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
Strong Acids (all)
Hydrochloric acid HCl (aq)
Hydrobromic acid HBr (aq)
Hydroiodic acid HI (aq)
Sulfuric acid H2SO4 (aq)
Nitric acid HNO3 (aq)
Perchloric acid HClO4 (aq)
Strong Bases (all)
Lithium hydroxide LiOH (aq)
Sodium hydroxide NaOH (aq)
Potassium hydroxide KOH (aq)
Calcium hydroxide Ca(OH)2 (aq)
Strontium hydroxide Sr(OH)2 (aq)
Barium hydroxide Ba(OH)2 (aq)
Weak Acids (examples)
Acetic acid HC2H3O2 (aq)
Cyanic acid HCN (aq)
Phosphoric acid H3PO4 (aq)
Organic acids contain ending group COOH
Weak Bases (examples)
Ammonia (ammonium hydroxide) NH3 (aq) actually NH4OH (aq)
Organic amines contain ending group -NHx
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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