Reactions in Aqueous Solutions

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Chapter 4

• Reactions in Aqueous Solutions

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Types of Chemical Reactions

• Chemical Reactions discussed in College Chemistry
  can be broken down into 3 main categories
• Precipitation reactions
• Acid-Base reactions
• Oxidation-Reduction (redox) reactions

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Types of Chemical Reactions

• Precipitation Reactions A process in which an
  insoluble solid (precipitate) drops out of the
  solution.
• Clear solutions of two ionic compounds when mixed
  form a cloudy solution (cloudiness indicates
  solid)

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Types of Reactions

• AcidBase Neutralization A process in which an
  acid reacts with a base to yield water plus an
  ionic compound called a salt.
• The driving force of this reaction is the
  formation of the stable water molecule.

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Types of Reaction

• Metathesis Reactions (Double Displacement
  Reaction) These are reactions where two
  reactants just exchange parts.
• AX BY ? AY BX

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Types of Reactions

• OxidationReduction (Redox) Reaction A process
  in which one or more electrons are transferred
  between reaction partners.
• The driving force of this reaction is the
  decrease in electrical potential.

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Precipitation Reactions

• Develop the reaction equation
• Balance the reaction equation
• Predict the state of matter of each species
  present

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Precipitation Reactions and Solubility Rules

• To predict whether a precipitation reaction will
  occur must be able to predict whether a compound
  is soluble or not
• Solubility rules

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Solubility Rules

• Salts - soluble
• All alkali metal and ammonium ion salts
• All salts of the NO3, ClO3, ClO4, C2H3O2, and
  HCO3 ions

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Solubility Rules

• Salts which are soluble with exceptions
• Cl, Br, I ion salts except with Ag, Pb2,
  Hg22
• SO42 ion salts except with Ag, Pb2, Hg22,
  Ca2, Sr2, Ba2

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Solubility Rules

• Salts which are insoluble with exceptions
• O2 OH ion salts except with the alkali metal
  ions, and Ca2, Sr2, Ba2 ions
• CO32, PO43, S2, CrO42, SO32 ion salts
  except with the alkali metal ions and the
  ammonium ion

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Precipitation Reactions and Solubility Rules

• Predict the solubility of
• (a) CdCO3 (b) MgO (c) Na2S (d) PbSO4 (e)
  (NH4)3PO4 (f) HgCl2

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Precipitation Reaction

• Precipitation reactions only occur if a solid is
  produced as a product.
• If all products are aqueous compounds then no
  reaction has taken place.

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Electrolytes in Aqueous Solution

• To discuss aqueous reactions, must understand how
  different compounds interact with water
• Electrolyte substance that when added to water
  dissociates to form ions allowing for
  conductivity
• Strong electrolytes completely dissociate
• Weak electrolytes partially dissociate
• Non-electrolyte no dissociation in water

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Aqueous Reactions and Net Ionic Equations

• Molecular equations substances involved in
  chemical reaction have been written using full
  formulas
• Complete ionic equation shows every species as
  it truly appears in the reaction vessel (aqueous
  strong electrolytes are shown as ions)
• Net ionic equation shows what is truly taking
  place in the reaction (all spectators are
  removed)
• Must have a net ionic reaction for the reaction
  to occur as written

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Complete Ionic Equation

• AgNO3(aq) KI(aq) ? KNO3(aq) AgI(s)

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Net Ionic Equation

• Shows species that undergo a change during the
  reaction
• Obtained by removal of spectator ions.

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Additional Examples

• Reaction of magnesium acetate and sodium
  hydroxide in solution.
• Reaction of lead nitrate with potassium
  dichromate in solution.

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Acids, Bases and Neutralization Reactions

• Acid / Base Definitions
• Arrhenius
• Acid donates a H (H3O)
• Base donates an OH-
• Bronsted-Lowry
• Acid donates a H
• Base H acceptor

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Acids, Bases and Neutralization Reactions
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Electrolytes in Aqueous Solution

• To discuss aqueous reactions, must understand how
  different compounds interact with water
• Electrolyte substance that when added to water
  dissociates to form ions allowing for
  conductivity
• Strong electrolytes completely dissociate
• Weak electrolytes partially dissociate
• Non-electrolyte no dissociation in water

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Acids, Bases and Neutralization Reactions

• Neutralization Reactions
• Complete ionic reactions
• Strong electrolytes (strong acids and bases) will
  dissociate entirely
• Weak electrolytes are not shown as dissociated
• Net ionic for strong acids reacting with strong
  bases will always be for the formation of water

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Acids, Bases and Neutralization Reactions

• Neutralization Reaction produces salt water.
• HA(aq) MOH(aq) ? H2O(l) MA(aq)
• Write ionic and net ionic equations for the
  following
• (a) HBr(aq) Ba(OH)2(aq) ?
• (b) HCl(aq) NH3(aq) ?

