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Chemical Equations Preparation for College Chemistry Columbia University Department of Chemistry
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The Chemical Equation Writing and Balancing Equations Information in an Equation Types of Chemical Equations Heat in Chemical Equations The Greenhouse Effect Chapter Outline
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The Chemical Equation Shorthand Expression for a Chemical Change Fe + Al 2 O 3 Products Al + Fe 2 O 3 Reactants Conditions (s) (l) (s) Physical State Stoichiometric Coefficients
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Writing Chemical Equations Mg 3 (PO 4 ) 2 + H 2 O Write the skeleton equation Mg(OH) 2 + H 3 PO 4 Identify the Reaction magnesium hydroxide + phosphoric acid magnesium phosphate + water 2 3 2 Find the Stoichiometric Coefficients (Balance) 3Mg PO 4 Mg PO 4 R P 12 H 14 O 12 H
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Types of Chemical Equations Combination: Decomposition Single -Displacement Double -Displacement A + B AB AB A + B A + BC AB + C AB + CD AD + CB
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Combination Reactions 2Mg(s) + O 2 (g) 2MgO(s) m metal + Oxygen metal oxide m nonmetal + Oxygen non metal oxide 2S (s) + 3O 2 (g) 2SO 3 (g) m metal + nonmetal Salt 2Na (s) + Cl 2 (g) 2NaCl(s) m metal oxide + water Metal Hydroxide m nonmetal oxide + water Oxy-acid MgO (s) + H 2 O(l) Mg(OH) 2 (s) SO 3 (g) + H 2 O(g) H 2 SO 4 (s)
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Decomposition Reactions m Metal oxides m Carbonates and Hydrogen carbonates m Other decomposition reactions 2HgO(s) 2Hg (l) + O 2 (g) 2PbO 2 (g) 2PbO (g) + O 2 (g) CaCO 3 (s)CaO (s) + CO 2 (g) 2NaHCO 3 (s)Na 2 CO 3 (s) + H 2 O(l) + CO 2 (g) KClO 3 (s)2KCl (s) + 3O 2 (g) NaNO 3 (s)NaNO 2 (s) + O 2 (g) 2H 2 O 2 (l)2H 2 O (l) + O 2 (g) 2NaN 3 (s)2Na (s) + 3N 2 (g)
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Single-Displacement Reactions Zn(s) + 2HCl(g) H 2 (g) + ZnCl 2 (s) m metal + acid Hydrogen + Salt m metal + waterHydrogen + metal hydroxide or oxide 2Na(s) + 2H 2 O(l) H 2 (g) + 2NaOH(aq) m metal + Salt Salt + metal Zn(s) + CuSO 4 (aq) ZnSO 4 (aq) + Cu(s) m halogen + halide salt Halide salt + Halogen Cl 2 (g) + 2NaBr(aq) 2NaCl(aq) + Br 2 (l)
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Double-Displacement Reactions NaCl(aq) + KNO 3 (aq) NaNO 3 (aq) + KCl(aq) AB + CD AD + CB Evolution of Heat (Neutralization Reactions) Formation of an Insoluble precipitate Gas Formation Physical Evidences for double-displacement
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+ Ionic Dissolution + - - - - + + + - - - + + + - - - - + + + - + + - - + -- + - - - + + + + -- ++++ --
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Precipitation Reactions NO 3 - All nitrates are soluble Appendix V p. A19 Solubility Rules Cl - AgCl, Hg 2 Cl 2, Pb 2 Cl 2 All chlorides are soluble, except SO 4 2- Most sulfates are soluble, except SrSO 4, PbSO 4 and BaSO 4 CaSO 4 is slightly soluble CO 3 2- All carbonates are insoluble, except Group I and NH 4 + OH - All hydroxides are insoluble, except group I Sr(OH) 2 and Ba(OH) 2. Ca(OH) 2 is slightly soluble S 2- All sulfides except Groups I and II and NH 4 + are insoluble
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Solubility Rules Used to predict results of precipitation reactions Example 1 What happens when solutions of Ba(NO 3 ) 2 and Na 2 CO 3 are mixed? Ions present: Ba 2+ (aq), NO 3 - (aq), Na + (aq), CO 3 2- (aq) Possible precipitates: BaCO 3, NaNO 3 According to solubility rules, BaCO 3 is insoluble Ba 2+ (aq) + CO 3 2- (aq) BaCO 3 (s)
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Solubility Rules Mix solutions of BaCl 2, NaOH Example 2 ions present: Ba 2+ (aq), Cl - (aq), Na + (aq), OH - (aq) possible precipitates: Ba(OH) 2, NaCl both are soluble; no reaction
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Net Ionic Equations (Spectator ions do not appear) Mix solutions of Cu(NO 3 ) 2, NaOH ions present: Cu 2+ (aq), NO 3 - (aq), Na + (aq), OH - (aq) possible precipitates: Cu(OH) 2, NaNO 3 NaNO 3 is soluble; Cu(OH) 2 is not. Net Ionic Equation: Cu 2+ (aq) + 2 OH - (aq) Cu(OH) 2 (s) Example Spectator ions: Na + (aq), NO 3 - (aq)
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Heat in Chemical Reactions Endothermic Reaction Potential Energy Reactants Products Net Energy absorbed Activation Energy Time
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Heat in Chemical Reactions Exothermic Reaction Potential Energy Reactants Products Net Energy released Activation Energy Time
