分析化学(11) Chpter 19-20 RedoxTitrations
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What is the value of Kw? H2O + e ½ H2 + OHH+ + e ½ H2
Eo = -0.8277 V Eo = 0.0000 V
Ecell = Ecathode - E anode = -0.8277 - 0.0000 = -0.8277 V
• (reduction at the cathode… remember overall equation H2O H+ + OH- ) Eo = RT/nF (ln K ) = 0.05916/n log K
• n = 1 here log K = Eo/0.05916 = -13.99
Байду номын сангаас
Kw = 1.02 x 10-14
What is Ksp for AgCl?
What two reactions give us the Ksp reaction for AgCl? (not electrochemistry just adding reactions) AgCl(s) + e- Ag(s) + Cl-(aq) Ag+(aq) + e- Ag(s) AgCl(s) Ag+(aq) + Cl-(aq) 0.2223 V 0.7996 V (Ksp)
Finding Keq from Eo
DGo = -nFEo
DG = DGo + RT ln Q At equilibrium DG = 0 and Q = Keq
Therefore DGo = - nFEo = -RT ln Keq Eo = RT/nF ln Keq
Actually finding Keq from Eo
Intro to Redox Titratons
• Imagine a titration of Fe+2 with some oxidizing agent. Set up the following cell:
Pt|H+(1.0 M), H2(1.0 atm)||Fe+2 (X M), Fe+3 (X M)|Pt
Chapter 19
Application of Standard Electrode Potentials
Calculating Cell Potential
Ecell = Ecathode – Eanode
• In General you can either: 1. break the cell into anode and cathode and use the above equation (use Nernst equation on each ½ cell) or 2. Calculate the overall cell reaction and use the Nernst Equation
What is E cathode?
• The cathode is an indicator electrode merely indicating the ratio of [Fe+2]/[Fe+3]. • E cath = EoFe+3/Fe+2 – 0.05916 log([Fe+2]/[Fe+3])
Calculating the Titration Curve: In acid-base and complexation titrations, the titration curve shows a change in pH (H3O+) or pMn+ upon addition of the titrant. In redox titrations, we monitor the change in electrochemical potential. The steps in calculating the titration curve are similar to those for an acid-base and complexation titrations. Steps: 1. Determine the equivalence volume. 2. Determine the electrochemical potential before the equivalence point. 3. Determine the electrochemical potential at the equivalence point. 4. Determine the electrochemical potential after the equivalence point.
• What will be the potential of the above cell?
Calculating the cell potential
E cell = E cathode – E anode
What is E anode? It is merely the SHE under standard conditions. Therefore, E anode = 0.000 V. and E cell = E cathode
Calculating a Titration Curve
• Now that we have our setup. Let’s calculate a titration curve where 100.0 mL of 0.050 M Fe(II) in sulfuric acid is titrated with 0.10 M Ce(IV).
• Ksp = 1.77 x 10-10
Finding the Endpoint Potentiometrically:
A method for locating the end point of a redox titration is to use an electrode to monitor the change in electrochemical potential as titrant is added to a solution of analyte. The end point can then be found from the titration curve.
Eo = -0.8277 V Eo = 0.0000 V
Ecell = Ecathode - E anode = -0.8277 - 0.0000 = -0.8277 V
• (reduction at the cathode… remember overall equation H2O H+ + OH- ) Eo = RT/nF (ln K ) = 0.05916/n log K
• n = 1 here log K = Eo/0.05916 = -13.99
Байду номын сангаас
Kw = 1.02 x 10-14
What is Ksp for AgCl?
What two reactions give us the Ksp reaction for AgCl? (not electrochemistry just adding reactions) AgCl(s) + e- Ag(s) + Cl-(aq) Ag+(aq) + e- Ag(s) AgCl(s) Ag+(aq) + Cl-(aq) 0.2223 V 0.7996 V (Ksp)
Finding Keq from Eo
DGo = -nFEo
DG = DGo + RT ln Q At equilibrium DG = 0 and Q = Keq
Therefore DGo = - nFEo = -RT ln Keq Eo = RT/nF ln Keq
Actually finding Keq from Eo
Intro to Redox Titratons
• Imagine a titration of Fe+2 with some oxidizing agent. Set up the following cell:
Pt|H+(1.0 M), H2(1.0 atm)||Fe+2 (X M), Fe+3 (X M)|Pt
Chapter 19
Application of Standard Electrode Potentials
Calculating Cell Potential
Ecell = Ecathode – Eanode
• In General you can either: 1. break the cell into anode and cathode and use the above equation (use Nernst equation on each ½ cell) or 2. Calculate the overall cell reaction and use the Nernst Equation
What is E cathode?
• The cathode is an indicator electrode merely indicating the ratio of [Fe+2]/[Fe+3]. • E cath = EoFe+3/Fe+2 – 0.05916 log([Fe+2]/[Fe+3])
Calculating the Titration Curve: In acid-base and complexation titrations, the titration curve shows a change in pH (H3O+) or pMn+ upon addition of the titrant. In redox titrations, we monitor the change in electrochemical potential. The steps in calculating the titration curve are similar to those for an acid-base and complexation titrations. Steps: 1. Determine the equivalence volume. 2. Determine the electrochemical potential before the equivalence point. 3. Determine the electrochemical potential at the equivalence point. 4. Determine the electrochemical potential after the equivalence point.
• What will be the potential of the above cell?
Calculating the cell potential
E cell = E cathode – E anode
What is E anode? It is merely the SHE under standard conditions. Therefore, E anode = 0.000 V. and E cell = E cathode
Calculating a Titration Curve
• Now that we have our setup. Let’s calculate a titration curve where 100.0 mL of 0.050 M Fe(II) in sulfuric acid is titrated with 0.10 M Ce(IV).
• Ksp = 1.77 x 10-10
Finding the Endpoint Potentiometrically:
A method for locating the end point of a redox titration is to use an electrode to monitor the change in electrochemical potential as titrant is added to a solution of analyte. The end point can then be found from the titration curve.