Driving a non-spontaneous Oxidation-Reduction...

1 January 13 Electrolysis

18.3 Electrolysis

Driving a non-spontaneous Oxidation-Reduction Reaction

Dr. Fred Omega Garces Chemistry 201 Miramar College


Voltaic Vs. Electrolytic Cells General Characteristics of voltaic and electrolytic cells. A voltaic cell generates energy from a spontaneous reaction (ΔG < 0), whereas an electrolytic cell requires energy to drive a non-spontaneous reaction (ΔG > 0). In both types of cell two electrodes dip into electrolyte solution, and an external circuit provides the means for electrons to flow. Oxidation takes place at the anode, and reduction takes place at the cathode, but the relative electrode changes are opposite in the two cells. The anode in the electrolytic cell is now the positive (+) electrode and the cathode in the electrolytic cell is now the negative (-) electrode.

Oxidation Reaction A- g A + e-

Oxidation Reaction X g X+ + e-

Reduction Reaction e- + Y+ g Y

Reduction Reaction e- + B+ g B

Overall (Cell) Reaction X + Y+ g X+ + Y, ΔG = 0

Overall (Cell) Reaction A- + B+ g A + B, Δ G> 0

Voltaic Cell Energy is released from spontaneous

redox reaction

System does work on load (surroundings)

Electrolytic Cell Energy is absorbed to drive

nonspontaneous redox reaction

Surrounding (power supply) do work on system (cell)


Electrolysis What is the reaction which occurs in an electrolysis process ?

Voltaic/Galvanic Cell - Produce electricity Electrolysis - Causes nonspontaneous reaction to occur by using external energy source.

Electrolytic Cell • Two electrodes (inert or non inert) • External potential (power) Source of electricity, battery (e- pump) g Drive a non spontaneous reaction

+ - pulls electron

Draws electron to positive (Oxidation) Anode (+) A g A- + e-

push electron Pumps electron to reaction (Reduction) Cathode (-) e- + B+ g B

e- g e- g


Question: Electrolysis NaCl What is the direction of e- flow ? Which is the Anode/ Cathode Which is the Oxidation / Reduction ? Which is the Positive / Negative terminal ? What are the Half rxns.

With NaCl Electrode Ions ... Anode Rxn Cathode Rxn E°

Pt(inert) Pt (inert) (inert) NaCl Na+ Na+ +e- g Na(s) -2.71 V Cl- 2Cl- g Cl2 + 2e- -1.36 V

To drive rxn, Battery requires 4.07 V Complications due to H2O undergoing electrolysis process. This reaction must be considered.

Electrolysis: Molten Salt


Water Complications in Electrolysis

In an electrolysis, the most easily oxidized and most easily reduced reaction occurs. When water is present in an electrolysis reaction, then water (H2O) can be oxidized or reduced according to the reaction shown.

Electrode Ions ... Anode Rxn Cathode Rxn E°

Pt (inert) H2O H2O(l)+ 2e- g H2(g)+ 2OH-(aq) -0.83 V

H2O 2 H2O(l) g 4e- + 4H+(g) + O2(g) -1.23 V

Net Rxn Occurring: 2 H2O g 2 H2(g)+ O2 (g) E° = - 2.06 V


Electrolysis of NaCl Question: i) What is the direction of e- flow ? ii) Which is the Anode/ Cathode iii) Which is the Oxidation / Reduction ? iv Which is the Positive / Negative terminal ? v) What are the Half rxns. Reaction which occurs is the reaction requiring least E° potential

Specie Ions ... Anode Rxn Cathode Rxn E° Pt (inert) (inert) NaCl Na+ Na+ + e- g Na(s) -2.71 V Cl- 2Cl- g Cl2 + 2e- -1.36 V H2O H2O H2O(l)+ 2e- g H2(g)+ 2OH-

(aq) - 0.83 V H2O H2O(l) g 4e- + 4H+

(aq) + O2(g) -1.23 V

Overall Rxn: Anode: 2 H2O(l) g 4e- + 4H+(aq) + O2(g) - 1.23 V

Cathode: H2O(l)+ 2e- g H2(g)+ 2OH-(aq) - 0.83 V

E°Cell = -2.06 V

Electrolysis of water is the reaction occurring.


