Section 6.4—Solubility & Precipitation

How can we make sure everything that’s added to the sports drink will dissolve?

A Review of Double-Replacement Reactions

NaCl + AgNO3 AgCl + NaNO3

Double Replacement Reactions

The cations from two compounds replace each other.

ClNa

Ag

O ON

O

ClAg

Na

O ON

O

Two ionic compounds switch ions

Double Replacement Reactions

A X B Z A XBZAA XX BB ZZ AA XXBBZZ

General format of a double replacement reaction:

Combine the cation of the first reactant with the anion of the second reactant

CaCl2 + AgNO3

1

Products of a Double Replacement

Combine the cation of the second reactant with the anion of the first reactant

CaCl2 + AgNO3

2

Products of a Double Replacement

& balance charges with subscripts when writing formulasRemember to write cations first …

AgCl

CaCl2 + AgNO3

3

Ca(NO3)2 +CaCl2 AgNO3+

Products of a Double Replacement

Only leave subscripts that are in the original compound there if they are a part of a polyatomic ion!

Precipitation Reactions

Precipitation Reactions

A precipitation reaction is when 2 soluble substances are mixed together and they form an insoluble substance

2 soluble chemicals: NaOH and Cu(NO3)2

Precipitation Reactions

A precipitation reaction is when 2 soluble substances are mixed together and they form an insoluble substance

2 soluble chemicals: NaOH and Cu(NO3)2

2 soluble chemical: NaNO3 1 insoluble chemical (the precipitate): Cu(OH)2

Why do some things dissolve and others don’t?

Remember the dissolving process?

Substances are dissolved by a process called hydrationThe solvent and solute need to break

intermolecular forces within themselvesNew intermolecular forces are formed between

the solvent and soluteThe solvent “carries off” the solute particles

Review--Dissolving Ionic Compounds

- +

+

+

+

-

--

-

+

-

OH H - +water Ionic compound

Water molecules are polar and they are attracted to the charges of the ions in an ionic compound.

When the intermolecular forces are stronger between the water and the ion than the intramolecular between the ions, the water carries away the ion.

- +

+

-

--

-

+

+

Review--Dissolving Ionic Compounds

+

-

OH H - +water Ionic compound

As more ions are “exposed” to the water after the outer ions were “carried off”, more ions can be “carried off” as well.

Review--Dissolving Ionic Compounds

+

-

OH H - +water Ionic compound

- +

+

+

+

-

--

-

These free-floating ions in the solution allow electricity to be conducted

What about with stronger ionic bonds?

2- 2+

2+

2+

2+

2-

2-2-

2-

+

-

OH H 2- 2+water Ionic compound

Ion charge can affect strength of ionic bond—the higher the charges, the stronger the bond.

(How closely the two ions can pack together also affects ionic bond strength)

+

-

OH H 2- 2+water Ionic compound

If the connection between the water and the ions is not similar in strength or stronger than the ion-ion and water-water connections that are being broken…

What about with stronger ionic bonds?

2- 2+

2+

2+

2+

2-

2-2-

2-

What about with stronger ionic bonds?

2- 2+

2+

2+

2+

2-

2-2-

2-

If the connection between the

water and the ions is not similar in strength or stronger than the ion-ion and water-water connections that are being broken…

The water won’t be able to carry the ions away…it won’t dissolve the solid.

+

-

OH H 2- 2+water Ionic compound

Solubility Rules

Solubility Rules Table

THESE ANIONS

FORMS SOLUBLE COMPOUNDS WITH THESE CATIONS

FORM INSOLUBLE COMPOUNDS WITH THESE CATIONS

NO3-

nitrateMost cations No common cations

CH3COO–

acetateMost cations Ag+

Cl- chloride

Most cations Ag+, Pb2+, Hg22+, Tl+

Br- bromide

Most cations Ag+, Pb2+, Hg22+, Tl+

I- iodide Most cations Ag+, Pb2+, Hg22+, Tl+

SO42-

sulfateMost cations Ba2+, Sr2+, Pb2+, Ag+,

Ca2+

CrO42-

chromateMost cations Ba2+, Sr2+, Pb2+, Ag+

S2- sulfide NH4+, cations of

column 1, cations of column 2

Most other cations

OH- hydroxide

NH4+, cations of

column 1, and Ba2+ and Sr2+

Most other cations

CO32-

carbonate

NH4+, cations of

column 1 except Li+

Most other cations

PO43-

phosphate

NH4+, cations of

column 1 except Li+

Most other cations

This table, found at the end of Chpt 6 and in the Appendix, can help you figure out which compounds dissolve (those that are soluble) and which form precipitate (insoluble)

Let’s Practice #1

Example:Decide whether each is soluble

or not

NaNO3

AgCH3COO

CaBr2

Ba(OH)2

Cu(OH)2

Let’s Practice #1

Example:Decide whether each is soluble

or not

NaNO3

AgCH3COO

CaBr2

Ba(OH)2

Cu(OH)2

Soluble

Not soluble

Soluble

Soluble

Not Soluble

Let’s Practice #2

Example:Write the

products for this reaction

Na2CrO4 (aq) + BaCl2 (aq)

Remember to indicate compounds that dissolve with “aq” for “aqueous” and compounds that don’t dissolve with “s” for “solid”

Let’s Practice #2

2 NaCl (aq) + BaCrO4 (s)

Example:Write the

products for this reaction

Na2CrO4 (aq) + BaCl2 (aq)

Remember to indicate compounds that dissolve with “aq” for “aqueous” and compounds that don’t dissolve with “s” for “solid”

Let’s Practice #3

Example:Write the

products for this reaction

NaCH3COO (aq) + KCl (aq)

Remember to indicate compounds that dissolve with “aq” for “aqueous” and compounds that don’t dissolve with “s” for “solid”

NaCH3COO (aq) + KCl (aq)

Let’s Practice #3

NaCl (aq) + KCH3COO (aq)

Example:Write the

products for this reaction

Remember to indicate compounds that dissolve with “aq” for “aqueous” and compounds that don’t dissolve with “s” for “solid”

When everything dissolves, there is no net reaction!