Reactions in Aqueous Solutions

Chapter 4

GENERAL PROPERTIES

Solution

Electrolyte

Hydration

HCl(l) H+(aq) + Cl−

(aq)

CH3COOH(aq) H+(aq) + CH3COO−(aq)

Review of ConceptsThe diagrams here show three compounds AB2 (a), AC2 (b), and AD2 (c) dissolved in water. Which is the strongest electrolyte and which is the weakest? (For simplicity, water molecules are not shown.)

PRECIPITATE REACTIONS

Double-displacement reaction

Solubility Rules

EquationsPb(NO3)2(aq) + 2 KI(aq) PbI2(s) + 2KNO3(aq)

Pb2+(aq) + 2NO3−(aq) + 2K+(aq) + 2I−(aq) PbI2(s) + 2K+

(aq) + 2NO3(aq)

Pb2+(aq) + 2I−(aq) PbI2(s)

Example

K3PO4(aq) + Ca(NO3)2(aq)

Example 4.2 page 125 in textbook

Review of ConceptsWhich of the diagrams here acuratly describes the reaction between Ca(NO3)2(aq) and Na2CO3(aq)? For simplicity, only the Ca2+ (yellow) and CO3

2− (blue) ions are shown.

ACID-BASE REACTIONS

ACID• Sour taste• Color changes in plant

dyes• React with metals to

produce H2 gas• React with carbonates

and bicarbonates to produce CO2 gas

• Aqueous acid solutions conduct electricity

• Taste bitter• Feel slippery• Color changes in plant

dyes• Aqueous base

solutions conduct electricity

BASE

General Properties

ACID• Proton donor

• Monoprotic• Diprotic• Triprotic

• Proton acceptorBASE

Brønsted Acid and Bases

STRONG ACIDS

HIHBrHClO4

HClH2SO4

HNO3

NaOHKOHLiOHRbOHCsOH                    

Ca(OH)2

Ba(OH)2

Sr(OH)2

STRONG BASES

Strong acids/bases are strong electrolytes and will completely dissociate in water.

Review of ConceptsWhich of the following diagrams best represents a weak acid? Very weak acid? Strong acid? The proton exists in water as the hydronium ion. All acids are monoprotic. (For simplicity, water molecules are not shown.)

• Reaction between an acid and a base • Generally aqueous solutions result in water and a

salt• Ex: HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

*this is a strong acid and strong base so they completely dissociate and the net ionic equation is H+(aq) + OH−(aq) H2O(l)

• Ex: HCN(aq) + NaOH(aq) NaCN(aq) + H2O(l)

*this is a weak acid and strong base so the acid does not completely ionize in water. When writing the ionic and net ionic equations you cannot break the weak acid apart! The net ionic equation isHCN(aq) + OH−(aq) CN−(aq) + H2O(l)

Acid-Base Neutralization

Gas formation

• Certain salts react with acids to produce gaseous products• HNO3 breaks down into H2O(l) + NO2(g) + NO(g)

• H2CO3 breaks down into H2O(l) + CO2(g)

• H2SO3 breaks down into H2O(l) + SO2(g)

• NH4OH breaks down into H2O(l) + NH3(g)

• H2S(g)

• CO2(g)

• H2(g)

• If you get one of these as a product in your molecular equation, they immediately breakdown as above

• Gasses do not ionize

Double Replacement Rxns Review

Driving Force How do you recognize it?

Precipitate

You must memorize the solubility rules. Any compound formed from two ions can be recognized as soluble (written as separate ions) or as a precipitate (written as a molecule).

Gas formed

You must memorize the combinations that decompose into gases (there are 4). You must also memorize the gases that form. For example, when you H2SO3 as a product, you must know it decomposes into H2O and SO2 gas.

Weak electrolyte

You must memorize the short list of strong acids and strong bases so you will recognize all the weak acids and bases that dissolve, but do not dissociate into ions. The weak base ammonia, NH3, is in this category. It exits in water as NH3(aq) and only slightly forms the ions NH4

+ + OH−

OXIDATION-REDUCTION REACTIONS

OXIDATION REACTION• Reaction that involves

the loss of electrons• Contains reducing

agent-donates electrons

• Involves the gain of electrons

• Contains oxidizing agent-accepts electrons

REDUCTION REACTION

Half-reaction OIL RIG

Oxidation Number

• Charge of the atom would have in a molecule if electrons were transferred completely

• Rules• Uncombined elements = 0• Neutral compounds sum = 0• Ion = ion charge (polyatomic ions sum to

charge)• Exceptions

• Hydrogen +1 w/ nonmetals, −1 w/ metals• Oxygen −2 except w/ fluorine (+2), in peroxides

(−1)• Fluorine ALWAYS −1

More common oxidation numbers

are in red.

Example

2 Mg(s) + O2(g) 2 MgO(s)

Types of Redox Reactions

• Combination 2 Al(s) + 3 Br2(l) 2 AlBr3(s)• Decomposition 2 NaH(s) 2 Na(s) + H2(g)• Combustion C2H8(g) + 5 O2(g) 3 CO2(g) +

4H2O(l)• Displacement

• Hydrogen Ca(s) + 2 H2O(l) Ca(OH)2(s) + H2(g)• Metal Zn(s) + 2 HCl(aq) ZnCl2(aq) + H2(g)• Halogen Cl2(g) +2 KBr(aq) 2 KCl(aq) + Br2(l)

• Disproportionation 2 H2O2(aq) 2H2O(l) + O2(g)

Activity Series

For Halogens:F2 > Cl2 > Br2

> I2

Elements most likely to undergo disproportionation

Concentration

Molarity = moles of solute

liters of solution

nV

M =

KCl(s) K+(aq) + Cl−(aq)

H2O

Ex: 1M KCl solution

Ex: 1M Ba(NO3)2 solution

Ba(NO3)2(s) Ba2+(aq) + 2 NO3−

(aq)

H2O

Example

How many grams of potassium dichromate (K2Cr2O7) are required to prepare a 250 mL solution whose concentration is 2.16M?

Dilutions

MiVi = MfVf

Example

Describe how you would prepare 5.00x102 mL of a 1.75M H2SO4 solution, starting with an 8.16M stock solution of H2SO4.

Review of Concepts

What is the final concentration of a 0.6M NaCl solution if its volume is doubled and the number of moles of solute is tripled?

Quantitative analysis

• Gravimetric analysis• Titrations

• Acid-base• redox

Gravimetric Analysis

ExampleA 0.5662 g sample of an ionic compound containing chloride ions and an unknown metal is dissolved in water and treated with and excess of AgNO3. if 1.0882 g of AgCl precipitate forms, what is the percent by mass of Cl in the original compound?

Acid-base titrations

Example

How many mL of a 0.610M NaOH solution are needed to neutralize 20.0 mL of a 0.245M H2SO4 solution?

Redox titrations

ExampleA 16.42 mL volume of 0.1327M KMnO4 solution is needed to oxidize 25.00 mL of a FeSO4 solution in an acidic medium. What is the concentration of the FeSO4 solution in molarity? The net ionic equation is

5Fe2+ + MnO4− + 8H+ Mn2+ + 5Fe3+ + 4H2O