Chapter 5 Molecular View of Reactions in Aqueous Solutions

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Chapter 5 Molecular View of Reactions in Aqueous
Solutions

• Chemistry The Molecular Nature of Matter, 6E
• Jespersen/Brady/Hyslop

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Reactions in Solution

• For reaction to occur
• Reactants needs to come into physical contact
• Happens best in gas or liquid phase
• Movement occurs
• Solution
• Homogeneous mixture
• 2 or more components mix freely
• Molecules or ions completely intermingled
• Contains at least 2 substances

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Definitions

• Solvent
• Medium that dissolves solutes
• Component present in largest amount
• Can be gas, liquid, or solid
• Liquids most common
• Aqueous solutionwater is solvent
• Solute
• Substance dissolved in solvent
• Solution is named by solute
• Can be gasCO2 in soda
• LiquidEthylene glycol in antifreeze
• SolidSugar in syrup

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Iodine Molecules in Ethanol
Crystal of solute placed in solvent
Solute molecules dispersed throughout solvent
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Solutions

• May be characterized using
• Concentration
• Solute-to-solvent ratio
• Percent Concentration

or
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Relative Concentration

• Dilute solution
• Small solute to solvent ratio
• Ex. Eyedrops
• Concentrated solution
• Large solute to solvent ratio
• Ex. Pickle brine
• Dilute solution contains less solute per unit
  volume than more concentrated solution

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Concentration

• Solubility
• Temperature dependent
• Saturated solution
• Solution in which no more solute can be dissolved
  at a given temperature
• Unsaturated solution
• Solution containing less solute than maximum
  amount
• Able to dissolve more solute

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Solubilities of Some Common Substances
Substance Formula Solubility (g/100 g water)
Sodium chloride NaCl 35.7 at 0C 39.1 at 100C
Sodium hydroxide NaOH 42 at 0C 347 at 100C
Calcium carbonate CaCO3 0.0015 at 25C
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Concentrations

• Supersaturated Solutions
• Contains more solute than required for saturation
  at a given temperature
• Formed by careful cooling of saturated solutions
• Unstable
• Crystallize out when add seed crystal results
  in formation of solid or precipitate (ppt.)

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Preciptates

• Precipitate
• Solid product formed when reaction carried out in
  solutions and one product has low solubility
• Insoluble product
• Separates out of solution
• Precipitation reaction
• Reaction that produces precipitate
• Pb(NO3)2(aq) 2KI(aq) ? PbI2(s) 2KNO3(aq)
• 1 mol Pb(NO3)2 ? 2 mol KI
• 0.100 mol Pb(NO3)2 ? 0.200 mol KI

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Electrolytes in Aqueous Solution

• Ionic compounds conduct electricity
• Molecular compounds dont conduct electricity
• Why?

Bright light
No light
Ions present
Molecular
CuSO4 water
Sugar water
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Ionic Compounds (Salts) in Water

• H2O molecules arrange themselves around ions
  remove them from lattice.
• Dissociation
• Break salts apart into ions when enter solution
• Separated ions
• Hydrated
• Conduct electricity
• Note Polyatomic ions remain intact
• Ex. KIO3 ? K IO3?

NaCl(s) ? Na(aq) Cl(aq)
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Molecular Compounds In Water

• When molecules dissolve in water
• Solute particles are surrounded by water
• Molecules are not dissociated

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Electrical Conductivity

• Electrolyte
• Solutes that yield electrically conducting
  solutions
• Separate into ions when enter into solution
• Strong electrolyte
• Electrolyte that dissociates 100 in water
• Yields aqueous solution that conducts electricity
  
• Good electrical conduction
• Ionic compounds
• Strong acids and bases
• Ex. NaBr, KNO3, HClO4, HCl

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Electrical Conductivity

• Weak electrolyte
• Aqueous solution that weakly conducts electricity
  due to low ionization
• Weak acids and bases
• Ex. Acetic acid (HC2H3O2), ammonia (NH3)
• Non-electrolyte
• Aqueous solution that doesnt conduct electricity
• Molecules remain intact in solution
• Ex. Sugar, alcohol

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Your Turn

• How many ions form on the dissociation of Na3PO4?
• 1
• 2
• 3
• 4
• 8

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Your Turn

• How many ions form on the dissociation of
  Al2(SO4)3?
• 2
• 3
• 5
• 9
• 14

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Equations for Dissociation Reactions

• Ionic compound dissolves to form hydrated ions
• Hydrated surrounded by water molecules
• In chemical equations, hydrated ions are
  indicated by
• Symbol (aq) after each ions
• Ions are written separately
• KBr(s) ?? K(aq) Br?(aq)
• Mg(HCO3)2(s) ?? Mg2(aq) 2HCO3?(aq)

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Learning Check

• Write the equations that illustrate the
  dissociation of the following salts
• Na3PO4(aq) ?
• Al2(SO4)3(aq) ?
• CaCl2(aq) ?
• Ca(MnO4)2(aq) ?

