Ions in Aqueous Solutions and Colligative Properties

Ch 14

Compounds in Aqueous Solutions

Sec 1

Dissociation

Is the separation of ions that occurs when an ionic compound dissolves in water

Ex) NaCl(s) Na+(aq) + Cl-(aq)

CaCl2 (s) Ca+2(aq) + Cl-(aq)

Dissociation

Assume 100% dissociation

NaCl(s) Na+(aq) + Cl-(aq)

1mole 1mole 1mol

CaCl2 (s) Ca+2(aq) + 2Cl-(aq)

1mol 1mol 2 mol

Molar amount of dissociated ions

Dissociation

Sample problem

Write the equation for the dissolution of aluminum sulfate, Al2(SO4)3 in water. How many moles of aluminum ions and sulfate ions are produced by dissolving 1 mole of aluminum sulfate? What is the total number of moles of ions produced by dissolving 1 mole of aluminum sulfate?

Precipitation Reactions Not all ionic compounds are

soluble (actually - all ionic compounds are soluble to some degree. However, some compounds of very low solubility are considered insoluble)

Insoluble Ions -

force of attraction of ions to each other is > the attraction of the ions to water molecules.

General guidelines ------>

Na2CO3 ?

Soluble or Insoluble ?

Ca3PO4 ?

Precipitation Reactions

What will happen if solution of 2 different soluble compounds are mixed?

If reactants ions recombine to form an insoluble compound, a double-replacement reaction and a precipitate (ppt) will occur.

So, What use is there in knowing the solubility of ionic compounds?

PRECIPITATE REACTIONS

Precipitation Reactions

Ex) will a ppt form from the mixing of Ammonium sulfide (NH4)2S and Cadmium nitrate Cd(NO3)2? 1st determine if both compounds are soluble

Yes 2nd Write the double replacement reaction for the

combination of the two compounds (NH4)2S(aq) + Cd(NO3)2(aq) NH4NO3(?) + CdS(?)

3rd use guidelines to determine the solubility of each product.

Net Ionic Equations

Includes only those compounds and ions that undergo a chemical change in a reaction in an aqueous solution.

Usually used to represent reaction of ions in aqueous solution.

Net Ionic Equations

How to write a net ionic equation 1st write an overall ionic equation

All soluble compounds are shown as dissociated ion is solution.

Cd+2(aq) +2NO3

-(aq)+2NH4

+(aq)+S-2

(aq)

CdS(s) +2NO3-(aq)+2NH4

+(aq)

2nd remove spectator ions from the equation. Cd+2

(aq)+ S-2(aq) CdS(s)

Net Ionic Equations

Practice Identify the ppt that forms when aqueous

solutions of zinc nitrate and ammonium sulfide are combined.

Write the double replacement reaction Write the overall ionic equation Write the net ionic equation.

Ionization

Some molecular compounds (aka - covalent compounds) can form ions in solution.

Ionization is the creation of ions where there were none (such as in a covalently bonded molecule).

Ex) HCl is a molecular compound, that contains a highly polar bond

HCl H+(aq) + Cl-(aq)

Hydronium Ion H3O+ Forms because the H+ ion attracts other molecules or

ions so strongly, it does not exist alone.

HCl(g) + H2O(l) --> H3O+(aq) + Cl-(aq)

Strong and Weak Electrolytes Strong electrolytes:

Any compound of which all or almost all of the dissolved compound exists as ions in an aqueous solution.

Ex) HCl, HBr, other acids, and all ionic compounds.

A strong electrolyte yields only ions in solution no matter how much dissolves.

AgCl(s) + H2O(l) Ag+(aq) + Cl-(aq)

Strong and Weak Electrolytes Weak Electrolytes

Is a compound of which a relatively small amount of the dissolved compound exists as ions in an aqueous solution. 2 Examples

1. HF(aq) + H2O(l) ↔ H3O+(aq) + F-

(aq) Strong attractive forces between H+ and F- results in more HF remaining

intact than ionizing into H3O+ and F-.

2. CH3COOH(aq) + H2O(l) ↔ CH3COO (aq) + H3O+(aq)

Only a very small portion of the acetic acid molecule ionizes.