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ChemWiki: The Dynamic Chemistry E-textbook > Analytical Chemistry > Electrochemistry > Connection between \(E_{cell}\), ∆G, and K

Connection between \(E_{cell}\), ∆G, and K

The connection between cell potential, Gibbs free energy and constant equilibrium are directly related in the following multi-part equation:

\[ \Delta G^o= -RT\ln K_{eq} = -nFE^o_{cell} \]

∆G: Gibbs Free Energy

∆G is the change of Gibbs (free) energy for a system and ∆G° is the Gibbs energy change for a system under standard conditions (1 atm, 298K). On an energy diagram, ∆G can be represented as:

delta_g_diagram.png

Where ∆G is the difference in the energy between reactants and products. In addition ∆G is unaffected by external factors that change the kinetics of the reaction. For example if Ea(activation energy) were to decrease in the presence of a catalyst or the kinetic energy of molecules increases due to a rise in temperature, the ∆G value would remain the same.

cell: Standard Cell Potential

cell is the electromotive force (also called cell voltage or cell potential) between two half-cells. The greater the E°cell of a reaction the greater the driving force of electrons through the system, the more likely the reaction will proceed (more spontaneous). E°cell is measured in volts (V). The overall voltage of the cell = the half-cell potential of the reduction reaction + the half-cell potential of the oxidation reaction. To simplify,

Ecell = Ereduction + Eoxidation   or   Ecell = Ecathode + Eanode

The potential of an oxidation reduction (loss of electron) is the negative of the potential for a reduction potential (gain of electron). Most tables only record the standard reduction half-reactions. In other words, most tables only record the standard reduction potential; so in order to find the standard oxidation potential, simply reverse the sing of the standard reduction potential.

*Note: The more positive reduction potential of reduction reactions are more spontaneous. When viewing a cell reduction potential table, the higher the cell is on the table, the higher potential it has as an oxidizing agent. 

Difference between Ecell and Eºcell

Eº cell is the standard state cell potential, which means that the value was determined under standard states. The standard states include a concentration of 1 Molar (mole per liter) and an atmospheric pressure of 1. Similar to the standard state cell potential, Eºcell, the Ecell is the non-standard state cell potential, which means that it is not determined under a concentration of 1 Molar and pressure of 1 atm. The two are closely related in the sense that the standard cell potential is used to calculate for the cell potential in many cases. 

\[ E_{cell}= E^o_{cell} -\dfrac{RT}{nF} ln\; Q \]

Other simplified forms of the equation that we typically see: 

\[ E_{cell}= E^o_{cell} -\dfrac{0.0257}{n} ln \; Q \]

or in terms of log base 10, instead of the natural logarithm (base e) 

\[ E_{cell}= E^o_{cell} - \dfrac{0.0592}{n} log_{10}\; Q \]

Both equations applies when the temperature is 25ºC. Deviations from 25ºC requires the use of the original equation. Essentially, Eº is E at standard conditions

Example 1

What is the value of Ecell for the voltaic cell below:

Pt(s)|Fe2+(0.1M),Fe3+(0.2M)||Ag+(0.1M)|Ag(s) 

Ecell= ?    

SOLUTION

To use the Nersnt equation, we need to establish Eocelland the reaction to which the cell diagram corresponds so that the form of the reaction quotient (Q) can be revealed. Once we have determined the form of the Nernst equation, we can insert the concentration of the species.

Solve:

 Eocell = Eocathode- Eo anode

          = EoAg/Ag  - EoFe3+/Fe2

          = 0.800V-0.771V = 0.029V

Now to determine Ecell for the reaction

Fe2+(0.1M) + Ag+(1.0M)  → Fe3+(0.20M) + Ag(s)

Use the Nernst equation

Ecell= 0.029V -(0.0592V/1)log [Fe3+]/[Fe2+][Ag]

         =0.029V - 0.0592V*log [0.2]/[0.1]*[1.0]

         =0.011V

K: The Equilibrium Constant

K is the equilibrium constant of a reaction  and is given by the reaction quotient:

\[ aA + bB \leftrightharpoons cC + dD \]

\[ K= \dfrac{[A]^a[B]^b}{[C]^c[D]^d} \]

The Relationship Between The Three

The relationship between ∆G, K, and E° cell can be represented by the following diagram.

triangle_diagram.png

where

  • R = 8.314 J mol C-1
  • T = Temp in K
  • n = moles of e- from balanced redox reaction
  • F = Faraday's constant  = 96,485 C/mol

cell can be calculated using the following formula:

            cell  = E° (cathode) – E° (anode) = (Reduction) – (Oxidation)

Example 2

Question Find the E° cell for the following coupled half-reactions

SOLUTION

1. Determine the cathode and anode in the reaction

Zn(s) ↔ Zn2+(aq) + 2e-                  Anode, Oxidation (since Zn(s) increase oxidation state from 0 to +2)

