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UNIT 5. REACTIONS IN AQUEOUS SOLUTIONS

Chapter 6. Some Types of Chemical Reactions in Chemistry by Whitten

GENERAL PROPERTIES OF AQUEOUS SOLUTIONS

A solution is a homogenous mixture of 2 or more substances.

The solute is the substance present in the smaller amount.

The solventis the substance present in the larger amount.

water-soluble solutes can be:

1. electrolytes aqueous solutions conduct electricity

a. strong electrolytesb. weak electrolytes

2. non-electrolytes aqueous solutions do not conduct electricity

What happens when a substance dissolves in water?

Dissociation:

process in which an ionic compound separates into its ions when dissolved in water NaCl(s) Na+(aq) + Cl-(aq)

Ionization:

process in which acids and bases separate or react with water to form ions in solution acids and bases are molecular compounds that behave like ionic compounds in water HCl(g) H+(aq) + Cl-(aq)

Hydration:

process in which an ion is surrounded bywater molecules arranged in a specific manner

Non-electrolytes do not conduct electricity because molecular compounds do not form ions in solution.

ELECTROLYTES

Ionic compounds that are soluble in water are strong electrolytes: 100% dissociation (strong electrolytes)

Strong and weak acids

strong acids

100% ionization (strong electrolytes)

HCl(g) H+

(aq) + Cl-(aq)

all the HCl molecules are ionized into H+ and Cl-weak acids partial ionization only (

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All weak inorganic acids ionize reversibly ( ) or in equilibrium reactions - Reversible Reactions This is why they ionize less than 100%.

Strong Bases

soluble ionic metal hydroxides (usually of groups I and II), NaOH, KOH, CsOH

some metal hydroxides are insoluble: Ca(OH)2, Cu(OH)2, Fe(OH)3

similarly to strong acids, strong bases ionize 100% in water; weak bases that

ionize slightly in water

strong electrolytes

Weak Basespartial ionization only; example: NH3 (weak electrolyte)

OXIDATION-REDUCTION REACTIONS

oxidation:

increase in oxidation number loss of electronreduction:

decrease in oxidation number gain of electron

oxidizing agent:

substance that is reduced oxidizes the other substancereducing agent:

substance that is oxidized reduces the other substance

Examples:

2NaI + Cl2 2NaCl + I2

2KMnO4 + 16HCl 2MnCl2 + 2KCl + 5Cl2 + 8H2O

2Mg + O2 2MgO

2H2SO3 + O2 4H+

+ 2SO4-2

H2SO3 is oxidized. The oxidation state of S in H2SO3 is +4. In SO42-, S has an oxidation state of +6.

O2 is reduced. Oxidation state of O in O2 is 0 In SO42-, O has an oxidation state of2.

H2SO3 is reducing agent. O2 is oxidizing agent.

Disproportionation reaction: 2H2O2 2H2O + O2

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COMBINATION REACTIONS

Occur when two or more substances combine to form a compound There are three basic types of combination reactions:

1. element + element

metal + nonmetal binary ionic compound

2Na(s) + Cl2 (g) 2NaCl(s)

nonmetal + nonmetal binary molecular compound

P4 (s) + 6Cl2 (g) 4PCl3 (l)

2. compound + element compound

PCl3 (l) + Cl2 (g) PCl5 (s)

3. compound + compound compound

CaO (s) + CO2 (g) CaCO3 (s)

DECOMPOSITION REACTIONSDecomposition reactions occur when one compound decomposes to form:

1. compound element + element

2H2O (l) 2H2 (g) + O2 (g)

2. compound compound + element

2KClO3 (s) 2KCl(s) + 3O2 (g)

3. compound compound + compound

CaCO3 (s) CaO (s) + CO2 (g)

DISPLACEMENT REACTIONS

Occur when one element displaces another element from a compound.

These are redox reactions in which the more active metal displaces the less active metal of

hydrogen from a compound in aqueous solution.

Activity series is given in Table 4-14.

1. more active metal + salt of less active metal

Zn(s) + CuSO4(aq) Cu(s) + ZnSO4(aq) re: activity series

2. active metal + non-oxidizing acidZn(s) + H2SO4(aq) ZnSO4(aq) + H2 (g)

3. active nonmetal + salt of less active nonmetal

Cl2 (g) + 2NaBr(aq) 2NaCl(aq) + Br2 (g) F2 > Cl2 > Br2 > I2

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METATHESIS REACTIONS

Occur when two ionic aqueous solutions are mixed and the ions switch partners.AX + BY AY + BX

Metathesis reactions remove ions from solution in two ways:1. form predominantly unionized molecules like H2O2. form an insoluble solid

Ion removal is the driving force of metathesis reactions.

ACID-BASE NEUTRALIZATION

the products are always a salt and water: acid + base salt + waterexample: HCl(aq) + NaOH(aq) NaCl(aq) + H2O (g)

PRECIPITATION REACTIONS

metathesis reactions in which an insoluble compound is formed precipitate: insoluble solid

AgNO3 (aq) + NaCl(aq) AgCl(s) + NaNO3 (aq)

Solubility Guidelines for Compounds in Aqueous Solutions (SOLUBILITY RULES)

1) Common inorganic acids and low-molecular-weight organic acids are water soluble.2) All common compounds of the Group IA metal ions and the ammonium ion are water soluble.

Li+, Na+, K+, Rb+, Cs+,and NH4+3) Common nitrates, acetates, chlorates, and perchlorates are water soluble.

NO3-, CH3COO-, ClO3-,and ClO4- Common chlorides are water soluble. ExceptionsAgCl, Hg2Cl2,& PbCl2 Common bromides and iodides behave similarly to chlorides. Common fluorides are water soluble ExceptionsMgF2, CaF2, SrF2, BaF2, and PbF2

4) Common sulfates are water soluble. Exceptions PbSO4, BaSO4, & HgSO4 Moderately soluble CaSO4, SrSO4,& Ag2SO4

5) Common metal hydroxides are water insoluble. ExceptionsLiOH, NaOH, KOH, RbOH & CsOH

7) Common carbonates, phosphates, and arsenates are water insoluble. CO32-, PO43-, & AsO43- Exceptions- IA metals and NH4+ BaCO3 is moderately soluble Moderately soluble MgCO3

8) Common sulfides are water insoluble. ExceptionsIA metals and NH4+ plus IIA metals

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WRITING NET IONIC EQUATIONS

Net ionic equation

Shows ions that participate in reaction and removes spectator ions.

Spectator ions do not participate in the reaction. In the total and net ionic equations the only common substances that should be written as ions are:

Strong acids Strong bases Soluble ionic salts

Example:

Write the net ionic equation for the reaction of silver nitrate with sodium chloride.

a. write the molecular equation:

AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq)

b. write the ionic equation:

Ag+

+ NO3-+ Na

++ Cl

-AgCl (s) + Na

++ NO3

-

c. eliminate the spectator ions: (Na+and NO3

-)

Ag+

+ NO3-+ Na

++ Cl

-AgCl (s) + Na

++ NO3

-

d. what is left is the net ionic equation:

Ag+

+ Cl-

AgCl (s)