3 Na(aq) PO43?(aq)
2 Al3(aq) 3 SO42?(aq)
Ca2(aq) 2 Cl?(aq)
Ca2(aq) 2 MnO4?(aq)
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Equations of Ionic Reactions

• Consider the reaction of Pb(NO3)2 with KI

PbI2(s)
Pb2
NO3
K
I
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Equations of Ionic Reactions

• When two soluble ionic solutions are mixed,
  sometimes an insoluble solid forms.
• Three types of equations used to describe
• Molecular Equation
• Substances listed as complete formulas
• Ionic Equation
• All soluble substances broken into ions
• Net Ionic Equation
• Only lists ions that actually take part in
  reaction

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Equations of Ionic Reactions

• 1. Molecular Equation
• Complete formulas for all reactants and products
• Formulas written with ions together
• Does not indicate presence of ions
• Gives identities of all compounds
• Good for planning experiments
• Ex.
• Pb(NO3)2(aq) 2KI(aq) ? PbI2(s) 2KNO3(aq)

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Equations of Ionic Reactions

• 2. Ionic Equation
• Emphasizes the reaction between ions
• All strong electrolytes dissociate into ions
• Used to visualize what is actually occurring in
  solution
• Insoluble solids written together as they dont
  dissociate to any appreciable extent
• Ex.
• Pb2(aq) 2NO3?(aq) 2K(aq) 2I?(aq) ??
  PbI2(s) 2K(aq) 2NO3?(aq)

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Equations of Ionic Reactions

• Spectator Ions
• Ions that dont take part in reaction
• They hang around and watch
• K NO3? in our example
• 3. Net Ionic Equation
• Eliminate all spectator ions
• Emphasizes the actual reaction
• Focus on chemical change that occurs
• Ex. Pb2(aq) 2I?(aq) ?? PbI2(s)

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Net Ionic Equations

• Many ways to make PbI2
• Pb(NO3)2(aq) 2KI(aq) ? PbI2(s)
  2KNO3(aq)
• Pb(C2H3O2)2(aq) 2NH4I(aq) ? PbI2(s)
  2NH4C2H3O2(aq)
• Different starting reagents
• Same net ionic equation
• Pb2(aq) 2I?(aq) ? PbI2(s)

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Converting Molecular Equations to Ionic Equations

• Strong electrolytes exist as dissociated ions in
  solution
• Strategy
• Identify strong electrolytes
• Use subscript coefficients to determine total
  number of each type of ion
• Separate ions in all strong electrolytes
• Show states as recorded in molecular equations

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Learning Check Convert Molecular to Ionic
Equations

• Write the correct ionic equation for each
• Pb(NO3)2(aq) 2NH4IO3(aq) ? Pb(IO3)2(s)
  2NH4NO3(aq)
• 2NaCl (aq) Hg2(NO3)2 (aq) ? 2NaNO3 (aq)
  Hg2Cl2 (s)

Pb2(aq) 2NO3(aq) 2NH4(aq) 2IO3(aq) ?
Pb(IO3)2(s) 2NH4(aq) 2NO3(aq)
2Na(aq) 2Cl(aq) Hg22(aq) 2NO3(aq) ?
2Na(aq) 2NO3(aq) Hg2Cl2(s)
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Your Turn

• Consider the following reaction
• Na2SO4(aq) BaCl2(aq) ? 2NaCl(aq) BaSO4(s)
• Which is the correct ionic equation?
• 2Na(aq) SO42(aq) Ba2(aq) Cl22(aq) ?
  2Na(aq) 2Cl(aq) BaSO4(s)
• 2Na(aq) SO42(aq) Ba2(aq) 2Cl(aq) ?
  2Na(aq) 2Cl(aq) BaSO4(s)
• 2Na(aq) SO42(aq) Ba2(aq) Cl22(aq) ?
  2Na(aq) 2Cl(aq) Ba2(s) SO42(s)
• Ba2(aq) SO42(aq) ? BaSO4(s)
• Ba2(aq) SO42(aq) ? Ba2(s) SO42(s)

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Converting Ionic Equations to Net Ionic Equations

• Strategy
• Identify spectator ions
• Eliminate from both sides
• Rewrite equation using only ions that actually
  react.
• Show states as recorded in molecular and ionic
  equations

