Reactions in Aqueous Solns

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Reactions in Aqueous Solns
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Aqueous Solns

• Water is the universal solvent -dissolves
  almost anything
• (aq) aqueous soln
• Soln consists of
• Solute -sub in lesser amount
• Solvent - sub in greater amount

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General Props of Aqueous Solns

• Electrolyte an aqueous soln th contains ions
  conducts electricity
• NaCl
• Nonelectrolyte an aqueous soln th does not form
  ions so does not conduct electricity
• C6H12O6

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Ionic Comps in Water

• Ionic solids dissociate into component ions in
  water.
• Water is a polar molecule th surrounds cations
  anions, dissolving ionic comps.
• Helps stabilize the ions in soln prevents
  recombination
• Ions become uniformly dispersed
• NaCl(s) H2O -gt Na (aq) Cl-(aq)

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Molecular Comps in Water

• Consists of intact molecules dispersed throughout
  the soln
• Usu. Nonelectrolytes
• Exceptions -gt acids
• HCl (aq) -gt H (aq) Cl- (aq)
• HNO3 (aq) -gt H (aq) NO3- (aq)

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Strong Weak Electrolytes

• Strong solutes in soln are completely or
  nearly completely ions
• HCl (aq) -gt H (aq) Cl- (aq)
• Weak exist mostly as molecules in soln w/
  only a small fraction forming ions
• HC2H3O2 (aq) ltgt H (aq) C2H3O2 - (aq)
• Chemical Equilibrium at the same time (rate)
  some molecules are ionizing while some are
  recombining

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Precipitation Rxns

• 2 aq solns mix from an insoluble product, a
  precipitate (ppt)
• Pb(NO3)2 (aq) 2KI (aq) -gt PbI2 (s) 2KNO3 (aq)
  
• Predict ppt prods by solubility guidelines
• See pg 178 in text or ref. tables
• Memorize these rules!!!

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Predicting Ppt Rxns

• 1. Mix 2 strong electrolytes.
• 2. Consider possible combos of cations anions.
• 3. Use sol guidelines to determine if any combos
  are insoluble.
• Mg(NO3)2 (aq) NaOH (aq) -gt

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Double Displacement Rxns

• Mg(NO3)2 (aq) 2NaOH (aq) -gt
• Mg(OH)2 (s) 2NaNO3 (aq)
• Notice th the cations exchange anions

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Ionic Equations

• Molecular equation
• Pb(NO3)2 (aq) 2KI (aq) -gt PbI2 (s) 2KNO3 (aq)
• Shows the complete molecular formula of the
  reactants products
• Complete Ionic Equation
• Pb 2 (aq) 2NO3- (aq) 2K (aq) 2I - (aq) -gt
  
• PbI2 2K (aq) 2NO3 - (aq)
• Shows all strong electrolytes as ions
• Spectator ions ions th appear in ID forms among
  both reactants prods

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Ionic Equations -cont.

• Net Ionic Equation omits the spectator ions
• Pb 2 (aq) 2I - (aq) -gt PbI2 (s)
• Includes only the ions molecules directly
  involved in the rxn

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Writing Net Ionic Equations

• 1. Write a balanced molecular equation for the
  rxn.
• 2. Rewrite the equation to show the ions th form
  in soln when ea strong electrolyte dissociates or
  ionizes.
• 3. Identify cancel spectator ions.
• CaCl2(aq) Na2CO3(aq) -gt CaCO3(s) 2NaCl(aq)

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Ionic Equations -cont.

• Ca2 (aq) 2Cl - (aq) 2 Na (aq) CO32-(aq)
  -gt CaCO3(s) 2Na 2Cl-(aq)
• Ca2 (aq) CO32-(aq) -gt CaCO3(s)

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Acid-Base Rxns

• Acids - subs th ionize in aq soln to form H (aq)
  ions (a proton)
• Monoprotic acids - yield 1 H (aq) per molecule
• HNO3, HCl
• Diprotic acids - yield 2 H (aq) per molecule
• H2SO4
• Ionization occurs in 2 steps
• H2SO4 (aq) -gtH (aq) HSO4- (aq)
• HSO4- (aq) ltgtH (aq) SO42- (aq)
• only the first ionization is complete

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Acid-Base Rxns -cont.

