Chemical EquationsChemical Equations

Preparation for College ChemistryColumbia UniversityDepartment of Chemistry

The Chemical Equation

Writing and Balancing Equations

Information in an Equation

Types of Chemical Equations

Heat in Chemical Equations

The Greenhouse Effect

Chapter Outline

The Chemical EquationThe Chemical EquationShorthand Expression for a Chemical Change

Fe + Al2O3

Products

Al + Fe2O3

Reactants

Conditions

(s) (l)(s) (s)

Physical State

Stoichiometric Coefficients

Writing Chemical EquationsWriting Chemical Equations

Mg3(PO4)2 + H2O

Write the skeleton equation

Mg(OH)2 + H3PO4

Identify the Reaction

magnesium hydroxide + phosphoric acid magnesium phosphate + water

2

3

2

Find the Stoichiometric Coefficients (Balance)

3Mg PO4

Mg PO4R

P

12 H14 O

14 O 12 H

Types of Chemical EquationsTypes of Chemical Equations

Combination:

Decomposition

Single -Displacement

Double -Displacement

A + B AB

AB A + B

A + BC AB + C

AB + CD AD + CB

Combination ReactionsCombination Reactions

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

metal + Oxygen metal oxide

nonmetal + Oxygen non metal oxide

2S (s) + 3O2(g) 2SO3 (g)

metal + nonmetal Salt

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

metal oxide + water Metal Hydroxide

nonmetal oxide + water Oxy-acid

MgO (s) + H2O(l) Mg(OH)2(s)

SO3 (g) + H2O(g) H2SO4(s)

Decomposition ReactionsDecomposition Reactions Metal oxides

Carbonates and Hydrogen carbonates

Other decomposition reactions

2HgO(s) 2Hg (l) + O2(g)

2PbO2(g) 2PbO (g) + O2(g)

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

2NaHCO3 (s) Na2CO3 (s) + H2O(l) + CO2(g)

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

NaNO3 (s) NaNO2 (s) + O2(g)

2H2O2 (l) 2H2O (l) + O2(g)

2NaN3 (s) 2Na (s) + 3N2(g)

Single-Displacement ReactionsSingle-Displacement Reactions

Zn(s) + 2HCl(g) H2(g) + ZnCl2(s)

metal + acid Hydrogen + Salt

metal + water Hydrogen + metal hydroxide or oxide

2Na(s) + 2H2O(l) H2(g) + 2NaOH(aq)

metal + Salt Salt + metal

Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)

halogen + halide salt Halide salt + Halogen

Cl2 (g) + 2NaBr(aq) 2NaCl(aq) + Br2(l)

Double-Displacement ReactionsDouble-Displacement Reactions

NaCl(aq) + KNO3(aq) NaNO3(aq) + KCl(aq)

AB + CD AD + CB

Evolution of Heat (Neutralization Reactions)

Formation of an Insoluble precipitate

Gas Formation

Physical Evidences for double-displacement

+

Ionic Dissolution

+

-

-

-

-

+

+

+

-

-

-+

+

+

-

-

-

-

+

+

+

-

+

+

-

-

+

-

-

+-

--+

+

+

+-

-

++

++

-

-

Precipitation ReactionsPrecipitation Reactions

NO3- All nitrates are soluble

Appendix V p. A19 Solubility Rules

Cl- AgCl, Hg2Cl2, Pb2Cl2 All chlorides are soluble, except

SO42- Most sulfates are soluble, except SrSO4, PbSO4 and BaSO4

CaSO4 is slightly soluble

CO32- All carbonates are insoluble, except Group I and NH4

+

OH- All hydroxides are insoluble, except group I Sr(OH) 2

and Ba(OH)2. Ca(OH) 2 is slightly soluble

S2- All sulfides except Groups I and II and NH4+

are insoluble

Solubility RulesSolubility RulesUsed to predict results of precipitation reactions

Example 1

What happens when solutions of Ba(NO3)2 and Na2CO3 are mixed?

