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Chemical Equations

Chemical Equations. Preparation for College Chemistry Columbia University Department 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.

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Chemical Equations

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  1. Chemical Equations Preparation for College Chemistry Columbia University Department of Chemistry

  2. The Chemical Equation Writing and Balancing Equations Information in an Equation Types of Chemical Equations Heat in Chemical Equations The Greenhouse Effect Chapter Outline

  3. Reactants • Products • Stoichiometric Coefficients • Conditions (l) (s) (s) (s) • Physical State Al + Fe2O3 Fe + Al2O3 2 2 The Chemical Equation Shorthand Expression for a Chemical Change

  4. Write the skeleton equation Mg(OH)2 + H3PO4 Mg3(PO4)2 + H2O 2 3 R Mg 12 H 3 2 PO4 14 O P 3Mg PO4 14 O 12 H Writing Chemical Equations • Identify the Reaction magnesium phosphate + water magnesium hydroxide + phosphoric acid • Find the Stoichiometric Coefficients (Balance) 6 2

  5. A + B AB AB A + B A + BC AB + C AB + CD AD + CB Types of Chemical Equations • Combination: • Decomposition • Single -Displacement • Double -Displacement

  6. metal + Oxygen metal oxide 2Mg(s) + O2(g) 2MgO(s) 2S (s) + 3O2(g) 2SO3 (g) 2Na (s) + Cl2(g) 2NaCl(s) • nonmetal + Oxygen • metal + nonmetal non metal oxide Salt • metal oxide + water Metal Hydroxide MgO (s) + H2O(l) Mg(OH)2(s) • nonmetal oxide + water Oxy-acid SO3 (g) + H2O(g) H2SO4(s) Combination Reactions

  7. 2HgO(s) 2Hg (l) + O2(g) 2PbO2(g) 2PbO (g) + O2(g) CaCO3 (s) KClO3 (s) NaNO3 (s) NaNO2 (s) + O2(g) CaO (s) + CO2(g) 2KCl (s) + 3O2(g) 2NaHCO3 (s) Na2CO3 (s) + H2O(l) + CO2(g) 2H2O2 (l) 2H2O (l) + O2(g) 2NaN3 (s) 2Na (s) + 3N2(g) Decomposition Reactions • Metal oxides • Carbonates and Hydrogen carbonates • Other decomposition reactions

  8. metal + acid Hydrogen + Salt Zn(s) + 2HCl(g) H2(g) + ZnCl2(s) • 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) Single-Displacement Reactions

  9. NaCl(aq) + KNO3(aq) NaNO3(aq) + KCl(aq) AB + CD AD + CB Double-Displacement Reactions Physical Evidences for double-displacement • Formation of an Insoluble precipitate • Evolution of Heat (Neutralization Reactions) • Gas Formation

  10. - - - - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + + + + + + + + Ionic Dissolution

  11. NO3- All nitrates are soluble All chlorides are soluble, except AgCl, Hg2Cl2, Pb2Cl2 Cl- Most sulfates are soluble, except SrSO4, PbSO4 and BaSO4 CaSO4 is slightly soluble SO42- All carbonates are insoluble, except Group I and NH4+ CO32- All hydroxides are insoluble, except group I Sr(OH) 2 and Ba(OH)2. Ca(OH) 2 is slightly soluble OH- S2- All sulfides except Groups I and II and NH4+are insoluble Precipitation Reactions Appendix V p. A19 Solubility Rules

  12. Ba2+(aq) + CO32-(aq) BaCO3(s) Solubility Rules Used 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

  13. Solubility Rules Example 2 Mix solutions of BaCl2, NaOH ions present: Ba2+(aq) , Cl-(aq), Na+(aq), OH-(aq) possible precipitates: Ba(OH)2, NaCl both are soluble; no reaction

  14. Net Ionic Equation: Cu2+ (aq)+ 2 OH- (aq) Cu(OH)2 (s) Net Ionic Equations (Spectator ions do not appear) Example 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. Spectator ions: Na+(aq), NO3-(aq)

  15. Potential Energy Time Heat in Chemical Reactions Endothermic Reaction Activation Energy Products Net Energy absorbed Reactants

  16. Potential Energy Time Heat in Chemical Reactions Exothermic Reaction Activation Energy Reactants Net Energy released Products

  17. Greenhouse Effect http://web1.infotrac-college.com/wadsworth/ session/61/39/3567398/27!xrn_2_0_A17279460 session/61/39/3567398/12!xrn_39_0_A20080477 session/61/39/3567398/25!xrn_4_0_A15273396 session/61/39/3567398/8!xrn_29_0_A20571782&bkm_8_29

  18. Oxidation Number Oxidation & Reduction Balancing Redox Reactions Redox Reactions (electron-transfer reactions)

  19. Oxidation number(oxidation state) # of e- lost, gained or unequally shared by the atom “pseudocharge” assigned according to arbitrary rules . (rules p.436) 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.