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Learning Check Convert Ionic Equation to Net
Ionic Equation

• Write the correct net ionic equation for each.
• Pb2(aq) 2NO3(aq) 2K(aq) 2IO3(aq)
  ?Pb(IO3)2(s) 2K(aq) 2NO3(aq)
• 2Na(aq) 2Cl(aq) Hg22(aq) 2NO3(aq) ?
  2Na(aq) 2NO3(aq) Hg2Cl2(s)

Pb2(aq) 2IO3(aq) ? Pb(IO3)2(s)
2Cl(aq) Hg22(aq) ? Hg2Cl2(s)
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Your Turn

• Consider the following molecular equation
• (NH4)2SO4(aq) Ba(CH3CO2)2(aq) ?
  2NH4CH3CO2(aq) BaSO4(s)
• Which is the correct net ionic equation?
• Ba2(aq) SO42(aq) ? BaSO4(s)
• 2NH4(aq) 2CH3CO2(aq) ? 2NH4CH3CO2(s)
• Ba2(aq) SO42(aq) ? BaSO4(aq)
• 2NH4(aq) Ba2(aq) SO42(aq) 2CH3CO2(aq) ?
  2NH4(aq) 2CH3CO2(aq) BaSO4(s)
• 2NH4(aq) 2CH3CO2(aq) ? 2NH4CH3CO2(aq)

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Criteria for Balancing Ionic and Net Ionic
Equations

• Material Balance
• There must be the same number of atoms of each
  kind on both sides of the arrow
• Electrical Balance
• The net electrical charge on the left must equal
  the net electrical charge on the right
• Charge does not have to be zero

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Learning Check Balancing Equations for Mass
Charge

• Balance Molecular Eqn. for mass
• 2Na3PO4(aq) 3Pb(NO3)2(aq) ? 6NaNO3(aq)
  Pb3(PO4)2(s)
• Can keep polyatomic ions together when counting
• Balance Ionic Eqn. for charge
• 6Na(aq) 2PO43?(aq) 3Pb2(aq) 6NO3?(aq)
  ? 6Na(aq) 6NO3?(aq) Pb3(PO4)2(s)
• Charge must add up to zero on both sides.
• Net Ionic Eqn. Balanced for both mass charge
• 3Pb2(aq) 2PO43?(aq) ?? Pb3(PO4)2(s)

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Acids Bases as Electrolytes

• Many common laboratory chemicals and household
  products
• Indicators
• Dye molecules that change color in presence of
  acids or bases
• Acids
• Turn blue litmus red
• Lemon juice, vinegar, H2SO4
• Bases
• Turn red litmus blue
• Drano (lye, NaOH), ammonia (NH3)

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Neutralization Reaction

• Important reaction of acids and bases
• Acid reacts with base to form water and salt
  (ionic compound).
• Acid base ?? salt H2O
• Ex. HCl(aq) NaOH(aq) ? NaCl(aq) H2O
• HBr(aq) LiOH(aq) ? LiBr(aq) H2O
• 11 mole ratio of acidbase gives neutral
  solution
• Ionization reactions
• Ions form where none have been before
• Reactions of acids or bases with water

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Arrhenius

• Acid-base neutralization is
• H(aq) OH(aq) ?? H2O
• In solution, H attaches itself to H2O to form
  H3O or hydronium ion in water
• H does not ever exist in aqueous solution
• When H3O reacts, it releases H
• H is active ingredient
• Often use just H for simplicity

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Arrhenius Acid

• Substance that reacts with water to produce the
  hydronium ion, H3O
• Acid H2O ?? Anion H3O
• HA H2O ?? A H3O
• HC2H3O2(aq) H2O ? H3O(aq) C2H3O2-(aq)

HCl(g) H2O
??
Cl(aq) H3O(aq)
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Acids Categorized by Number of Hs

• Monoprotic Acids
• Furnish only one H
• HNO3(aq) H2O ?? H3O(aq) NO3(aq)
• HC2H3O2(aq) H2O ? H3O(aq) C2H3O2(aq)
• Polyprotic acids
• Furnish more than one H
• Diprotic acids furnish two H
• H2SO3(aq) H2O ?? H3O(aq) HSO3(aq)
• HSO3(aq) H2O ?? H3O(aq) SO32(aq)

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Acids Catagorized by Number of Hs

