Chapter 13 properties of solutions
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Chemistry, The Central Science , 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten. Chapter 13 Properties of Solutions. Adapted by SA Green from: John D. Bookstaver St. Charles Community College St. Peters, MO  2006, Prentice Hall, Inc. Solutions.

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Chapter 13 Properties of Solutions

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Chapter 13 properties of solutions

Chemistry, The Central Science, 10th edition

Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten

Chapter 13Properties of Solutions

Adapted by SA Green from:

John D. Bookstaver

St. Charles Community College

St. Peters, MO

 2006, Prentice Hall, Inc.


Solutions

Solutions

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

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


Solutions1

Solutions

How does a solid dissolve into a liquid?

What ‘drives’ the dissolution process?

What are the energetics of dissolution?


How does a solution form

How Does a Solution Form?

  • Solvent molecules attracted to surface ions.

  • Each ion is surrounded by solvent molecules.

  • Enthalpy (DH) changes with each interaction broken or formed.

Ionic solid dissolving in water


How does a solution form1

How Does a Solution Form?

  • Solvent molecules attracted to surface ions.

  • Each ion is surrounded by solvent molecules.

  • Enthalpy (DH) changes with each interaction broken or formed.


How does a solution form2

How Does a Solution Form

The ions are solvated (surrounded by solvent).

If the solvent is water, the ions are hydrated.

The intermolecular force here is ion-dipole.


Energy changes in solution

Energy Changes in Solution

To determine the enthalpy change, we divide the process into 3 steps.

  • Separation of solute particles.

  • Separation of solvent particles to make ‘holes’.

  • Formation of new interactions between solute and solvent.


Enthalpy changes in solution

Enthalpy Changes in Solution

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

Start

End

Start

End


Enthalpy changes during dissolution

Enthalpy changes during dissolution

DHsoln = DH1 + DH2 + DH3

The enthalpy of solution, DHsoln, can be either positive or negative.

DHsoln (MgSO4)= -91.2 kJ/mol --> exothermic

DHsoln (NH4NO3)= 26.4 kJ/mol --> endothermic


Why do endothermic processes sometimes occur spontaneously

Why do endothermic processes sometimes occur spontaneously?

Some processes, like the dissolution of NH4NO3 in water, are spontaneous at room temperature even though heat is absorbed, not released.


Enthalpy is only part of the picture

Enthalpy Is Only Part of the Picture

Entropy is a measure of:

  • Dispersal of energy in the system.

  • Number of microstates (arrangements) in the system.

    b. has greater entropy,  is the favored state

(more on this in chap 19)


Entropy changes during dissolution

Entropy changes during dissolution

Each step also involves a change in entropy.

  • Separation of solute particles.

  • Separation of solvent particles to make ‘holes’.

  • Formation of new interactions between solute and solvent.


Chapter 13 properties of solutions

SAMPLE EXERCISE 13.1Assessing Entropy Change

In the process illustrated below, water vapor reacts with excess solid sodium sulfate to form the hydrated form of the salt. The chemical reaction is

Does the entropy of the system increase or decrease?


Dissolution vs reaction

dry

Dissolution vs reaction

NiCl2(s)

Ni(s) + HCl(aq)

NiCl2(aq) + H2(g)

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

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


Degree of saturation

Degree of saturation

  • Saturated solution

    • Solvent holds as much solute as is possible at that temperature.

    • Undissolved solid remains in flask.

    • Dissolved solute is in dynamic equilibrium with solid solute particles.


Degree of saturation1

Degree of saturation

  • Unsaturated Solution

    • Less than the maximum amount of solute for that temperature is dissolved in the solvent.

    • No solid remains in flask.


Degree of saturation2

Degree of saturation

  • Supersaturated

    • Solvent holds more solute than is normally possible at that temperature.

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


Degree of saturation3

Degree of saturation

Unsaturated, Saturated or Supersaturated?

 How much solute can be dissolved in a solution?

More on this in Chap 17

(solubility products, p 739)


Factors affecting solubility

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.


Factors affecting solubility1

Factors Affecting Solubility

The stronger the intermolecular attractions between solute and solvent, the more likely the solute will dissolve.

Example: ethanol in water

Ethanol = CH3CH2OH

Intermolecular forces = H-bonds; dipole-dipole; dispersion

Ions in water also have ion-dipole forces.


Factors affecting solubility2

Factors Affecting Solubility

Glucose (which has hydrogen bonding) is very soluble in water.

Cyclohexane (which only has dispersion forces) is not water-soluble.


Factors affecting solubility3

Factors Affecting Solubility

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

  • Vitamin C is soluble in water.


Chapter 13 properties of solutions

Which vitamin is water-soluble and which is fat-soluble?


Gases in solution

Gases in Solution

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

    Why?

  • Larger molecules have stronger dispersion forces.


Gases in solution1

Gases in Solution


Gases in solution2

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.

Increasing pressure above solution forces more gas to dissolve.


Henry s law

Henry’s Law

Sg = kPg

where

  • Sg is the solubility of the gas;

  • k is the Henry’s law constant for that gas in that solvent;

  • Pg is the partial pressure of the gas above the liquid.


Henry s law1

Henry’s Law

k for N2 at 25°

=6.8 x 10-4 mol/L atm

Sg = kPg


Temperature

Temperature

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


Temperature1

Temperature

  • The opposite is true of gases. Higher temperature drives gases out of solution.

    • Carbonated soft drinks are more “bubbly” if stored in the refrigerator.

