C H E M I S T R Y

Chapter 4Reactions in Aqueous Solution

H3O1+(aq) + Cl1-(aq)HCl(aq) + H2O(aq)

Acid (Arrhenius): A substance that dissociates in water to produce hydrogen ions, H1+:

H1+(aq) + A1-(aq)HA(aq)

In water, acids produce hydronium ions, H3O1+:

H1+(aq) + Cl1-(aq)HCl(aq)

Ammonia, commonly called “ammonium hydroxide” is a base:

M1+(aq) + OH1-(aq)MOH(aq)

Na1+(aq) + OH1-(aq)NaOH(aq)

NH41+(aq) + OH1-(aq)NH3(aq) + H2O(aq)

Base (Arrhenius): A substance that dissociates in water to produce hydroxide ions, OH1-:

Weak acids and weak bases are weak electrolytes.Strong acids and strong bases are strong electrolytes.

MA + H2OHA + MOH

These acid-base neutralization reactions are double-replacement reactions just like the precipitation reactions:

or

MA + HOHHA + MOH

WaterAcid Base Salt

Write the chemical formulas of the products (use proper ionic rules for the salt).

1.

Write the molecular, ionic, and net ionic equations for the reaction of aqueous HBr and aqueous Ba(OH)2.

H2OHBr(aq) + Ba(OH)2(aq) + BaBr2

SaltAcid Base Water

Write the molecular, ionic, and net ionic equations for the reaction of aqueous NaOH and aqueous HF.

Write the chemical formulas of the products (use proper ionic rules for the salt).

1.

SaltAcid Base Water

H2OHF(aq) + NaOH(aq) + NaF

ExamplesPredict the product and write a molecular

equation, ionic equation and net ionic equation for the following reactionsK2CO3(aq) + NiCl2(aq)

HNO3(aq) + LiOH(aq)

HCN(aq) + Mg(OH)2

2Fe2O3(s)4Fe(s) + 3O2(g) Rusting of iron:an oxidation of Fe

4Fe(s) + 3CO2(g)2Fe2O3(s) + 3C(s)Manufacture of iron: a reduction of Fe

1. An atom in its elemental state has an oxidation number of 0.

Rules for Assigning Oxidation Numbers

Oxidation Number (State): A value which indicates whether an atom is neutral, electron-rich, or electron-poor.

Na H2 Br2 S Ne

Oxidation number 0

2. A monatomic ion has an oxidation number identical to its charge.

Na1+

+1

Ca2+

+2

Al3+

+3

Cl1-

-1

O2-

-2

b) Oxygen usually has an oxidation number of -2.

H O1-

-2+1

HH Ca

-1-1 +2

3. An atom in a polyatomic ion or in a molecular compound usually has the same oxidation number it would have if it were a monatomic ion.a) Hydrogen can be either +1 or -1.

OH O

-1+1 -1

HH O

+1+1 -2

H

+1

H Cl

+1 -1

c) Halogens usually have an oxidation number of -1.3.

ClCl O

+1+1 -2

Cr2O72-

-2+1 x

4. The sum of the oxidation numbers is 0 for a neutral compound and is equal to the net charge for a polyatomic ion.

x = +6

2(x) + 7(-2) = -2 (net charge)

H2SO3

x -2

x = +4

2(+1) + x + 3(-2) = 0 (net charge)

ExampleDetermine the oxidation number for each

atom in the following compounds/moleculesCO2

CCl4

CoSO4

K2O2

Oxidation: losing one or more electrons decreasing in oxidation number Reducing agent

2Fe23O2(g)+4Fe(s) O3 (s)

+30

oxidation

reduction

-20

Reduction: gaining one or more electron increasing in oxidation number

Oxidizing agent

Reducing Agent

Causes reductionLoses one or more electronsUndergoes oxidationOxidation number of atom increases

Oxidizing Agent

•Causes oxidation •Gains one or more electrons•Undergoes reduction•Oxidation number of atom decreases

ExampleIdentify each of the following as

1) oxidation or 2) reduction.

__A. Sn(s) Sn4+(aq) + 4e−

__B. Fe3+(aq) + 1e− Fe2+(aq)__C. Cl2(g) + 2e− 2Cl-(aq)

18

Writing Oxidation and Reduction Reactions

Write the separate half oxidation and reduction reactions for the following equation.

2Cs(s) + F2(g) 2CsF(s)

3 Na(l) + AlCl3(l) 3 NaCl(l) + Al(l)

19

2Ag1+(aq) + Cu(s)2Ag(s) + Cu2+(g)

Cu2+(aq) + 2Ag(s)Cu(s) + 2Ag1+(g)

Which one of these reactions will occur?

Fe2+(aq) + Cu(s)Fe(s) + Cu2+(aq)

Elements that are higher up in the table are more likely to be oxidized.

Thus, any element higher in the activity series will reduce the ion of any element lower in the activity series.

2Ag1+(aq) + Cu(s)2Ag(s) + Cu2+(g)

Cu2+(aq) + 2Ag(s)Cu(s) + 2Ag1+(g)

Which one of these reactions will occur?

ExamplePredict whether the following redox reactions

will occurred or not. If so, predict the products

Zn(s) + FeCl2(aq)

Ni(s) + Mg(NO3)2(aq)

If the unknown concentration is the potassium permanganate solution, MnO4

1-, it can be slowly added to a known amount of oxalic acid, H2C2O4, until a faint purple color persists.

Titration: A procedure for determining the concentration of a solution by allowing a carefully measured volume to react with a solution of another substance (the standard solution) whose concentration is known.

5H2C2O4(aq) + 2MnO41-(aq) + 6H1+(aq)

10CO2(g) + 2Mn2+(aq) + 8H2O(l)

5H2C2O4(aq) + 2MnO41-(aq) + 6H1+(aq)

10CO2(g) + 2Mn2+(aq) + 8H2O(l)

A solution is prepared with 0.2585 g of oxalic acid, H2C2O4. 22.35 mL of an unknown solution of potassium permanganate are needed to titrate the solution. What is the concentration of the potassium permanganate solution?

Moles ofH2C2O4

Mass ofH2C2O4

Moles ofKMnO4

Molarity ofKMnO4

Mole Ratio

Molarity of KMnO4

Molar Mass of H2C2O4

ExampleA 0.0484M standard solution of potassium

permanganate was titrated against 25.00mL of an iron (II) sulfate solution. The equivalence point, as indicated by a faint pink color, was reached when 15.50mL of potassium permanganate solution had been added. Calculate the concentration of the iron (II) sulfate solution