• Polyprotic acids
• Triprotic acids furnish three H
• H3PO4 ?? H2PO4 ?? HPO42 ?? PO43
• Stepwise equations
• H3PO4(aq) H2O ?? H3O(aq) H2PO4(aq)
• H2PO4(aq) H2O ?? H3O(aq) HPO42(aq)
• HPO42(aq) H2O ?? H3O(aq) PO43(aq)
• Net
• H3PO4(aq) 3H2O ?? 3H3O(aq) PO43(aq)

H
H
H
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Acidic Anhydrides

• Nonmetal Oxides
• Act as Acids
• React with water to form molecular acids that
  contain hydrogen
• SO3(g) H2O ?? H2SO4(aq)
• sulfuric acid
• N2O5(g) H2O ?? 2HNO3(aq)
• nitric acid
• CO2(g) H2O ?? H2CO3(aq)
• carbonic acid

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Arrhenius Bases

• Ionic compounds that contain hydroxide ion, OH,
  or oxide ion, O2.
• or
• Molecular compounds that react with water to give
  OH.
• 1. Ionic compounds containing OH or O2
• a. Metal Hydroxides
• Dissociate into metal hydroxide ions
• NaOH(s) ?? Na(aq) OH(aq)
• Mg(OH)2(s) ?? Mg2(aq) 2OH(aq)

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Ionic Oxides

• b. Basic Anhydrides
• Soluble metal oxides
• Undergo ionization (hydrolysis) reaction to form
  hydroxide ions
• Oxide reacts with water to form metal hydroxide
• CaO(s) H2O ?? Ca(OH)2(aq)
• Then metal hydroxide dissociates in water
• Ca(OH)2(aq) ?? Ca2(aq) 2OH(aq)

H2O
2OH
O2
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Strong vs. Weak Electrolyte
HCl(aq)
NH3(aq)
CH3COOH(aq)
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Strong Acids
HClO4(aq) perchloric acid
HClO3(aq) chloric acid
HCl(aq) hydrochloric acid
HBr(aq) hydrobromic acid
HI(aq) hydroiodic acid
HNO3(aq) nitric acid
H2SO4(aq) sulfuric acid

• Dissociate completely when dissolved in water
• Ex. HBr(g) H2O ?? H3O(aq) Br(aq)
• Good electrical conduction
• Any acid not on this list, assume weak

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Arrhenius Bases

• 2. Molecular Bases
• Undergo ionization (hydrolysis) reaction to form
  hydroxide ions
• Base H2O ?? BaseH(aq) OH(aq)
• B H2O ?? BH(aq) OH(aq)
• NH3(aq) H2O ?? NH4(aq) OH(aq)

??
NH4
NH3
OH
H2O
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Strong Bases

• Bases that dissociate completely in water
• Soluble metal hydroxides
• KOH(aq) ?? K(aq) OH(aq)
• Good electrical conductors
• Behave as (aq) ionic compounds
• Common strong bases are
• Group IA metal hydroxides
• LiOH, NaOH, KOH, RbOH, CsOH
• Group IIA metal hydroxides
• Ca(OH)2, Sr(OH)2, Ba(OH)2

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Weak Acids

• Any acid other than 7 strong acids
• Only ionize partially (lt100)
• Organic acids
• HC2H3O2(aq) H2O ?? H3O(aq) C2H3O2(aq)
• Ex. HCO2H(aq) H2O ? H3O(aq) HCO2(aq)

Acetic Acid Molecule,HC2H3O2
Acetate ion, C2H3O2
Only this H comes off as H
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Why is Acetic Acid Weak?
H2O C2H3O2(aq) ? HC2H3O2(aq) H3O(aq)
H3O(aq) C2H3O2(aq) ? HC2H3O2(aq) H2O
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Dynamic Equilibrium

• 2 opposing reactions occurring at same rate
• Also called Chemical equilibrium
• Equilibrium
• Concentrations of substances present in solution
  do not change with time
• Dynamic
• Both opposing reactions occur continuously
• Represented by double arrow
• HC2H3O2(aq) H2O H3O(aq)
  C2H3O2(aq)
• Forward reaction Forms ions
• Reverse reaction Removes ions

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Weak Bases

• Molecular bases
• Do not dissociate
• Accept H from water inefficiently
• Accept H from acids preferentially
• NH3(aq) HCl(aq) ?? NH4Cl(aq)
• Ex.
• NH3(aq) H2O ?? NH4(aq) OH?(aq)
• Or for general base
• B(aq) H2O ?? BH(aq) OH?(aq)

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Equilibrium for Weak Base

• Forward reaction

Reverse reaction

• Net is dynamic equilibrium
• NH3(aq) H2O NH4(aq) OH?(aq)

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Position of Equilibrium

• Extent of completion
• Depends on electrolyte

• Strong electrolyte
• Large ionizes
• ???? dominant
• Mostly products
• Strong acids bases
• Little back reaction
• Write eqn. as ??

