Reactions in Aqueous Solutions

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Reactions in Aqueous Solutions

• Chapter 7

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Aqueous Solutions

• Water is the dissolving medium, or solvent.

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Driving Forces for Chemical Reactions

• 1. Formation of a precipitate (solid).
• 2. Formation of molecular substance (water).
• 3. Formation of a gas.
• 4. Transfer of electrons (Redox).

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Types of Solution Reactions

• Precipitation reactions
• AgNO3(aq) NaCl(aq) ? AgCl(s) NaNO3(aq)
• Acid-base reactions
• NaOH(aq) HCl(aq) ? NaCl(aq) H2O(l)

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Electrolytes

• Strong - conduct current efficiently - many ions
  in solution.
• NaCl, KNO3, HNO3, NaOH
• Weak - conduct only a small current - few ions in
  solution,
• HC2H3O2, aq. NH3, tap H2O
• Non - no current flows - no ions in solution.
• pure H2O, sugar solution, glycerol

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In what two ways can a solid ionic compound be
made to conduct electricity?

• Dissolve it in water.
• Melt or fuse it.
• Ions must be free to move
• (mobile) in order to conduct electricity!

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Dissociation

• ionic compounds
• metal nonmetal (Type I II)
• metal polyatomic anion
• Ammonium compounds
• Acids
• When ionic compounds dissolve in water the anions
  and cations are separated from each other this
  is called dissociation
• We know that ionic compounds dissociate when they
  dissolve in water because the solution conducts
  electricity

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Dissociation

• potassium chloride dissociates in water into
  potassium cations and chloride anions
• KCl(aq) K (aq) Cl- (aq)
• copper(II) sulfate dissociates in water into
  copper(II) cations and sulfate anions
• CuSO4(aq) Cu2(aq) SO42-(aq)

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Dissociation

• potassium sulfate dissociates in water into
  potassium cations and sulfate anions
• K2SO4(aq) 2 K (aq) SO42-(aq)

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Ionic Compounds in Solution
In aqueous solution, soluble ionic
compounds exist in the form of ions.
K2CrO4(aq) Ba(NO3)2(aq) ----gt BaCrO4(s)
2KNO3(aq)
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Figure 7.1 The precipitation reaction that
occurs when yellow potassium chormate, K2CrO4
(aq), is mixed with a colorless barium nitrate
solution, Ba(NO3)2 (aq)
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Solubility

• 1. A soluble solid readily dissolves in water--
  designated with (aq).
• 2. A slightly soluble solid only dissolves to a
  tiny extent in water--designated with (s).
• 3. An insoluble solid does not dissolve to any
  appreciable extent in water--designated with (s).

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Simple Rules for Solubility

• 1. Most nitrate (NO3?), acetate (C2H3O2-),
  chlorate (ClO3-) salts are soluble.
• 2. Most alkali (group 1A) salts and NH4 are
  soluble.
• 3. Most Cl?, Br?, and I? salts are soluble (NOT
  Ag, Pb2, Hg22)
• 4. Most sulfate salts are soluble (NOT BaSO4,
  PbSO4, HgSO4, CaSO4)
• 5. Most OH? salts are only slightly soluble
  (NaOH, KOH are soluble, Ba(OH)2, Ca(OH)2 are
  marginally soluble)
• 6. Most S2?, CO32?, CrO42?, PO43? salts are only
  slightly soluble.

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Precipitation Reactions

• In all precipitation reactions, the ions of one
  substance are exchanged with the ions of another
  substance when their aqueous solutions are mixed
• At least one of the products formed is insoluble
  in water
• KI(aq) AgNO3(aq) ? KNO3(aq) AgI?s?

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Describing Reactions in Solution

• 1. Molecular equation (reactants and products as
  compounds)
• AgNO3(aq) NaCl(aq) ? AgCl(s) NaNO3(aq)
• 2. Complete ionic equation (all strong
  electrolytes shown as ions)
• Ag(aq) NO3?(aq) Na(aq) Cl?(aq) ?
• AgCl(s) Na(aq) NO3?(aq)

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Describing Reactions in Solution (continued)

• 3. Net ionic equation (show only components that
  actually react)
• Ag(aq) Cl?(aq) ? AgCl(s)
• Na and NO3? are spectator ions.