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Greenhouse Effect session/61/39/3567398/8!xrn_29_0_A20571782&bkm_8_29 session/61/39/3567398/12!xrn_39_0_A20080477 session/61/39/3567398/25!xrn_4_0_A15273396 http://web1.infotrac-college.com/wadsworth/ session/61/39/3567398/27!xrn_2_0_A17279460
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Redox Reactions (electron-transfer reactions) Oxidation Number Oxidation & ReductionBalancing Redox Reactions
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Oxidation number(oxidation state) 1. ON of an element in an elementary substance is zero 2. H ON = +1, except in metal hydrides NaH, CaH 2 What is it? 3. O ON = -2 in most compounds, -1 in peroxides Na 2 O 2, +2 in OF 2 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
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Oxidation number. Calculation 6. In a compound: Determine the ON of Cr in Cr 2 O 7 2- 7. In a PAI: Cr 2 O 7 2- Determine the ON of As in K 3 AsO 4 K 3 AsO 4 +1 -2 As + (-2) x 4 +(+1) x 3 = 0 -2 ? ? 2Cr + (-2) x 7 = -2 2Cr = +12 Cr = +6 As = +5
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Oxidation & 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 -57 -3 -2 6 5 4 3 2 1 0 ON O 2 (g) + H 2 (g) 2H 2 O(l) REDUCING AGENT OXIDIZING AGENT
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Balancing Redox Equations Two Methods q Oxidation number method q Ion-electron method (Molecular redox equations) (Ionic redox equations)
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Oxidation number method KMnO 4 + HCl + H 2 S KCl + MnCl 2 + S + H 2 O -2 +1 +1-2+1 +7 +2 0 Reduction Oxidation Oxidation: Reduction: Mn +7 +5e- Mn +2 S -2 S 0 + 2e- x 5 2Mn +7 + 5 S -2 2Mn +2 + 5 S 0 2 6 5 2 2 5 8 x 2 2Mn +7 + 5S -2 + 10e-2Mn +2 + 5 S0+ 5 S0 + 10e-
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Ion-electron method (rules p. 443-444) q Acidic Medium H + (aq) q Basic Medium OH - (aq) Mass and charge must balance Neutralization: H + (aq) + OH - (aq) H 2 O(l)
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Ion-electron method (Acidic Medium) K + (aq) + MnO 4 - (aq) + H + (aq) + Cl - (aq ) + 2H + (aq) + S 2- (aq) = write the molecular equation in ionic form KMnO 4 + HCl + H 2 S KCl + MnCl 2 + S + H 2 O Oxidation Eliminating spectator ions (appear in both sides of the equation) K + (aq) + Cl - (aq ) + Mn 2+ (aq) + 2Cl - (aq ) ) + S 0 (s) + H 2 O MnO 4 - (aq) + H + (aq) + S 2- (aq)Mn 2+ (aq) + S 0 (s) Reduction Net ionic Equation
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Oxidation: Reduction: x 2 Write the two half reactions Balance elements other than O and H Balance O and H, acidic medium: + 8H + + 4H 2 O Balance each half reaction electrically with electrons: MnO 4 - Mn +2 x 5S -2 S 0 + 5e- + 2e- Equalize loss and gain of e- Add the half equations 2MnO 4 - +16H + +5 S -2 2Mn +2 + 5S 0 + 8H 2 O
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Ion-electron method (Basic Medium) Oxidation Reduction SbO 2 - (aq) + ClO 2 (aq) Sb(OH) 6 - (aq) + ClO 2 - (aq) Oxidation: Reduction: Write the two half reactions SbO 2 Sb(OH) 6 - ClO 2 ClO 2 -
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Balance elements other than O and H Balance O and H, ACIDIC medium, + 2H + + 4H 2 O NEUTRALIZE: add OH- in both sides of the equation Oxidation:SbO 2 - Sb(OH) 6 - + 2OH - + 4H 2 OSbO 2 - Sb(OH) 6 - + 2H 2 O+ 2OH - Balance each half reaction electrically with electrons: SbO 2 - Sb(OH) 6 - + 2H 2 O+ 2OH - + 2e- Reduction:ClO 2 ClO 2 - + e- Equalize loss and gain of e- x 2 SbO 2 - + 2OH - + 2H 2 O + 2ClO 2 2ClO 2 - + Sb(OH) 6 -
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Ba Ba +2 + 2e- Zn Zn 2+ + 2e- Cr Cr 3+ + 3e- Pb Pb 2+ + 2e- Fe Fe 2+ + 2e- Ag Ag + + e- As As 3+ + 3e- H 2 2H + + 2e- Cu Cu 2+ + 2e- Ni Ni 2+ + 2e- Sn Sn 2+ + 2e- Ca Ca 2+ + 2e- Na Na + + e- Mg Mg 2+ + 2e- Al Al 3+ + 3e- K K + + e- Hg Hg 2+ + 2e- Au Au 3+ + 3e- Activity Series of Metals (table 17.3) Ease of oxidation
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Activity Series of Metals Useful to Predict the Course of Chemical Reactions � Na(s) + HCl(aq) � Hg + AgNO 3 NaCl(aq) + H 2 ? ? No Reaction Na(s) + 2H + (aq) 2Na + (aq) + H 2 Net Ionic Reaction: � Cr(s) + Sn(SO 4 )(aq) Sn + Cr 2 (SO 4 ) 3 ? Cr(s) + 3Sn 2+ (aq) 2Cr 3+ (aq) + Sn Net Ionic Reaction:
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Electrolytic Cells Anode: the OXIDATION SITE Cathode: the REDUCTION SITE Use electrical energy to produce a chemical reaction Voltaic (Galvanic) Cells Use chemical reactions to produce electrical energy Applications Corrosion
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