CHEMICAL REACTIONS Chapter 3

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CHEMICAL REACTIONS Chapter 3. Reactants: Zn + I 2. Product: ZnI 2. Chemical Equations. Depict the kind of reactants and products and their relative amounts in a reaction. 4 Al(s) + 3 O 2 (g)  2 Al 2 O 3 (s) The numbers in the front are called stoichiometric coefficients
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CHEMICAL REACTIONSChapter 3Reactants: Zn + I2Product: ZnI2Chemical EquationsDepict the kind of reactants and products and their relative amounts in a reaction.4 Al(s) + 3 O2(g)  2 Al2O3(s)The numbers in the front are calledstoichiometric coefficientsThe letters (s), (g), and (s) are the physical states of compounds.Reaction of Phosphorus with Cl2Notice the stoichiometric coefficients and the physical states of the reactants and products.Reaction of Iron with Cl2Notice the stoichiometric coefficients and the physical states of the reactants and products.Chemical Equations4 Al(s) + 3 O2(g) 2 Al2O3(s)This equation means4 Al atoms + 3 O2 molecules  2 “molecules” of Al2O34 moles of Al + 3 moles of O2 2 moles of Al2O3Chemical EquationsPLAY MOVIE
  • Because the same atoms are present in a reaction at the beginning and at the end, the amount of matter in a system does not change.
  • The Law of the Conservation of Matter
  • 2HgO(s) 2 Hg(liq) + O2(g)Lavoisier, 1788Chemical EquationsBecause of the principle of the conservation of matter, an equation must be balanced.It must have the same number of atoms of the same kind on both sides.PLAY MOVIEBalancing Equations___ Al(s) + ___ Br2(s)  ___ Al2Br6(s)Balancing EquationsPLAY MOVIE____C3H8(g) + _____ O2(g)  _____CO2(g) + _____ H2O(g)____B4H10(g) + _____ O2(g)  ___ B2O3(g) + _____ H2O(g)Chemical Equilibrium• Chemical reactions are reversible.• Ammonia can be produced from the elements in the Haber process N2(g) + 3 H2(g)  2 NH3(g)• But NH3 can also be decomposed to the elements 2 NH3(g)  N2(g) + 3 H2(g) • In a process to make NH3, the reaction can come eventually to equilbrium. N2(g) + 3 H2(g)  2 NH3(g)• Double arrows indicate equilibriumReaction ReversibilityStalactites and stalagmites in caves depend on a reversible chemical reactionCa2+(aq) + 2 HCO3–(aq)CaCO3(s) + CO2(g) + H2O(s)Reaction ReversibilityChemical EquilibriumOnce equilibrium is achieved, reaction continues, but there is no net change in amounts of products or reactants. K+(aq) + MnO4-(aq)Reactions in Aqueous SolutionMany reactions involve ionic compounds, especially reactions in water — aqueous solutions.KMnO4 in waterPLAY MOVIEPLAY MOVIEAn Ionic Compound, CuCl2, in WaterAqueous SolutionsHow do we know ions are present in aqueous solutions?The solutions conduct electricity!They are called ELECTROLYTESHCl, CuCl2, and NaCl are strong electrolytes. They dissociate completely (or nearly so) into ions.Aqueous SolutionsHCl, CuCl2, and NaCl are strong electrolytes. They dissociate completely (or nearly so) into