Honors Chemistry

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Honors Chemistry

• Chapter 4 Reactions in Aqueous Solutions

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4.1 Aqueous Solutions

• Solution homogeneous mixture
• Solute gets dissolved, often smaller quantity
• Solvent dissolves solute, often greater
  quantity
• Aqueous solution solvent is water!
• Electrolyte conducts electricity when dissolved
  in water
• Strong electrolyte good conductor
• Weak electrolyte poor conductor
• Nonelectrolyte does not conduct when dissolved

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4.1 Aqueous Solutions

• Dissolution process
• Solvation solute surrounded by solvent
  molecules
• Hydration solvation with water molecules
• Strong
• NaCl (s) ? Na (aq) Cl- (aq)
• Weak
• HF (g) ?? H (aq) F- (aq)
• Equilibrium

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4.2 Precipitation Reactions

• Formation of an insoluble product
• Solubility Rules
• Always soluble
• Alkalai metal salts and ammonium salts
• Nitrates, bicarbonates, and chlorates
• Usually soluble
• Halides (except Ag, Hg2, Pb2)
• Sulfates (except Ag, Ca2, Sr2, Ba2, Hg2,
  Pb2)
• Usually insoluble
• Carbonates, phosphates, chromates, sulfides,
  hydroxides

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4.2 Ionic Equations

• Molecular Equation
• Formulas written out as normal
• Pb(NO3)2 (aq) 2 KI (aq) ? PbI2 (s) 2 KNO3
  (aq)
• Ionic Equation
• Dissolved substances shown as free ions
• Pb2 (aq) 2 NO3- (aq) 2 K (aq) 2 I- (aq) ?
  PbI2 (s) 2 K (aq) 2 NO3- (aq)
• Net Ionic Equation
• Remove spectator ions
• Pb2 (aq) 2 I- (aq) ? PbI2 (s)

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4.2 Ionic Equations

• Try this
• Write a molecular equation, ionic equation, and
  net ionic equation for the reaction between
  silver nitrate and iron (III) chloride
• 3 AgNO3 (aq) FeCl3 (aq) ? 3 AgCl (s) Fe(NO3)3
  (aq)
• 3 Ag (aq) 3 NO3- (aq) Fe3 (aq) 3 Cl- (aq)
  ? 3 AgCl (s) Fe3 (aq) 3 NO3- (aq)
• Ag (aq) Cl- (aq) ? AgCl (s)

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4.3 Acid-Base Reactions

• Arrhenius definition
• Acid
• produces H ion in aqueous solution
• HCl (g) ? H (aq) Cl- (aq)
• CH3COOH (l) ?? H (aq) CH3COO- (aq)
• Base
• produces OH- ion in aqueous solution
• NaOH (s) ? Na (aq) OH- (aq)
• NH3 (g) H2O (l) ?? NH4 (aq) OH- (aq)

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4.3 Acid-Base Reactions

• Properties of Acids
• Sour taste (vinegar, citrus fruits)
• React with metals to liberate hydrogen gas
• Zn (s) 2 HCl (aq) ? ZnCl2 (aq) H2 (g)
• React with carbonates and bicarbonates
• 2 HCl (aq) CaCO3 (s) ? CaCl2 (aq) H2O (l)
  CO2 (g)
• Properties of Bases
• Bitter taste
• Feel slippery (saponification)
• Indicators change color in acid / base solutions

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4.3 Brønsted Theory

• Define based on movement of H ion
• Acid proton donor
• Base proton acceptor
• HCl (g) H2O (l) ? H3O (aq) Cl- (aq)
• acid base conjugate acid conjugate
  base
• H3O is the hydronium ion
• Same as H (aq)
• Shows a water of hydration
• NH3 (aq) H2O (l) ?? NH4 (aq) OH- (aq)
• base acid CA CB

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4.3 Brønsted Theory

• Strong acid
• HNO3 H2O ? H3O NO3-
• Weak acid
• HBr H2O ?? H3O Br-
• Diprotic acid
• H2SO4 H2O ? H3O HSO4-
• HSO4- H2O ?? H3O SO42-
• Triprotic acid
• 3 ionizations, e. g., H3PO4

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4.3 Autoprotolysis of Water

• HOH (l) ?? H (aq) OH- (aq)
• Water is both a weak acid and a weak base
• Using Brønsted theory,
• H2O H2O ?? H3O (aq) OH- (aq)
• B A CA CB
• H3O 1.0 x 10-7 M in pure water
• Define pH -log H3O
• pH 7 in pure water
• Acid lower pH Base higher pH

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4.3 Acid-Base Neutralization

• Acid base ? salt water
• HCl (aq) NaOH (aq) ? NaCl (aq) H2O
• Ionic equation
• H (aq) Cl- (aq) Na (aq) OH- (aq) ? Na
  (aq) Cl- (aq) H2O (l)
• Net ionic equation
• H (aq) OH- (aq) ? H2O (l)
• All neutralizations reduce to this
• Try this H2SO4 (aq) Mg(OH)2 (aq) ?

