 # Compounds in Aqueous Solutions. Total Ionic Equations Once you write the molecular equation (synthesis, decomposition, etc.), you should check for reactants.

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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: K 2 CrO 4 + Pb(NO 3 ) 2  PbCrO 4 + 2 KNO 3 SolubleSolubleInsoluble Soluble Total Ionic Equation: 2 K + + CrO 4 -2 + Pb +2 + 2 NO 3 -  PbCrO 4 (s) + 2 K + + 2 NO 3 -

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 + + CrO 4 -2 + Pb +2 + 2 NO 3 -  PbCrO 4 (s) + 2 K + + 2 NO 3 - Net Ionic Equation: CrO 4 -2 + Pb +2  PbCrO 4 (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. AgNO 3 + PbCl 2  Molecular: 2 AgNO 3 + PbCl 2  2 AgCl + Pb(NO 3 ) 2 Total Ionic: 2 Ag + + 2 NO 3 - + Pb +2 + 2 Cl -  2 AgCl (s) + Pb +2 + 2 NO 3 - 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. Chapter 13 Ionization

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. Chapter 13 The H 3 O + 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. Chapter 13

lowering

Vapor Pressure Reduction Vapor Pressure of concentrated solution is lower because solute particles interfere with vaporization ΔT b = K b m Also results in boiling point elevation: Pure Solvent Solution

Solvent Particles travel from Right to Left Less concentrated to more concentrated

© 2009, Prentice-Hall, Inc. Boiling Point Elevation The change in boiling point is proportional to the molality of the solution:  T b = K b m i where: K b = molal boiling point elevation constant, a property of the solvent. m = molality of the solvent i = Van’t Hoff constant  T b is added to the normal boiling point of the solvent.

© 2009, Prentice-Hall, Inc. Freezing Point Depression The change in freezing point can be found similarly:  T f = K f m i where: K f = molal freezing point depression constant, a property of the solvent. m = molality of the solvent i = Van’t Hoff constant  T f is subtracted from the normal boiling point of the solvent.

© 2009, Prentice-Hall, Inc. 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.

© 2009, Prentice-Hall, Inc. 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.

© 2009, Prentice-Hall, Inc. 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.

© 2009, Prentice-Hall, Inc. 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 ΔT b = K b m

Freezing Point Depression ΔT f = K f m

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