Ions present: Ba2+ (aq), NO3

-(aq), Na+

(aq), CO32-

(aq)

Possible precipitates: BaCO3, NaNO3

According to solubility rules, BaCO3 is insoluble

Ba2+(aq) + CO32-(aq) BaCO3(s)

Solubility RulesSolubility Rules

Mix solutions of BaCl2, NaOH

Example 2

ions present: Ba2+(aq) , Cl-

(aq), Na+(aq), OH-

(aq)

possible precipitates: Ba(OH)2, NaCl

both are soluble; no reaction

Net Ionic Equations Net Ionic Equations (Spectator ions do not appear)(Spectator ions do not appear)

Mix solutions of Cu(NO3)2, NaOH

ions present: Cu2+(aq), NO3 -

(aq), Na+(aq), OH-

(aq)

possible precipitates: Cu(OH)2, NaNO3

NaNO3 is soluble; Cu(OH)2 is not.

Net Ionic Equation:

Cu2+ (aq)

+ 2 OH- (aq) Cu(OH)2 (s)

Example

Spectator ions: Na+(aq), NO3 -

(aq)

Heat in Chemical ReactionsHeat in Chemical ReactionsEndothermic Reaction

Po t

e nti

al E

ne r

g y

Reactants

Products

Net Energy absorbed

ActivationEnergy

Time

Heat in Chemical ReactionsHeat in Chemical ReactionsExothermic Reaction

Po t

e nti

al E

ne r

g y

Reactants

Products

Net Energy released

ActivationEnergy

Time

Greenhouse EffectGreenhouse Effect

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Redox Reactions Redox Reactions (electron-transfer reactions)(electron-transfer reactions)

Oxidation Number

Oxidation & Reduction

Balancing Redox Reactions

Oxidation number(oxidation state)Oxidation number(oxidation state)

1. ON of an element in an elementary substance is zero

2. H ON = +1, except in metal hydrides NaH, CaH2 What is it?

3. O ON = -2 in most compounds, -1 in peroxides Na2O2 , +2 in OF2

4. ON of metallic elements in ionic compounds is positive.

5. Negative ON is assigned to the most electronegative element in a covalent compound.

“pseudocharge” assigned according to arbitrary rules . (rules p.436)

# of e- lost, gained or unequally shared by the atom

Oxidation number. CalculationOxidation number. Calculation

6. In a compound:

Determine the ON of Cr in Cr2O72-

ON i = charge PAIi∑

ON i =0i∑

7. In a PAI:

Cr2O72-

Determine the ON of As in K3AsO4

K3AsO4

+1 -2

As + (-2)x4 +(+1)x3 = 0

-2?

?

2Cr + (-2)x7 = -2 2Cr = +12 Cr = +6

As = +5

Oxidation & ReductionOxidation & Reduction

oxid. # H increases from 0 to +1 (oxidizes)

oxid. # O decreases from 0 to -2 (reduces)

Oxidation (lost of electrons)

Reduction (gain of electrons)

-7 -4-6 -5 7-3 -2 -1 6543210ON

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

OXIDIZING AGENT

Balancing Redox EquationsBalancing Redox Equations

Two Methods

Oxidation number method

Ion-electron method

(Molecular redox equations)

(Ionic redox equations)

Oxidation number methodOxidation number method

KMnO4 + HCl + H2S KCl + MnCl2 + S + H2O

-2+1 +1 -1 +1 -2 +1 -1 -1+7 +2

0

Reduction

Oxidation

Oxidation:

Reduction: Mn+7 +5e- Mn+2

S-2 S0 + 2e- x 5

2Mn+7 + 5 S-2 2Mn+2 + 5 S0

2 6 5 2 2 5 8

x 2

2Mn+7 + 5S-2 + 10e- 2Mn+2 + 5 S0 + 10e-

Ion-electron method Ion-electron method (rules p. 443-444)(rules p. 443-444)

Acidic Medium H+(aq)

Basic Medium OH-(aq)

Mass and charge must balance

Neutralization:

H+(aq) + OH-(aq) H2O(l)