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4.4 Redox Reactions

• Oxidation-reduction reaction
• Electron transfer reactions
• Oxidation loss of e- (higher charge)
• Reduction gain of e- (lower charge)
• 2 Ca (s) O2 (g) ? 2 CaO (s)
• Ca goes from neutral to 2 ..... Oxidation
• O goes from neutral to 2- ..... Reduction

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4.4 Half-Reactions

• Separate oxidation from reduction
• 2 Ca (s) O2 (g) ? 2 CaO (s)
• 2 Ca ? 2 Ca2 4 e- oxidation
• O2 4 e- ? 2 O2- reduction
• Ca is reducing agent
• Donates e-, gets oxidized
• O is oxidizing agent
• Accepts e-, gets reduced

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4.4 Half-Reactions

• Try this...
• Write half reactions forZn 2 HCl ? ZnCl2 H2
• Zn 2 H 2 Cl- ? Zn2 2 Cl- H2
• Cl- ion is a spectator
• Ox Zn ? Zn2 2 e-
• Red 2 H 2 e- ? H2
• Zn is reducing agent, H is oxidizing agent

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4.4 Oxidation Numbers

• Charge an atom has within a molecule
• Free element ox 0
• Monatomic ions ox charge
• Oxygen always 2- (except peroxides)
• Hydrogen 1 (except in alkalai hydrides)
• Fluorine always -1
• Neutral molecule all ox s add up to 0
• Ox s can occasionally be fractions

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4.4 Oxidation Numbers

• Try this
• Find oxidation numbers for all elements
• Al2O3
• H2SO4
• PO43-
• Fe3O4

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4.4 Types of Redox Reactions

• Combination Reactions (synthesis)
• Two or more substances combine, single product
• S O2 ? SO2
• Decomposition Reactions
• Compound breaks down into components
• 2 KClO3 ? 2 KCl 3 O2
• Displacement Reactions
• One ion displaces another in a compound
• Zn 2 HCl ? ZnCl2 H2

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4.4 Displacement Reactions

• Hydrogen Displacement
• IA and IIA metals displace H from water
• 2 Na 2 HOH ? 2 NaOH H2
• Most metals displace H from acid
• Metal Displacement
• Cu AgNO3 ? Cu(NO3)2 Ag
• Activity series shows who can displace whom
• p. 134
• Cu is higher in series and so can displace Ag

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4.4 Redox Reactions

• Halogen Displacement
• F gt Cl gt Br gt I
• 2 NaCl F2 ? 2 NaF Cl2
• NaCl Br2 ? NR
• Disproportionation Reactions
• Same element is oxidized and reduced
• 2 H2O2 ? 2 H2O O2
• O starts with 1- charge
• Ends with 2- charge in H2O and 0 charge in O2

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4.5 Concentration of Solutions

• Molarity (M)
• Number of moles solute in 1 L of solution
• n M ---- V
• Find the Molarity of 15 g NaCl in 250 mL.
• 15 g 1 mol------ x --------- 0.256
  mol 1 58.5 g
• M 0.256 mol / 0.250 L 1.03 M

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4.5 Concentration of Solutions

• How would you make 175 mL of a 0.500 M solution
  of CaCl2?
• n0.500 M -----------
  0.175 L
• n 0.0875 mol
• 0.0875 mol 111 g--------------- x
  -------- 9.71 g 1 1 mol

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4.5 Dilution of Solutions

• Reduce the concentration by adding water
• Moles of solute stay constant
• From M n/V, we get n MV
• Since n for soln 1 equals n for soln 2,
• M1V1 M2V2
• Try this...
• How would you prepare 500 mL of a 0.75 M HCl
  solution from a 11.7 M stock HCl solution?

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4.6 Gravimetric Analysis

• Lab technique based on measuring mass
• Used to find percent composition
• E.g., find Cl in NaCl
• AgNO3 NaCl ? NaNO3 AgCl
• Collect AgCl precipitate and mass
• Determine mass of Cl in precipitate
• Use it to find percent Cl in original sample
• Well do this in lab

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4.7 Acid-Base Titration

• Titration
• solution of known concentration is reacted with
  solution of unknown concentration
• Reaction must go to completion
• Calculate the unknown concentration
• Standard Solution
• Solution whose concentration is accurately known
• KHP (Potassium Hydrogen Phthalate)
• Used to standardize base solution
• Base solution then used to titrate an unknown acid

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4.7 Acid-Base Titration

• Equivalence Point
• Point where acid and base have neutralized
• Added in exact stoichiometric ratio
• Need an indicator to show end point
• Titration Equation
• mol acid mol base at equivalence point
• MaVa MbVb
• If we know the Molarity and volume of base, we
  can measure the volume of acid used and calculate
  its Molarity

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4.8 Redox Titration

• Titration can be done with redox reactions
• Often need to use a reactant whose color changes
  during the reaction
• Takes the place of an indicator
• Works just like acid-base titration
• End of chapter 4 Finally!