Electrolysis of MgCl2

Specie Ions ... Anode Rxn Cathode Rxn E° Pt (inert) (inert) MgCl2 Mg2+ Mg2+ +2e- g Mg(s) -2.37 V

Cl- 2Cl- g Cl2 + 2e- -1.36 V H2O H2O H2O(l)+ 2e- g H2(g)+ 2OH-

(aq) -0.83 V

H2O 2 H2O(l) g 4e- + 4H+(g) + O2(g) -1.23 V

Overall Rxn: Anode: 2 H2O(l) g 4e- + 4H+(aq) + O2(g) - 1.23 V

Cathode: H2O(l)+ 2e- g H2(g)+ 2OH-(aq) -0.83 V

E°Cell = -2.06 V

∴ MgCl2 undergoes no chemical change. Electrolysis of water is the reaction occurring.

What is the reaction in an electrolysis reaction of MgCl2. Reaction which occurs is the reaction requiring least E° potential


Electrolysis with non-inert Electrode

What is the reaction in an electrolysis reaction of NaCl with iron nails

Specie Ions ... Anode Rxn Cathode Rxn E° Fe Fe g Fe+2 + 2e- +0.44 V NaCl Na+ Na+ + e- g Na(s) -2.71 V

Cl- 2Cl- g Cl2 + 2e- -1.36 V H2O H2O H2O(l)+ 2e- g H2(g)+ 2OH-

(aq) -0.83 V H2O 2 H2O(l) g 4e- + 4H+

(g) + O2(g) -1.23 V

Overall Rxn: Anode: Fe g Fe+2 + 2e- 0.44 V

Cathode: H2O(l)+ 2e- g H2(g)+ 2OH-(aq) -0.83 V

E°Cell = -.39 V

∴Iron oxidize at the anode and water is reduced at the cathode. A potential of 0.39 V is used to drive this reaction.


Electrolysis of Copper A net reaction of zero, yet a process does take place.

A concentration cell based on the Cu/Cu2+ half-reaction. A, Even though the half-reactions involve the same components, the cell operates because the half-cell concentrations are different. B, The cell operates spontaneously until the half-cell concentrations are equal. Note the change in electrodes (exaggerated here for clarity) and the equal color of solutions.


Electro-Plating Example 20.97 BLB: Calculate the minimum applied voltage required to cause the following electrolysis reaction to occur, assuming that the anode is platinum and the cathode is nickel: Ni2+(aq)+ 2Br-

(aq) g Ni(s) + Br2(l)

Specie Ions ... Anode Rxn Cathode Rxn E° Pt (Inert) (Inert) Ni+2 Ni2+

Br- H2O H2O

H2O




Specie Ions ... Anode Rxn Cathode Rxn E° Pt (Inert) (Inert) Ni+2 Ni2+ Ni+2 +2 e- g Ni(s) -0.28 V

Br- 2Br- g Br2 + 2e- -1.065 V H2O H2O H2O(l)+ 2e- g H2(g)+ 2OH-

(aq) -0.83 V H2O 2 H2O(l) g 4e- + 4H+

(g) + O2(g) -1.23 V




Specie Ions ... Anode Rxn Cathode Rxn E° Pt (Inert) (Inert) Ni+2 Ni2+ Ni+2 +2 e- g Ni(s) -0.28 V

Br- 2Br- g Br2 + 2e- -1.065 V H2O H2O H2O(l)+ 2e- g H2(g)+ 2OH-

(aq) -0.83 V H2O 2 H2O(l) g 4e- + 4H+

(g) + O2(g) -1.23 V

Overall Rxn: Cathode: Ni+2 +2 e- g Ni(s) -0.28 V

Anode: 2Br- g Br2 + 2e- 1.06 V

E°Cell = -1.35 V

∴A larger voltage than the E°Cell = 1.35 V is required to overcome cell resistance (overvoltage) if rxn is to occur at reasonable rate.


Overvoltage At anode (Oxidation) E°

2H2O g 4H+ + O2 + 4e- - 1.23 V

Actually the voltage necessary to drive this reaction is greater than the theoretical value.

Overvoltage - Voltage necessary to overcome activation energy

Reason: i) Slow e- transfer at electrode surface

ii) Process (Kinetics) of bond breaking.

The actual overvoltage depends on the type of electrode and can only be determine experimentally.

For water an additional 0.4V is necessary for oxidation, therefore, instead of -1.23 V for water reduction, -1.6 V is necessary.


Electrolysis Reactions order

A Solution contains the ions H+, Cu2+, Ca2+ and Ni2+, each at a concentration of 1.0 M.