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


Chap 13 ways of expressing concentrations of solutions

Chap 13:Ways of Expressing Concentrations of Solutions


Mass percentage

mass of A in solution

total mass of solution

Mass Percentage

Mass % of A =

 100


Parts per million and parts per billion

mass of A in solution

total mass of solution

mass of A in solution

total mass of solution

Parts per Million andParts per Billion

Parts per Million (ppm)

ppm =

 106

Parts per Billion (ppb)

 109

ppb =


Mole fraction x

moles of A

total moles in solution

XA =

Mole Fraction (X)

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


Molarity m

mol of solute

L of solution

M =

Molarity (M)

  • You will recall this concentration measure from Chapter 4.

  • Because volume is temperature dependent, molarity can change with temperature.


Molality m

mol of solute

kg of solvent

m =

Molality (m)

Because neither moles nor mass change with temperature, molality (unlike molarity) is not temperature dependent.


Moles mass

Mass/Mass

Moles/Mass

Moles/Moles

Moles/L


Chapter 13 properties of solutions

SAMPLE EXERCISE 13.4 Calculation of Mass-Related Concentrations

(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?

PRACTICE EXERCISE

(a) Calculate the mass percentage of NaCl in a solution containing 1.50 g of NaCl in 50.0 g of water. (b) A commercial bleaching solution contains 3.62 mass % sodium hypochlorite, NaOCl. What is the mass of NaOCl in a bottle containing 2500 g of bleaching solution?

PRACTICE EXERCISE

A commercial bleach solution contains 3.62 mass % NaOCl in water. Calculate (a) the molality and (b) the mole fraction of NaOCl in the solution.


Colligative properties

Colligative Properties

  • Colligative properties depend only on the number of solute particles present, not on the identity of the solute particles.

  • Among colligative properties are

    • Vapor pressure lowering

    • Boiling point elevation

    • Melting point depression

    • Osmotic pressure


Vapor pressure

Vapor Pressure

As solute molecules are added to a solution, the solvent become less volatile (=decreased vapor pressure).

Solute-solvent interactions contribute to this effect.


Vapor pressure1

Vapor Pressure

Therefore, the vapor pressure of a solution is lower than that of the pure solvent.


Raoult s law

Raoult’s Law

PA = XAPA

where

  • XA is the mole fraction of compound A

  • PA is the normal vapor pressure of A at that temperature

    NOTE: This is one of those times when you want to make sure you have the vapor pressure of the solvent.


Chapter 13 properties of solutions

SAMPLE EXERCISE 13.8 Calculation of Vapor-Pressure Lowering

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 of pure water at 25°C is 23.8 torr (Appendix B).

PRACTICE EXERCISE

The vapor pressure of pure water at 110°C is 1070 torr. A solution of ethylene glycol and water has a vapor pressure of 1.00 atm at 110°C. Assuming that Raoult’s law is obeyed, what is the mole fraction of ethylene glycol in the solution?


Boiling point elevation and freezing point depression

Boiling Point Elevation and Freezing Point Depression

Solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.


Boiling point elevation

Boiling Point Elevation

The change in boiling point is proportional to the molality of the solution:

Tb = Kb  m

where Kb is the molal boiling point elevation constant, a property of the solvent.

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


Freezing point depression

Freezing Point Depression

  • The change in freezing point can be found similarly:

    Tf = Kf  m

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

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


Boiling point elevation and freezing point depression1

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

Tb = Kb  m

Tf = Kf  m

Boiling Point Elevation and Freezing Point Depression


Colligative properties of electrolytes

Colligative Properties of Electrolytes

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

e.g. NaCl dissociates to form 2 ion particles; its limiting van’t Hoff factor is 2.


Colligative properties of electrolytes1

Colligative Properties of Electrolytes

However, a 1 M solution of NaCl does not show twice the change in freezing point that a 1 M solution of methanol does.

It doesn’t act like there are really 2 particles.


Van t hoff factor

van’t Hoff Factor

One mole of NaCl in water does not really give rise to two moles of ions.


Van t hoff factor1

van’t Hoff Factor

Some Na+ and Cl− reassociate as hydrated ion pairs, so the true concentration of particles is somewhat less than two times the concentration of NaCl.


The van t hoff factor

The van’t Hoff Factor

  • Reassociation is more likely at higher concentration.

  • Therefore, the number of particles present is concentration dependent.


The van t hoff factor1

The van’t Hoff Factor

We modify the previous equations by multiplying by the van’t Hoff factor, i

Tf = Kf  m i

i = 1 for non-elecrtolytes


Osmosis

Osmosis

  • Semipermeable membranes allow some particles to pass through while blocking others.

  • In biological systems, most semipermeable membranes (such as cell walls) allow water to pass through, but block solutes.


Osmosis1

Osmosis

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

Water tries to equalize the concentration on both sides until pressure is too high.


Osmotic pressure

 =()RT = MRT

n

V

Osmotic Pressure

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

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.


Osmosis in blood cells

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.


Osmosis in cells

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.


Molar mass from colligative properties

Molar Mass from Colligative Properties

We can use the effects of a colligative property such as osmotic pressure to determine the molar mass of a compound.


Colloids

Colloids:

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


Tyndall effect

Tyndall Effect

  • Colloidal suspensions can scatter rays of light.

  • This phenomenon is known as the Tyndall effect.


Colloids in biological systems

Colloids in Biological Systems

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


Colloids in biological systems1

Colloids in Biological Systems

Sodium stearate is one example of such a molecule.


Colloids in biological systems2

Colloids in Biological Systems

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


End chap 13

END Chap 13


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