Ion-electron method Ion-electron method (Acidic Medium)(Acidic Medium)

K+ (aq) + MnO4 - (aq) + H+ (aq) + Cl

- (aq ) + 2H+ (aq) + S2-(aq) =

write the molecular equation in ionic form

KMnO4 + HCl + H2S KCl + MnCl2 + S + H2O

Oxidation

Eliminating spectator ions (appear in both sides of the equation)

K+ (aq) + Cl - (aq ) + Mn2+ (aq) + 2Cl

- (aq ) ) + S0(s) + H2O

MnO4 - (aq) + H+ (aq) + S2-(aq) Mn 2+ (aq) + S0 (s)

Reduction

Net ionic Equation

Oxidation:

Reduction: x 2

Write the two half reactions

Balance elements other than O and H

Balance O and H, acidic medium:

+ 8H+ + 4H2O

Balance each half reaction electrically with electrons:

MnO4- Mn+2

x 5S-2 S0

+ 5e-

+ 2e-

Equalize loss and gain of e-

Add the half equations

2MnO4- + 16H+ + 5 S-2 2Mn+2 + 5S0 + 8H2O

Ion-electron method Ion-electron method (Basic Medium)(Basic Medium)

Oxidation

Reduction

SbO2 - (aq) + ClO2 (aq) Sb(OH)6

- (aq) + ClO2

- (aq)

Oxidation:

Reduction:

Write the two half reactions

SbO2 Sb(OH)6 -

ClO2 ClO2 -

Balance elements other than O and H

Balance O and H, ACIDIC medium,

+ 2H+ + 4H2O

NEUTRALIZE: add OH- in both sides of the equation

Oxidation: SbO2 - Sb(OH)6

- + 2OH-+ 2OH-

+ 4H2OSbO2 - Sb(OH)6

- + 2H2O+ 2OH-

Balance each half reaction electrically with electrons:

SbO2 - Sb(OH)6

-

+ 2H2O + 2OH- + 2e-

Reduction: ClO2 ClO2 -+ e-

Equalize loss and gain of e-

x 2

SbO2 - + 2OH

- + 2H2O + 2ClO2 2ClO2 - + Sb(OH)6

-

Ba Ba+2 + 2e-

Zn Zn2+ + 2e-Cr Cr3+ + 3e-

Pb Pb2+ + 2e-

Fe Fe2+ + 2e-

Ag Ag + + e-

As As3++ 3e-

H2 2H + + 2e-

Cu Cu2+ + 2e-

Ni Ni2+ + 2e-Sn Sn2+ + 2e-

Ca Ca2+ + 2e-Na Na+ + e-Mg Mg2+ + 2e-Al Al3+ + 3e-

K K+ + e-

Hg Hg2++ 2e-Au Au3+ + 3e-

Activity SeriesActivity Seriesof Metalsof Metals(table 17.3)(table 17.3)

Eas

e o f

ox i

da t

ion

Eas

e of

ox i

da t

ion

Activity Series of MetalsActivity Series of Metals

Useful to Predict the Course of Chemical Reactions

� Na(s) + HCl(aq)

� Hg + AgNO3

NaCl(aq) + H2?

?No Reaction

Na(s) + 2H+(aq) 2Na+(aq) + H2

Net Ionic Reaction:

� Cr(s) + Sn(SO4 )(aq) Sn + Cr2 (SO4)3?

Cr(s) + 3Sn2+(aq) 2Cr3+(aq) + Sn

Net Ionic Reaction:

Electrolytic CellsElectrolytic Cells

Anode: the OXIDATION SITEAnode: the OXIDATION SITE Cathode: the REDUCTION SITECathode: the REDUCTION SITE

Use electrical energy to produce a chemical reactionUse electrical energy to produce a chemical reaction

Voltaic (Galvanic) CellsVoltaic (Galvanic) Cells

Use chemical reactions to produce electrical energyUse chemical reactions to produce electrical energy

ApplicationsApplications

CorrosionCorrosion