ions.PLAY MOVIEAqueous SolutionsAcetic acid ionizes only to a small extent, so it is a weak electrolyte.CH3CO2H(aq)CH3CO2-(aq) + H+(aq)PLAY MOVIEAqueous SolutionsAcetic acid ionizes only to a small extent, so it is a weak electrolyte.CH3CO2H(aq)CH3CO2-(aq) + H+(aq)Aqueous SolutionsSome compounds dissolve in water but do not conduct electricity. They are called nonelectrolytes.Examples include: sugar ethanol ethylene glycolWater Solubility of Ionic CompoundsIf one ion from the “Soluble Compd.” list is present in a compound, the compound is water soluble.Iron pyrite, a sulfideOrpiment, arsenic sulfideAzurite, a copper carbonateWater Solubility of Ionic CompoundsCommon minerals are often formed with anions that lead to insolubility: sulfide fluoride carbonate oxideChemical Reactions in WaterPb(NO3) 2(aq) + 2 KI(aq) PbI2(s) + 2 KNO3 (aq) We will look at EXCHANGEREACTIONSThe anions exchange places between cations.PLAY MOVIEPrecipitation ReactionsThe “driving force” is the formation of an insoluble compound — a precipitate.Pb(NO3)2(aq) + 2 KI(aq)  2 KNO3(aq) + PbI2(s)BaCl2(aq) + Na2SO4(aq) BaSO4(s) + 2 NaCl(aq) Net Ionic EquationsPLAY MOVIEPb(NO3)2(aq) + K2CrO4(aq)  PbCrO4(s) + 2 KNO3(aq)This is the “complete equation”Because Pb(NO3)2 and K2CrO4 are strong electrolytes we should writePb2+(aq) + 2 NO3-(aq) + 2 K+(aq) + CrO42-(aq) PbCrO4(s) + 2 K+(aq) + 2 NO3-(aq) This is the “ionic equation”Question: do we need to include the K+ and NO3- ions?Net Ionic EquationsIonic equation: Pb2+(aq) + 2 NO3-(aq) + 2 K+(aq) + CrO42-(aq) PbCrO4(s) + 2 K+(aq) + 2 NO3-(aq) The NO3- and K+ ions are SPECTATOR IONS — they do not participate. Could have used Na+ instead of K+.We leave the spectator ions out —Pb2+(aq) + CrO42-(aq)  PbCrO4(s) to give the NET IONIC EQUATIONACIDSAn acid  H3O+ in waterPLAY MOVIEThe Nature of AcidsPLAY MOVIEHNO3ACIDSAn acid  H3O+ in waterSome strongacids areHCl hydrochloricH2SO4 sulfuricHClO4 perchloricHNO3 nitricAcetic acidWeak AcidsWEAK ACIDS = weak electrolytesCH3CO2H acetic acidH2CO3 carbonic acidH3PO4 phosphoric acidHF hydrofluoric acidPLAY MOVIEACIDSNonmetal oxides can be acidsCO2(aq) + H2O(s)  H2CO3(aq)SO3(aq) + H2O(s)  H2SO4(aq)and can come from burning coal and oil.BASESTable 3.2Base  OH- in waterNaOH(aq)  Na+(aq) + OH-(aq)NaOH is a strong basePLAY MOVIEAmmonia, NH3An Important BasePLAY MOVIEBASESMetal oxides are basesCaO(s) + H2O(s)  Ca(OH)2(aq)Metals from Groups 1A and 2ACaO in water. Indicator shows solution is basic.Know the strong acids & bases!Acid-Base Reactions
  • The “driving force” is the formation of water.
  • NaOH(aq) + HCl(aq) NaCl(aq) + H2O(liq)
  • Net ionic equation
  • OH-(aq) + H3O+(aq)  2 H2O(l)
  • This applies to ALL reactions of STRONG acids and bases.
  • PLAY MOVIESee Active Figure 3.14Acid-Base Reactions
  • A-B reactions are sometimes called NEUTRALIZATIONSbecause the solution is neither acidic nor basic at the end.
  • The other product of the A-B reaction is a SALT, MX.