  20. Oxidation number. Calculation ? Determine the ON of As in K3AsO4 +1 -2 K3AsO4 6. In a compound: As = +5 As + (-2)x4 +(+1)x3 = 0 ? -2 Determine the ON of Cr in Cr2O72- Cr2O72- 7. In a PAI: 2Cr + (-2)x7 = -2 Cr = +6 2Cr = +12

  21. Oxidation (lost of electrons) ON Reduction (gain of electrons) 6 -3 4 1 -2 -1 0 3 2 5 -5 -4 7 -6 -7 O2(g) + H2(g) 2H2O(l) Oxidation & Reduction oxid. # H increases from 0 to +1 (oxidizes) REDUCING AGENT OXIDIZING AGENT oxid. # O decreases from 0 to -2 (reduces)

  22. Balancing Redox Equations • Oxidation number method (Molecular redox equations) Two Methods • Ion-electron method (Ionic redox equations)

  23. KMnO4 + HCl + H2S KCl + MnCl2 + S + H2O Mn+7 +5e- Mn+2 x 5 S-2 S0 + 2e- x 2 2Mn+7 + 5 S-2 2Mn+2 +5S0 Oxidation number method Oxidation 2 6 5 2 2 5 8 -2 -1 +1 +1 -1 +1 -2 +1 -1 0 +7 +2 Reduction Reduction: Oxidation: 2Mn+7 + 5S-2 + 10e- 2Mn+2 +5S0 + 10e-

  24. H+(aq) + OH-(aq) H2O(l) Ion-electron method (rules p. 443-444) Mass and charge must balance • Acidic Medium H+(aq) • Basic Medium OH-(aq) Neutralization:

  25. KMnO4 + HCl + H2S KCl + MnCl2 + S + H2O MnO4 -(aq) + H+ (aq) +S2-(aq) Mn 2+ (aq) + S0 (s) Ion-electron method (Acidic Medium) write the molecular equation in ionic form K+ (aq) + MnO4 -(aq) + H+ (aq) + Cl-(aq ) + 2H+ (aq) +S2-(aq) = K+ (aq) + Cl-(aq ) + Mn2+ (aq) + 2Cl-(aq ) ) + S0(s) + H2O Eliminating spectator ions (appear in both sides of the equation) Net ionic Equation Oxidation Reduction

  26. MnO4-Mn+2 x 2 S-2 S0 x 5 Write the two half reactions Reduction: + 8H+ + 5e- + 4H2O + 2e- Oxidation: 2MnO4- + 16H+ + 5 S-2 2Mn+2 + 5S0 + 8H2O • Balance elements other than O and H • Balance O and H, acidic medium: • Balance each half reaction electrically with electrons: • Equalize loss and gain of e- • Add the half equations

  27. SbO2 - (aq) + ClO2(aq) Sb(OH)6 - (aq) + ClO2 -(aq) SbO2 Sb(OH)6 - ClO2 ClO2 - Ion-electron method (Basic Medium) Oxidation Reduction Write the two half reactions Oxidation: Reduction:

  28. x 2 + 2OH- + 2OH- Oxidation: SbO2-Sb(OH)6- SbO2-Sb(OH)6- + 4H2O SbO2-Sb(OH)6- + 2H2O + 2OH- Reduction: ClO2 ClO2 - SbO2- + 2OH- + 2H2O + 2ClO2 2ClO2 - + Sb(OH)6- • Balance elements other than O and H • Balance O and H, ACIDIC medium, • NEUTRALIZE: add OH- in both sides of the equation • Balance each half reaction electrically with electrons: • Equalize loss and gain of e- + 4H2O + 2H+ + 2OH- + 2H2O + 2e- + e-

  29. BaBa+2 + 2e- CaCa2+ + 2e- MgMg2+ + 2e- CrCr3+ + 3e- KK+ + e- FeFe2+ + 2e- NaNa+ + e- CuCu2+ + 2e- AlAl3+ + 3e- ZnZn2+ + 2e- NiNi2+ + 2e- SnSn2+ + 2e- Ease of oxidation PbPb2+ + 2e- H2 2H+ + 2e- AsAs3++ 3e- AgAg+ + e- HgHg2++ 2e- AuAu3+ + 3e- Activity Seriesof Metals(table 17.3)

  30. Na(s) + HCl(aq) Na(s) + 2H+(aq) Cr(s) + 3Sn2+(aq) • Cr(s) + Sn(SO4 )(aq) • Hg + AgNO3 Activity Series of Metals Useful to Predict the Course of Chemical Reactions NaCl(aq) + H2 ? Net Ionic Reaction: 2Na+(aq) + H2 ? Sn + Cr2 (SO4)3 Net Ionic Reaction: 2Cr3+(aq) + Sn No Reaction ?

  31. Applications • Electrolytic Cells Use electrical energy to produce a chemical reaction • Voltaic (Galvanic) Cells Use chemical reactions to produce electrical energy Anode: the OXIDATION SITE Cathode: the REDUCTION SITE • Corrosion

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