Compounds in Aqueous Solutions

Total Ionic Equations• Once you write the molecular equation (synthesis,

decomposition, etc.), you should check for reactants and products that are soluble or insoluble.

• We usually assume the reaction is in water• We can use a solubility table to tell us what

compounds dissolve in water.• If the compound is soluble (does dissolve in water),

then splits the compound into its component ions• If the compound is insoluble (does NOT dissolve in

water), then it remains as a compound

General Solubility Guidelines

Total Ionic Equations

Molecular Equation:K2CrO4 + Pb(NO3)2 PbCrO4 + 2 KNO3

Soluble Soluble Insoluble Soluble

Total Ionic Equation:2 K+ + CrO4 -2 + Pb+2 + 2 NO3

-

PbCrO4 (s) + 2 K+ + 2 NO3-

Net Ionic Equations

• These are the same as total ionic equations, but you should cancel out ions that appear on BOTH sides of the equation

Total Ionic Equation:2 K+ + CrO4 -2 + Pb+2 + 2 NO3

- PbCrO4 (s) + 2 K+ + 2 NO3

-

Net Ionic Equation:CrO4 -2 + Pb+2 PbCrO4 (s)

Net Ionic Equations

• Try this one! Write the molecular, total ionic, and net ionic equations for this reaction: Silver nitrate reacts with Lead (II) Chloride in hot water.

AgNO3 + PbCl2

Molecular: 2 AgNO3 + PbCl2 2 AgCl + Pb(NO3)2

Total Ionic:2 Ag+ + 2 NO3

- + Pb+2 + 2 Cl- 2 AgCl (s) + Pb+2 + 2 NO3-

Net Ionic: Ag+ + Cl- AgCl (s)

• Ions are formed from solute molecules by the action of the solvent in a process called ionization.

• When a molecular compound dissolves and ionizes in a polar solvent, ions are formed where none existed in the undissolved compound.

• Hydrogen chloride, HCl, is a molecular compound that ionizes in aqueous solution. • HCl contains a highly polar bond.

aq + aq2H O –HCl H ( ) Cl ( )

Chapter 13Ionization

The Hydronium Ion• Some molecular compounds ionize in an aqueous

solution to release H+.

• The H+ ion attracts other molecules or ions so strongly that it does not normally exist alone.

aq + aq2H O –3HCl H O ( ) Cl ( )

Chapter 13

• The H3O+ ion is known as the hydronium ion.

Strong and Weak Electrolytes• Electrolytes are substances that yield ions and conduct an

electric current in solution.

• The strength with which substances conduct an electric current is related to their ability to form ions in solution.

• Strong and weak electrolytes differ in the degree of ionization or dissociation.

Chapter 13

Models for Strong and Weak Electrolytes and Nonelectrolytes

Strong Electrolytes

• A strong electrolyte is any compound whose dilute aqueous solutions conduct electricity well; this is due to the presence of all or almost all of the dissolved compound in the form of ions.

• To whatever extent they dissolve in water, they yield only ions.

• HCl, HBr, HI• All soluble ionic compounds

Chapter 13

[HF] >> [H+] and [F–]

Weak Electrolytes• A weak electrolyte is any compound whose dilute

aqueous solutions conduct electricity poorly; this is due to the presence of a small amount of the dissolved compound in the form of ions.

• Some molecular compounds form aqueous solutions that contain not only dissolved ions but also some dissolved molecules that are not ionized.

aq + aq–3HF H O ( ) F ( )

Chapter 13

lowering

Vapor Pressure Reduction

Vapor Pressureof concentratedsolution is lowerbecause solute particles interferewith vaporization

ΔTb = Kbm

Also results in boiling point elevation:

Pure Solvent Solution

Solvent Particles travel from Right to LeftLess concentrated to more concentrated

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Boiling Point Elevation

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

Tb = Kb m i

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

m = molality of the solvent i = Van’t Hoff constant

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

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Freezing Point Depression• The change in freezing point can be found similarly:

Tf = Kf m iwhere: Kf = molal freezing point depression constant, a property of the solvent.

m = molality of the solvent i = Van’t Hoff constant

• Tf is subtracted from the normal boiling point of the solvent.

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Boiling Point Elevation and Freezing Point Depression

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

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Colligative Properties of Electrolytes

Since these properties 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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Colligative Properties of Electrolytes

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

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van’t Hoff Factor

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

• Some Na+ and Cl- reassociate for a short time, so the true concentration of particles is somewhat less than two times the concentration of NaCl.

• Reassociation is more likely at higher concentration.

© 2009, Prentice-Hall, Inc.

van’t Hoff Factor

Vapor Pressure Reduction/Boiling Point Elevation

ΔTb = Kbm

Freezing Point Depression

ΔTf = Kfm