ELECTROCHEMISTRY

Electrical and Chemical Energy Interconversion

REDOX REACTION

• Reduction• Oxidation• Reducing Agent• Oxidizing Agent

ELECTRICAL CONDUCTION

• Electrons• Conducting medium• electron transfer (flow)• conductance (mho)• Resistance (ohm)

Electrochemical Cell

ZnSO4 CuSO4 Cl- SALT BRIDGE K+

Zn Cu

V

e-

ANODE CATHODE

ELECTRODES

• Active Electrodes • Inert Electrodes

CELLSCell Cell Reaction Ecell

GALVANIC SPONTANEOUS +

ELECTROLYTIC NON-SPONTANEOUS

-

SAMPLE REACTION

Zn(s) Zn2+ + 2e-

2e- + Cu2+ Cu(s)

Zn(s) + Cu2+ Zn2+ +Cu(s)

E°cell = E°cathode - E°anode

SPONTANEITY

E°cell > 0SPONTANEOUS at standard state

E°cell < 0NON SPONTANEOUS at standard state

w = -nFEΔG = -nFE

ΔG° = -nFE°

n = moles of electronsF = Faraday’s constant = 96485 C/mole of e-

SAMPLE PROBLEM

Evaluate the standard cell potential of each of the following cells and indicate whether the cell is GALVANIC or ELECTROLYTIC.

1) AuCl4- + Fe2+ Au(s) + 4Cl- +Fe3+

2) K+ + H2O(l) O2(g) + H+ + K(s)

SEATWORK

Complete the cell reaction for:

Pb(s) + MnO4- Pb2+ + Mn2+

Identify the oxidation and reduction half reaction and evaluate the Standard Cell Potential.

CELL REPRESENTATION

Anode | Oxidation || Reduction | ReductionAnode Before Cathode!

Zn(s) | Zn2+(aq) || Cu2+

(aq) | Cu(s)

SEATWORK

5Pb(s) + 16H+ + 2MnO4- Pb2+ 2Mn2+ +8H2O

Write the cell representation for the following reaction.

QUIZ

Pt(s) | Fe2+,Fe3+ || Cu2+ |Cu(s)

1) Draw the cell. (Include the direction of the flow of electrons)

2) Identify the half-cell reaction and the complete cell reaction.

3) Evaluate the standard electron cell potential.4) Identify whether the cell is Galvanic or

Electrolytic.

EFFECT OF CONCENTRATION

ΔG = ΔG° + RT ln Q-nFEcell = -nFE°cell + RT ln Q

Ecell = E°cell – RT/nF ln Q

At 25°CE = E°cell – 0.05916/n log Q

SAMPLE PROBLEM

Is the cellPt(s) | Fe2+ (0.10M),Fe3+ (0.06M)|| Cu2+ (0.05M)|

Cu(s)

Galvanic or Electrolytic?

SEATWORK

Is the cellPb(s) | Pb2+(0.0025M) || H+(0.0020M), MnO4

- (0.0030M), Mn2+ (0.010M) | Pt(s)

Galvanic or Electrolytic?

SAMPLE PROBLEM

Given the cell:Pt | Fe2+, Fe3+ || Mn2+, H+, MnO4- | Pt

[Fe2+] = 0.20M[Fe3+] = 0.0020M[Mn2+] = 0.0010M[MnO4-] = 0.60M

Calculate the pH at which the reaction will occur spontaneously.

APPLICATION

On the basis of electrochemistry, provide an explanation on how galvanized iron (iron

coated with zinc) prevents corrosion of iron.

EVALUATION OF EQUILIBRIUM CONSTANT

ΔG = 0ΔG = ΔG° + RT ln Q

ΔG° = -RT ln Keq

-nFE° = -RT ln Keq

Keq = e ^ (nFE°/RT)Keq = 10 ^ (nE°/0.0592)

SAMPLE PROBLEM

Calculate the Ksp of AgCl

AgCl(s) + e- Ag(s) + Cl-(aq) E°=0.22V

Ag(s) Ag+(aq) + e- E°=0.80V

Electrical Conduction

• Electron / Metallic Conduction – metals• Electrolytic Conduction – salts in solution

(electrolytes in general)

Conductance (L) = 1/resistanceMolar conductance (siemens) = 1000Ls/C

ELECTROLYSIS

A+

B-

- +

MECHANISM OF ELECTROLYSIS

1. Migration of ions to electrode1. Cation to cathode2. Anion to Anode

2. Reaction1. Cathode – reduction2. Anode - Oxidation

FACTORS

• Nature of Electrodes– Active– Inert

• Nature of Electrolytes– Molten (one anion, one cation)– Aqueous (competition )

FARADAY’s Law of ELECTROLYSIS

Q = ItQ – charge in Coulombs

I – current in Amperet – time in seconds

1 mol of e- carries a charge of 96485CF = 96485 C / mol e-

SAMPLE PROBLEM

1) 500 C of electricity is applied to a molten solution of NaI.

1) What are the substances formed at each electrode? Write the reactions.

2) Determine the mass of products produced.3) How about for aqueous solution of NaI?4) How about when a current of 10 amperes is

applied to molten NaI for 1 hour.

SEATWORK

Calculate the mass (if solid) or volume at STP (if liquid) of products from the electrolysis of

molten CaCl2 between Platinum electrodes using 5.0A for 8 mins.

QUIZ

A spoon with an area of 3.5cm2 is plated with silver from a silver nitrate aqueous solution

using a current of 0.60A for 1.6 hours.A) How many grams of silver are plated if the

current efficiency is 100%? Recalculate for a current efficiency of only 80%.

B) What is the thickness of the silver plate formed if the density of silver is 10.5 g/cm3?