• Bases - subs th accept H produce OH- when
  dissolved in water
• NaOH, KOH, Ca(OH)2 (strong)
• Dissociate to produce OH-
• Ammonia is also a base accepts H
• NH3(aq) H2O(aq) ltgt NH4(aq) OH-(aq) (weak)

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Strong Weak Acids Bases

• STRONG Completely ionize in soln
• HCl, H2SO4, HNO3
• LiOH, NaOH, KOH, RbOH, CsOH (group 1) Ca(OH)2,
  Sr(OH) 2, Ba(OH) 2 (Group 2)
• WEAK Partly ionize
• Most acids
• NH 3(aq)
• Other metal hydroxides are insoluble in water

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Identifying Strong Weak Electrolytes

• Memorize the strong acids bases
• Remember NH3(aq) is a weak base
• Is sub ionic or molecular?
• Ionic -gt strong electrolyte
• Molecular -gt Acid? if not a strong one, probably
  a weak electrolyte
• Molecular -gt not an acid or NH3(aq), probably a
  nonelectrolyte

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Neutralization Rxns Salts

• Acidic characteristics sour, turn litmus
  red, pH lt 7
• Basic characteristics bitter, slippery, turn
  litmus blue, pH gt 7
• Neutralization Rxn acid base -gt sub w/
  neither A or B characteristics produces water
  salt (ionic)
• HCl(aq) NaOH(aq) -gt H2O(aq) NaCl(aq)

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Neutralization Rxns Salts -cont

• H(aq) Cl-(aq) Na (aq) OH- (aq) -gt
  H2O(aq) Na (aq) Cl- (aq)
• Net Ionic Equation
• H(aq) OH- (aq) -gt H2O(aq)
• Neutralization rxn double displacement rxn

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Acid-Base Rxn w/ Gas Formation

• HCO3- and CO32- each react w/ acids to form gases
  w/ low water solubility
• HCl(aq) NaHCO3 (aq) -gtCO2 (g) NaCl(aq)
  H2O(l)
• )

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Oxidation-Reduction Rxns (Re-dox)

• E-s are transferred
• LEO goes GER
• Lose Electrons Oxidized
• Gain Electrons Reduced
• Metals - lose e-s
• Ca -gt Ca 2
• Nonmetals gain e-s
• O2 -gt 2 O2-

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Oxidation Numbers

• Allows us to keep track of e-s
• Oxidation the actual charge of a monatomic
  ion otherwise, arbitrary charge using a set of
  rules
• Oxidation Ox increases
• Reduction Ox decreases
• Always go together if an element is oxidized,
  another is reduced.

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Rules for assigning Ox s

• 1. Free element 0.
• 2. Monoatomic ion charge of ion.
• 3. Nonmetals are usu negative
• Oxygen -2
• Hydrogen is 1 when bonded to nonmetals
• HCl
• Hydrogen is -1 when bonded to metals
• LiH

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Rules for assigning Ox s -cont.

• Fluorine -1
• Other halogens are -1 unless bonded w/ O, then
  they are .
• 4. Sum of Ox s of all atoms in a neutral comp
  0. NaCl-
• 5. The sum of the Ox s in a PA ion the charge
  of the ion. H3O

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Oxidation of Metals by Acids Salts

• Single Displacement Rxns
• A BX -gt AX B
• Zn 2HBr -gt ZnBr2 H2
• Oxi Changes
• Zn 0 - gt 2 loses 2 e-s oxidation
• Zn -gt Zn 2 2e-s
• H1 -gt 0 gains 1 e- reduction
• 2 H1 2e-s -gt H2

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The Activity Series

• A list of metals arranged in order of decreasing
  ease of oxidation
• Active metals from grps 1 and 2 at top
• Noble metals at bottom are very stable and form
  comps less readily
• Any metal on the list can be oxidized by ions of
  elements below it
• A metal above will displace a metal below it

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Concentrations of Solutions

• Behavior of solns depends upon their nature and
  concentration
• Concentration -the amt of solute dissolved in a
  given quantity of solvent or solution.
• Express quantitatively
• Molarity(M) moles of solute
• volume of soln in L

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Concentration of Electrolytes

• Electrolytes dissociate in soln releasing free
  ions
• Electrolyte conc may be expresses in terms of the
  compound or its free ions
• 1.0 M NaCl 1.0M Na and 1.0M Cl -
• 1.0 M Na2SO4 2.0M Na and 1.0M SO4 -2

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Dilution

• Mconc x V conc Mdil x Vdil
• How many mL of 3.0M H2SO4 are needed to make 450
  mL of .10 M H2SO4?

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Soln concentration -cont

• Calculate the molarity of a soln made by
  dissolving 23.4 g of Na2SO4 in enough water to
  form 125 mL of soln.
• Convert given (g/mL) to moles/L
• 23.4g Na2SO4 x 1mole Na2SO4 x 1000mL
• 125 mL 142g Na2SO4 1L
• 1.32M Na2SO4