• Weak electrolyte
• Small ionizes
• ??? dominant
• Mostly reactants
• Weak acids and bases
• Lots of back reaction
• Write eqn. as

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Learning Check

• Write the ionization equation for each of the
  following with water
• Weak base methylamine, CH3NH2.
• Weak acid nitrous acid, HNO2.
• Strong acid chloric acid, HClO3.
• Strong base strontium hydroxide, Sr(OH)2.

HClO3(aq) H2O ?? H3O(aq) ClO3(aq)
Sr(OH)2(aq) ?? Sr2(aq) 2 OH(aq)
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Your Turn

• Which of the following is a weak acid?
• HCl
• HNO3
• HClO4
• HC2H3O2
• H2SO4

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Your Turn

• Which of the following is not a strong base?
• NaOH
• CH3NH2
• Cs2O
• Ba(OH)2
• CaO

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Your Turn

• Which of the following is not a product of the
  reaction
• NH3(aq) HCN(aq) ???
• CN(aq)
• NH4(aq)
• NH3CN(s)
• H2O
• HCN

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AcidBase Nomenclature

• System for naming acids and bases
• Acids
• Hydrogen compounds of non-metals binary acids
• Hydrogen compounds of oxoanions Oxoacids
• Naming acid salts
• Bases
• Metal Hydroxides and oxides ionic
• Molecular molecular names

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Naming Acids

• A. Binary Acids hydrogen nonmetal
• Take molecular name
• Drop gen from H name
• Merge hydro with nonmetal name
• Replace ide with ic acid

Name of Molecular compound Name of Molecular compound Name of Aqueous Binary Acid Name of Aqueous Binary Acid
HCl(g) hydrogen chloride HCl(aq) hydrochloric acid
H2S(g) hydrogen sulfide H2S(aq) hydrosulfuric acid
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Naming Acids

• B. Oxo Acids
• Acids with hydrogen, oxygen and another nonmetal
  element
• Most of the polyatomic ions in Table 3.5
• To name
• Based on parent oxoanion name
• Take parent ion name
• Anion ends in ate change to ic (more O's)
• Anion ends in ite change toous (less O's)
• End name with acid to indicate H

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Oxoacids (Aqueous)

• Named according to the anion suffix
• Anion ends in -ite, acid name is -ous acid
• Anion ends in -ate, acid name is -ic acid

Name of Parent Oxoanion Name of Parent Oxoanion Name of Oxoacid Name of Oxoacid
NO3? HNO3
SO42? H2SO4
ClO2? HClO2
PO32? H2PO3
nitrate
nitric acid
sulfate
sulfuric acid
chlorite
chlorous acid
phosphite
phosphorous acid
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Learning Check Name Each Aqueous Acid

• HNO2
• HCN
• HClO4
• HF
• H2CO3

• nitrous acid
• hydrocyanic acid
• perchloric acid
• hydrofluoric acid
• carbonic acid

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Your Turn

• Which of the following is the correct name for
  HClO4 (aq)?
• chloric acid
• hydrochloric acid
• perchloric acid
• hypochlorous acid
• chlorous acid

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Your Turn

• Which of the following is the correct name for
  H2SO3(aq)?
• sulfuric acid
• sulfurous acid
• hydrosulfuric acid
• hydrosulfurous acid
• hydrogen sulfite acid

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Acid Salts

• If polyprotic acids are neutralized stepwise
• Can halt neutralization before all Hs are
  removed
• Must specify of H's that remain on salt
• Acid salt
• Ion containing H and anion
• Contains anion capable of furnishing additional
  hydrogen ions
• H2SO4(aq) KOH(aq) ?? KHSO4(aq) H2O(l)
• acid salt

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Naming Acid SaltsPolyprotic

• Must specify number of hydrogens still attached
  to the anion
• Can be neutralized by additional base
• Ex. Na2HPO4
• NaH2PO4
• KHSO4
• Some acid salts have common names
• NaHCO3

sodium hydrogen phosphate
sodium dihydrogen phosphate
potassium hydrogen sulfate
sodium hydrogen carbonate or sodium bicarbonate
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C. Naming Bases

• Oxides Hydroxides
• Ionic compounds
• Named like ionic compounds
• Ca(OH)2 calcium hydroxide
• Li2O lithium oxide
• Molecular Bases
• Named like molecules
• NH3 ammonia
• CH3NH2 methylamine
• (CH3)2NH dimethylamine
• (CH3)3N trimethylamine