolatile nonelectrolyte was prepared by dissolving 0.250 g of the substance in 40.0 g of CCl4. The boiling point of the resultant solution was 0.357 C higher than that of the pure solvent. Calculate the molar mass of the solute.


Exercise12 Molar Mass from Osmotic Pressure

The osmotic pressure of an aqueous solution of a certain protein was measured to determine the protein’s molar mass. The solution contained 3.50 mg of protein dissolved in sufficient water to form 5.00 mL of solution. Theosmotic pressure of the solution at 25 C was found to be 1.54 torr. Treating the protein as a nonelectrolyte, calculate its molar mass.

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Colloids

Suspensions of particles larger than individual ions or molecules, but too small to be settled out by gravity, are called colloids.


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Colloids in Biological Systems

Some molecules have a polar, hydrophilic (water-loving) end and a nonpolar, hydrophobic (water-hating) end.


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Colloids in Biological Systems

These molecules can aid in the emulsification of fats and oils in aqueous solutions.


chapter 13 properties of solutions dr. subhash c. goel south ga statecollege douglas, ga 31533 ©...

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