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Solutions of
AgNO3
Na2SO4
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If AgNO3 is mixed with Na2SO4 what ions are most
abundant in the solution?
AgNO3
Na2SO4
With what ions is the solution saturated?
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2AgNO3(aq) Na2SO4(aq) ? 2 NaNO3 (aq)
Ag2SO4(s)
Molecular Equation
2Ag(aq) 2 NO3-(aq) 2Na(aq) SO42-(aq) ?
2Na(aq) 2 NO3- (aq) Ag2SO4(s)
Overall Ionic Equation
2Ag(aq) SO42-(aq) ? Ag2SO4(s)
Net Ionic Equation
Silver Sulfate Precipitate
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Acids

• The nature of acids was discovered by Svante
  Arrhenius.
• Acids are characterized by
• a sour taste (lemons -- citric acid).
• producing H ions (protons) in aqueous solution.
• turn blue litmus red.

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Acids

• Strong acids - dissociate completely (nearly 100
  ) to produce H in solution
• HCl, H2SO4, HNO3, HBr, HI, HClO4
• Weak acids - dissociate to a slight extent
  (approximately 1 ) to give H in solution
• HC2H3O2, HCOOH, HNO2, H2SO3

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Figure 7.5 When gaseous HCl is dissolved in
water, each molecule dissociates to produce H
and Cl- ions
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Bases

• Bases are characterized by
• bitter taste (soap).
• feel slippery.
• produce hydroxide (OH-) ions in aqueous solution.
• turn red litmus blue.
• A basic solution is said to be alkaline since
  they often contain one of the alkali metals --Na,
  K, Li, etc.

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Bases

• Strong bases - react completely with water to
  give OH? ions.
• sodium hydroxide
• Weak bases - react only slightly with water to
  give OH? ions.
• ammonia

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Acid-Base Reactions

• Acid Base ----gt Salt water

Molecular Equation HCl(aq) NaOH(aq) ----gt
NaCl(aq) HOH(l)
Complete Ionic Equation H(aq) Cl-(aq)
Na(aq) OH-(aq) -----gt Na(aq)
Cl-(aq) HOH(l)
Net Ionic Equation H(aq) OH-(aq) -----gt HOH(l)
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Acid-Base Neutralization

• The net ionic equation for the neutralization of
  any strong acid and base will always be
• H(aq) OH-(aq) -----gt HOH(l)

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Salts

• Salts are ionic compounds consisting of
• a. metal nonmetal -- KCl
• b. metal polyatomic ion -- CuSO4
• c. polyatomic ion nonmetal -- NH4Cl
• d. two polyatomic ions -- (NH4)2SO4

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Oxidation-Reduction Reactions(Redox Reactions)

• Redox reaction -- involves the transfer of
  electrons
• loss and gain of electrons must be exactly
  equal.
• loss and gain of electrons must be simultaneous.

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Oxidation

• loss of electrons
• metal atoms -- Na, Ca, K
• Nao ---gt Na1 1e-
• Cao ---gt Ca2 2e-
• nonmetal ions -- Cl-, S2-, O2-
• Cl1- ---gt Clo 1e-
• S2- ---gt So 2 e-

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Reduction

• gain of electrons.
• nonmetal atoms.
• Oo 2e- ---gt O2-
• Fo 1e- ---gt F1-
• metal ions.
• K1 1e- ---gt Ko
• Ba2 2e- ---gt Bao

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Figure 7.7 When powdered aluminum and iodine
(shown in the foreground) are mixed (and a little
water added), they react vigorously
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Oxidation Reduction Half-Reactions

• Always add electrons (negative) to the more
  positive side of the equation.
• The charge on both sides of an equation must be
  equal.
• Oxidation -- Nao ----gt Na1 1 e-
• Reduction -- Cl2 2 e- ----gt 2 Cl-

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OIL RIG

• Oxidation Is Loss.
• Reduction Is Gain.

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Redox Reactions

• Metal-nonmetal reactions are always redox
  reactions.
• Any reaction that has a free element (such as O2)
  as a reactant or product are redox.
• All single replacement reactions are redox.
• All combustion reactions are redox.

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Synthesis I (Composition)

• A X ----gt AX ALWAYS REDOX
• Element Element -----gt Binary Compound
• Fe(s) S(s) ----gt FeS(s)
• 4 Al(s) 3 O2(g) ----gt 2 Al2O3(s)

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Synthesis II (Composition)

• A X ----gt AX NOT REDOX
• Compound Compound ----gt Compound (3
  or more elements)
• Ammonia Acid ----gt Ammonium Salt
• NH3(g) HCl(g) ----gt NH4Cl(s)
• 2 NH3(aq) H2SO4(aq) ----gt (NH4)2SO4(aq)

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Synthesis II (Composition)Continued