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Neutralization Reactions

• 2HBr (aq) Ba(OH)2 (aq) ? BaBr2(aq) 2
  H2O (l)

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Neutralization Reactions

• HCN (aq) NaOH (aq) ? NaCN (aq) H2O (l)

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Problem

• Select the correct set of products for the
  following reaction.Ba(OH)2(aq) HNO3(aq) ? 
• A.  BaN2(s) H2O(l)
• B.  Ba(NO3)2(aq) H2O(l)
• C.  Ba(s) H2(g) NO2(g)
• D.  Ba2O(s) NO2(g) H2O(l)
• E.  No reaction occurs

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Problem

• Select the net ionic equation for the reaction
  between lithium hydroxide and hydrobromic
  acid.LiOH(aq) HBr(aq) ? H2O(l) LiBr(aq) 
• a.  LiOH(aq) ? Li(aq) OH-(aq)
• b.  HBr(aq) ? H(aq) Br-(aq)
• c.  H(aq) OH-(aq) ? H2O(l)
• d.  Li(aq) Br-(aq) ? LiBr(aq)
• e.  Li(aq) OH-(aq) H(aq) Br-(aq) ? H2O(l)
  LiBr(aq)

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Oxidation-Reduction Reactions

• Redox reactions are those involving the oxidation
  and reduction of species (element or ion of an
  element).
• Oxidation and reduction must occur together. They
  cannot exist alone.
• Two important types
• Single displacement reactions (activity series)
• Combustions reaction of a substance with O2

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Oxidation Reduction Reactions

• Oxidation
• Is
• Loss (of electrons)
• Anode Oxidation
• Reducing Agent

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Oxidation Reduction Reactions

• Reduction
• Is
• Gain (of electrons)
• Cathode Reduction
• Oxidizing Agent

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Redox Reactions

• Assigning Oxidation Numbers All atoms have an
  oxidation number regardless of whether it
  carries an ionic charge.
• 1. An atom in its elemental state has an
  oxidation number of zero.
• 2. An atom in a monatomic ion has an oxidation
  number identical to its charge.

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Redox Reactions

• 3. An atom in a polyatomic ion or in a molecular
  compound usually has the same oxidation number it
  would have if it were a monatomic ion.
• A. Hydrogen can be either 1 or 1.
• B. Oxygen usually has an oxidation number of 2.
• In peroxides, oxygen is 1.
• C. Halogens usually have an oxidation number of
  1.
• When bonded to oxygen, chlorine, bromine, and
  iodine have positive oxidation numbers.

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Redox Reactions

• 4. The sum of the oxidation numbers must be zero
  for a neutral compound and must be equal to the
  net charge for a polyatomic ion.
• A. H2SO4 2(1) (?) 4(2) 0 net charge
• ? 0 2(1) 4(2) 6
• B. ClO4 (?) 4(2) 1 net charge
• ? 1 4(2) 7

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Problem

• Sodium tripolyphosphate is used in detergents to
  make them effective in hard water. Calculate the
  oxidation number of phosphorus in Na5P3O10. 
• A.  3
• B.  5
• C.  10
• D.  15
• E.  none of these is the correct oxidation number

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Problem

• The oxidation numbers of P, S and Cl in H2PO2-,
  H2S and KClO4 are, respectively 
• A.  -1, -1, 3
• B.  1, -2, 7
• C.  1, 2, 7
• D.  -1, -2, 7
• E.  -1, -2, 3

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Redox Reactions

• 5. Whenever one atom loses electrons (is
  oxidized), another atom must gain those electrons
  (be reduced).
• A substance which loses electrons (oxidized) is
  called a reducing agent. Its oxidation number
  increases.
• A substance which gains electrons (reduced) is
  called the oxidizing agent. Its oxidation number
  decreases.

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Redox Reactions

• For each of the following, identify which species
  is the reducing agent and which is the oxidizing
  agent.
• Ca(s) 2 H(aq) ? Ca2(aq) H2(g)
• 2 Fe2(aq) Cl2(aq) ? 2 Fe3(aq) 2 Cl(aq)
• SnO2(s) 2 C(s) ? Sn(s) 2 CO(g)
• Sn2(aq) 2 Fe3(aq) ? Sn4(aq) 2 Fe2(aq)

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Problem

• Identify the oxidizing agent in the following
  redox reaction.Hg2(aq) Cu(s) ? Cu2(aq)
  Hg(l) 
• A.  Hg2(aq)
• B.  Cu(s)
• C.  Cu2(aq)
• D.  Hg(l)
• E.  Hg2(aq) and Cu2(aq)

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Problem

• Sodium thiosulfate, Na2S2O3, is used as a "fixer"
  in black and white photography. Identify the
  reducing agent in the reaction of thiosulfate
  with iodine.2S2O32-(aq) I2(aq) ? S4O62-(aq)
  2I-(aq) 
• a.  I2(aq)
• b.  I-(aq)
• c.  S2O32-(aq)
• d.  S4O62-(aq)
• e.  S2O32-(aq) and I-(aq)

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The Activity Series of the Elements
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The Activity Series of the Elements

• Activity series looks at the relative reactivity
  of a free metal with an aqueous cation.
• Fe(s) Cu2(aq) ? Fe2(aq) Cu(s)
• Zn(s) Cu2(aq) ? Zn2(aq) Cu(s)
• Cu(s) 2 Ag(aq) ? 2 Ag(s) Cu2(aq)
• Mg(s) 2 H(aq) ? Mg2(aq) H2(g)

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The Activity Series of the Elements
Given the following three reactions, determine
the activity series for Cu, Zn, Fe. Fe(s)
Cu2(aq) ? Fe2(aq) Cu(s) Zn(s) Cu2(aq) ?
Zn2(aq) Cu(s) Fe(s) Zn2(aq) ? NR
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Optional Homework

• Text 4.24, 4.30, 4.32, 4.34, 4.36, 4.40, 4.42,
  4.46, 4.48, 4.52, 4.54, 4.58, 4.60, 4.62, 4.64,
  4.66, 4.68, 4.92, 4.100, 4.102, 4.108
• Chapter 4 Homework

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Required Homework

• Assignment 4