  • HX + MOH MX + H2OMn+ comes from base &Xn- comes from acidThis is one way to make compounds!Gas-Forming ReactionsThis is primarily the chemistry of metal carbonates.CO2 and water  H2CO3 H2CO3(aq) + Ca2+ 2 H+(aq) + CaCO3(s) (limestone)Adding acid reverses this reaction.MCO3 + acid  CO2 + saltGas-Forming ReactionsCaCO3(s) + H2SO4(aq)  2 CaSO4(s) + H2CO3(aq) Carbonic acid is unstable and forms CO2 & H2OH2CO3(aq) CO2 + water(Antacid tablet has citric acid + NaHCO3)PLAY MOVIEOxidation-Reduction ReactionsSection 3.9Thermite reactionFe2O3(s) + 2 Al(s)2 Fe(s) + Al2O3(s)EXCHANGE: Precipitation ReactionsEXCHANGEGas-FormingReactionsEXCHANGEAcid-BaseReactionsREACTIONSREDOX REACTIONSREDOX REACTIONSREDOX = reduction & oxidation O2(g) + 2 H2(g) 2 H2O(s)REDOX REACTIONSREDOX = reduction & oxidationCorrosion of aluminum2 Al(s) + 3 Cu2+(aq)  2 Al3+(aq) + 3 Cu(s)REDOX REACTIONSCu(s) + 2 Ag+(aq)  Cu2+(aq) + 2 Ag(s)In all reactions IF something has been oxidized then something has also been reducedPLAY MOVIEREDOX REACTIONSCu(s) + 2 Ag+(aq)  Cu2+(aq) + 2 Ag(s)FuelsWhy Study Redox ReactionsBatteriesCorrosionManufacturing metalsREDOX REACTIONSRedox reactions are characterized byELECTRON TRANSFER between an electron donor and electron acceptor.Transfer leads to— 1. increase in oxidation number of some element = OXIDATION
  • decrease in oxidation number of some element = REDUCTION
  • O I L R I GO I L R I G
  • Oxidation
  • It
  • Loses (electrons)
  • Reduction
  • It
  • Gains (electrons)
  • OXIDATION NUMBERSThe electric charge an element APPEARS to have when electrons are counted by some arbitrary rules:1. Each atom in free element has ox. # = 0. Zn O2 I2 S82. In simple ions, ox. # = charge on ion. -1 for Cl- +2 for Mg2+OXIDATION NUMBERS3. O has ox. # = -2(except in peroxides: in H2O2, O = -1)4. Ox. # of H = +1(except when H is associated with a metal as in NaH where it is -1)5. Algebraic sum of oxidation numbers = 0 for a compound = overall charge for an ionRecognizing a Redox ReactionCorrosion of aluminum2 Al(s) + 3 Cu2+(aq)  2 Al3+(aq) + 3 Cu(s)Al(s)  Al3+(aq) + 3 e-
  • Ox. # of Al increases as e- are donated by the metal.
  • Therefore, Al is OXIDIZED
  • Alis the REDUCING AGENT in this balanced half-reaction.
  • Recognizing a Redox ReactionCorrosion of aluminum2 Al(s) + 3 Cu2+(aq)  2 Al3+(aq) + 3 Cu(s)Cu2+(aq) + 2 e-  Cu(s)
  • Ox. # of Cu decreases as e- are accepted by the ion.
  • Therefore, Cu is REDUCED
  • Cu is the OXIDIZING AGENT in this balanced half-reaction.
  • Recognizing a Redox ReactionNotice that the 2 half-reactions add up to give the overall reaction —if we use 2 mol of Al and 3 mol of Cu2+. 2 Al(s) 2 Al3+(aq) + 6 e- 3 Cu2+(aq) + 6 e- 3 Cu(s) -----------------------------------------------------------2 Al(s) + 3 Cu2+(aq) 2 Al3+(aq) + 3 Cu(s)Final eqn. is balanced for mass and charge.Examples of Redox ReactionsPLAY MOVIEMetal + halogen2 Al + 3 Br2Al2Br6Examples of Redox ReactionsPLAY MOVIENonmetal (P) + OxygenP4O10Metal (Mg) + OxygenMgOPLAY MOVIERecognizing a Redox ReactionSee Table 3.4In terms of oxygen gain lossIn terms of halogen gain lossIn terms of electrons loss gainReaction TypeOxidationReductionMetals (Cu) are reducing agentsMetals (Na, K, Mg, Fe) are reducing agentsHNO3 is an oxidizing agentCommon Oxidizing and Reducing AgentsSee Table 3.4Cu + HNO3Cu2+ + NO22 K + 2 H2O 2 KOH + H2Examples of Redox ReactionsMetal + acidMg + HClMg = reducing agentH+ = oxidizing agentPLAY MOVIEMetal + acidCu + HNO3Cu = reducing agentHNO3 = oxidizing agentPLAY MOVIE
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