• Water An Oxide
• Rule 1 --Water Metal Oxide ----gt Metal
  Hydroxide (Base)
• HOH(l) CaO(s) ----gt Ca(OH)2(s)
• HOH(l) Na2O(s) ----gt 2 NaOH(aq)
• Rule 2 --Water Nonmetal Oxide ----gt Acid
• HOH(l) SO3(g) ----gt H2SO4(aq)
• HOH(l) N2O5(g) ----gt 2 HNO3(aq)

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Single Replacement I

• A BX ----gt AX B ALWAYS REDOX
• Element Compound ----gt Different Element
  Different Compound
• Metal Reactivities
• K gt Na gt Ca gt Mg gt Al gt Zn gt Cr gt Fe gt Cd gt Co gt
  Ni gt Sn gt Pb gt H gt Sb gt Cu gt Hg gt Ag gt Pt gt Au

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Single Replacement I(Continued)

• A is a metal.
• A is more reactive than B
• A BX ----gt AX B
• Cu(s) 2 AgNO3(aq) ----gtCu(NO3)2(aq) 2 Ag(s)
• Zn(s) 2 HCl(aq) ----gt ZnCl2(aq) H2(g)
• Rule 3 -- Active Metal Water ----gt Metal
  Hydroxide Hydrogen
• Ca(s) 2 HOH(l) ----gt Ca(OH)2(s) H2(g)

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Single Replacement I(Continued)

• A is a metal.
• A is less reactive than B
• A BX ----gt No Reaction (NR)
• Cu(s) HCl(aq) ----gt NR

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Single Replacement II

• Y BX ----gt BY X ALWAYS REDOX
• Element Compound ----gt Different Element
  Different Compound
• Nonmetal Reactivities
• F gt O gt Cl gt Br gt I

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Single Replacement II(Continued)

• Y is a nonmetal.
• Y is more reactive than X
• Y BX ----gt BY X
• Cl2(g) 2 KBr(aq) ----gt 2 KCl(aq) Br2(aq)
• Y is less reactive than X
• Y BX ----gt No Reaction
• Cl2(g) KF(aq) ----gt NR

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Single Replacement(Continued)

• Remember!!
• Metals replace metals.
• Mg(s) 2 Ag(NO3)2(aq) ----gt 2 Ag(s)
  Mg(NO3)2(aq)
• Nonmetals replace nonmetals.
• Cl2(g) 2 KBr(aq) ----gt Br2(aq) 2 KCl(aq)

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Figure 7.6 The thermite reaction gives off so
much heat that the iron formed is molten
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Decomposition I

• AX ----gt A X ALWAYS REDOX
• AX is a binary compound.
• Binary Compound ----gt Element Element
• elect
• 2 NaCl(l) ----gt 2 Na(l) Cl2(g)
• elect
• 2 HOH(l) ----gt 2 H2(g) O2(g)

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Decomposition II

• AX -----gt A X MAY OR MAY NOT BE REDOX
• AX is a ternary compound.
• Rule 4 -- Metal Chlorates ----gt Metal Chlorides
  Oxygen
• ?
• 2 KClO3(s) ----gt 2 KCl (s) 3 O2(g) REDOX

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Decomposition II(Continued)

• Rule 5 -- Metal Carbonates ----gt Metal Oxides
  Carbon Dioxide
• ?
• CuCO3(s) ----gt CuO (s) CO2(g) NOT REDOX

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Decomposition II(Continued)

• Rule 6 -- Metal Hydroxides ----gt Metal Oxides
  Water
• ?
• Ca(OH)2(s) ----gt CaO (s) HOH(g) NOT REDOX

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Decomposition II(Continued)

• Rule 7 -- Acid ----gt Nonmetal Oxide Water
• ?
• H2SO4(aq) ----gt SO3(g) HOH(g) NOT REDOX

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Double Displacement (Double Replacement)

• AX BY ----gt AY BX NEVER REDOX
• Compound Compound ----gt 2 New Compounds
• Acid-Base Precipitation Reactions (Chapter 7)
• H2SO4(aq) 2 NaOH(aq) ----gt Na2SO4(aq) 2
  HOH(l)
• NaCl(aq) AgNO3(aq) ----gt NaNO3(aq) AgCl(s)

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Combustion

• AX O2 ALWAYS REDOX
• AX is a hydrocarbon.
• ?
• CxHy O2(g) ----gt CO2(g) HOH(g)
• ?
• CH4(g) 2 O2(g) ----gt CO2(g) 2 HOH(g)
• ?
• 2 C6H6(l) 15 O2(g) ----gt 12 CO2(g) 6 HOH(g)