EXP' 14A: HYDROLYSIS OF SALTS

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EXP. 14-A HYDROLYSIS OF SALTS

• 1. Salt H2O ? Cation(aq) Anion(aq)
• NaC2H3O2? C2H3O2-(aq) Naaq)
• 2. In Hydrolysis,
• Cation/Anion H2O ? H/OH- Conjugate
• C2H3O2- H2O ? HC2H3O2(aq) OH-(aq)

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Hydrolysis Rules

• 1. Acidic pH
• Transition Metal Cations
• Polyatomic Cations
• NH4(aq) H2O ? NH3(aq) H (aq)
• Fe2(aq)H2O(l)? FeH2O)(OH) (aq) H (aq)

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Hydrolysis Rules

• 2. Basic pH
• Polyatomic anions other than NO3-
• C2H3O2- H2O ? HC2H3O2 OH-

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Hydrolysis Rules

• 3. Will not cause hydrolysis.
• NaCl, CaCl2
• Cations from groups 1A and 2A
• Anions from Group 7A

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Hydrolysis Rules

• 4. Will cause hydrolysis. Cant project pH.
• NH4C2H3O2

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Exp 14-A Overview of Protocol

• 1. Will use indicators to determine pH of each
  salt solution and deionized water.
• Will select indicator/indicators which give
  approximate pH.
• 2. From pH values, can calculate H or OH-

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Exp 14-A Overview of Protocol

• 3. Will then calculate either the Ka or the Kb
  value for the ion causing hydrolysis.
• 4. Spectator Ions are those ions which should
  not cause hydrolysis.
• Na

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Exp 14-A Overview of Protocol

• 1. Indicators used to estimate pH
• Methyl Orange
• Methyl Red
• Bromthymol Blue
• Phenol Red (yellow to red pH range 6.6-8.0)
• Phenolphthalein
• Alizarin Yellow-R

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Exp 14-A Overview of Protocol

• Determination of pH of Na2CO3 Solution
• Methyl Orange tube is yellow.
• Methyl Red tube is yellow.
• Bromthymol Blue tube is blue.
• Phenol Red tube is red.
• Phenolphthalein tube is red.
• Alazarin Yellow tube is orange.

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Determination of pH of Na2CO3 Solution

• Reject Methyl Orange
• Reject Methyl Red
• Reject Bromthymol Blue
• Reject Phenol Red
• Reject Phenolphthalein
• Select Alazarin Yellow pH of Na2CO3 is between
  10.1 and 12.0. Choose Mean 11.1

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Hydrolysis Equation for Na2CO3

• CO32- H2O ? HCO3- OH-
• Kb HCO3- OH-/CO32-
• Then Calculate Value of Kb. Need exact
  concentrations.

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Calculation of Kb Value

• pH value for CO32- solution 11.1
• pH pOH 14, pOH 14.0-11.1 2.9
• OH- antilog (-pOH) antilog (2.9) 1.26E-3
• HCO3- OH- 1.26E-3

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Calculation of Kb Value

• Starting CO32- Na2CO3 0.1 M
• At equilibrium, CO32- starting-change
• At equilibrium, CO32- 0.1M x
• x OH-
• At equilibrium, CO32- 0.1M 1.26E-3 M
  9.9E-2M

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Calculation of Kb Value

• Kb HCO3- OH-/CO32-
• Kb (1.26E-3) (1.26E-3)/(9.9E-2)
• Kb for CO32- 1.6E-5

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Comparison to Literature Value

• CO32- comes from H2CO3 which is a polyprotic weak
  acid.
• H2CO3 ? H HCO3-
• Ka1 4.3E-7
• HCO3- ? H CO32-
• Ka2 5.6E-11

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Comparison to Literature Value

• Hydrolysis Equation for CO32- is
• CO32- H2O ? HCO3- OH-
• Literature Value for Kb for CO32- can be
  calculated.
• (Ka) (Kb) Kw

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Comparison to Literature Value

• CO32- is conjugate base form of HCO3-
• HCO3- ? H CO32-
• Ka2 5.6E-11
• Literature Kb Value for Kw/Ka for HCO3-

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Comparison to Literature Value for CO32-

• Literature Kb Value for Kw/Ka for HCO3-
• Literature Kb Value
• 1.0E-14/5.6E-11 1.8E-4
• Kb Value calculated from data 1.6E-5

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Transition Metal Cations Lewis Acids

• Lewis Acids
• Transition Metal Cations can act as Lewis Acids.
• Fe2, Zn2, Cu2

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Transition Metal Cations Lewis Acids

• Fe2(aq)H2O(l)? FeH2O)(OH) (aq) H (aq)
• CationZ H2O(l)? Cation H2O)(OH)Z-1(aq) H
  (aq)