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