Determine the sequence of ions to be reduce at the Cathode ?

Reduction at Cathode E°

1st) 2H+ g H2 0.00 V

2nd) Cu+2 g Cu 0.34 V

3rd) Ca2+ g Ca -2.87 V

4th) Ni2+ g Ni 0.25 V


Electrolysis Reactions A Solution contains the ions H+, Cu2+, Ca2+ and Ni2+, each at a concentration of 1.0 M. Determine the sequence of ions to be reduce at the Cathode ? Reduction at Cathode E° 1st) Cu+2 g Cu 0.34 V 2nd) Ni2+ g Ni 0.25 V 3rd) 2H+ g H2 0.00 V

4th) Ca2+ g Ca -2.87 V *

* This reaction cannot be reduce in aqueous solution since H2O will be reduced first H2O reduction:

H2O(l)+ 2e- g H2(g) + 2OH-(aq) -0.83 V


Electrolysis Stoichiometry The amount of metal plated can be calculated based on stoichiometry. 1 Ampere = Coulomb per time (C/s) 1 Faraday = 96485 C / mol e-

Q 100: Silver is electroplated. i) What mass of silver plates if a current of 6.8A flows through the cell in 72 min. ii) What electrode will the silver plate-out.

Reaction: Ag+

(aq) + e- g Ag(s) (Reduction occurs at the cathode for electrolysis)

Given: Goal:6.8C = 1s Mass Ag (g)96485 C = 1 mol e-72 min = 432 sec107.87 g Ag = 1 mol Ag

72 min •60 sec1 min

•6.8 C1 sec

• 1 mol e-96485 C

•1 mol Ag1 mol e-

•107.87 g Ag1 mol Ag

=

Mass Ag = 32.84 g of Silver


Electrolysis Stoichiometry The amount of metal plated can be calculated based on stoichiometry. 1 Ampere = Coulomb per time (C/s) 1 Faraday = 96485 C / mol e-

Q 99: Copper can be electroplated at the cathode of an electrolysis cell by the half-reaction:

Reaction: Cu+2(aq) + 2e- g Cu (s) (Reduction occurs at the cathode for electrolysis)

How much time would it take to plate 325 mg of copper with 5.6 A?

Given: Goal:5.6 C = 1s Time for electrolysis 96485 C = 1 mol e-325 mg = .325 g63.55 g Cu = 1 mol Cu

0.325 g Cu •1 mol Cu63.55 g

•2 mol e-1 mol Cu

•96485 C1 mol e-

• 1 sec5.6 C

=

Time electrolysis = 176 sec = 180 sec


Voltaic Vs. Electrolytic Cells Voltaic: Generate E° (+) Conductivity

e- flow (external circuit) migration of ions (in cell) complete circuit (bridge)

Anode (-) Oxidation Cathode (+) Reduction

Electrolytic: Consumes E° (-) Conductivity

e- flow (pump by battery) migration of ions (in cell) Lowest E° reaction occurs

Anode (+) Oxidation Cathode (-) Reduction

Oxidation Anode Reduction Cathode M g M+ + e- M+ + e- g M Neg - Terminal Pos - Terminal

- +

Reduction Cathode Oxidation Anode M+ + e- g M M g M+ + e- Neg - Terminal Pos - Terminal

- +


Summary

General characteristics of voltaic and electrolytic cells. A voltaic cell generates energy from a spontaneous reaction (ΔG < 0), whereas an electrolytic cell requires energy to drive a nonspontaneous reaction (Δ G>0). In both types of cell, two external circuits provides the means or electrons to flow. Oxidation takes place all the anode, and reduction takes place at the cathode, but the relative electrode changes are opposite in the two cells.

Oxidation Reaction A- g A + e-

Oxidation Reaction X g X+ + e-

Reduction Reaction e- + Y+ g Y

Reduction Reaction e- + B+ g B

Overall (Cell) Reaction X + Y+ g X+ + Y, ΔG > 0

Overall (Cell) Reaction A- + B+ g A + B, Δ G> 0

Voltaic Cell Energy is released from spontaneous redox

reaction

System does work on load (surroundings)

Electrolytic Cell Energy is absorbed to drive

nonspontaneous redox reaction

Surrounding (power supply) do work on system (cell)

Driving a non-spontaneous Oxidation-Reduction...

Documents

Transcript of Driving a non-spontaneous Oxidation-Reduction...