Cu2+(aq) + 2e- ↔ Cu(s)                  Cathode, Reduction (since Cu2+(aq) decreases oxidation state from +2 to 0)

2. Determine the E° cell values using the standard reduction potential table

Zn(s) ↔ Zn2+(aq) + 2e-                  -0.763

Cu2+(aq) + 2e- ↔ Cu(s)                  +0.340

3. Use E° cell = E°cathode - E°anode

= 0.340 - (-0.763)

= 1.103 V

Problems

1.     Find E° cell for 2Br-(aq) + I2(s) Br2(s) + 2I-(aq)

2.     Find E° for Sn(s) ↔ Sn2+(aq) + 2e-

3.     Find E° cell for Zn(s) | Zn2+(aq) || Cr3+(aq) , Cr2+(aq)

(See answer key for solutions)

*If the E° values of the reaction is negative, then the reaction is NOT spontaneous and therefore the reverse reaction is occuring and the electrons are flowing in the opposite direction.

* These values are tabulated in the standard reduction potential table.

 

Summary Table

Eo cell Delta G Q & K Relationship Reaction Direction Spontaneity (as written)
>0 - Q<K Forward Spontaneous
<0 + Q>K Backward Non-spontaneous
=0 =0 Q=K No Reaction N/A
Example: Using ∆G=-RTlnK

Given K = 2.81x10-16 for a following reaction Cu2+(aq) + Ag(s)  ↔ Cu(s) + 2Ag+,  find ∆G.

Solution:

Use the following formula: ∆G=-RTlnK

= 8.314 x 298 x ln(2.81x10-16) = -8.87x105

= 8.871 kJ

Example: Using ∆G=-nFE°cell

Find ∆G for the following reaction:

2Al(s) +3Br2(l) ↔ 2Al3+(aq, 1M) + 6Br-(aq, 1M)

 

Solution:

Step 1: Separate the reaction into its two half reactions

2Al(s) ↔ 2Al3+(aq)

3Br2(l) ↔ 6Br-(aq, 1M)

Step 2: Balance the half equations using O, H, and charge using e-

2Al(s) ↔ 2Al3+(aq) +6e-           

6e- + 3Br2(l) ↔ 6Br-(aq)

Step 3: From the balanced half reactions, we can conclude the number of moles of e- for use later in the calculation of ∆G. Determine the E° values using the standard reduction potentials, using the E° cell table.

2Al2(s) ↔ Al3+(aq) +6e-      -1.676V

3Br2(l) + 6e- ↔ 6Br-(aq)     +1.065V

Step 4: Determine cell = E°cathode - E°anode.

= 1.065 - (-1.676)

= 2.741 V

Step 5: Once E° cell has be calculated and the number of moles of electrons have been determined, we can use ∆G = -nFE°cell

= (-6 mol e-)(96458 C/mol e-)(2.741 V)

= -1586kJ

This equation can be used to calculate E° cell given K or K given E°cell. If T=298 K, the RT is a constant then the following equation can be used: cell= (0.025693V/n) ln K

Example: Using E° cell=(RT/nF) lnK

Given the E° cell for the reaction Cu(s) + 2H+(aq) ↔ Cu2+(aq) +H2(g) is -0.34V, find the equilibrium constant (K) for the reaction.

SOLUTION

Step 1: Split into two half reaction

Cu(s) ↔ Cu2+(aq)

2H+(aq)↔ H2(g)

Step 2: Balance the half reactions with charges to determine n

Cu(s) ↔ Cu2+(aq) + 2e-

2H+(aq) +2e- ↔ H2(g)

Therefore n=2

Step 3: From the example above, E° cell = -0.34V

-0.34 = (0.025693/2) lnK

K = e(-0.34 x 2/0.025693) 

K = 3.19 x 10-12

Example 1: I see you

Given the following reaction determine ∆G, K, and Eocellfor the following reaction at standard conditions? Is this reaction spontaneous?

Mn2+(aq) + K(s) ↔ MnO2(s) + K(aq)

SOLUTION

Step 1:  Separate and balance the half reactions. Label which one is reduction and which one is oxidation. Find the corresponding E° values for the half reactions.

MnO2(s) + 4H+(aq) + 2e ↔ -Mn2+(aq) + 2H2O(l)        Reduction      +1.23V

K+(aq) + e- ↔ K(s)                                                 Oxidation      -2.92V

Step 2:  Write net balanced reaction in acidic solution, and determine the E° cell.

E° cell = E°cathode- E°anode = +1.23 - (-2.92) = 4.15

Mn2+(aq) + 2K+(aq) + 2H2O(l) ↔ MnO2(s) + 4H+(aq) + K(s)              E° cell = 4.15V

Step 3: Find ∆G for the reaction.

Use ∆G = -nFE°cell

= -2 mol e- x 96458 C x 4.15 = -800.60kJ

Therefore, since Eocell is positive and ∆G is negative, this reaction is spontaneous.

Problems

4. Find ∆G for the following combined half reactions:

F2(g) + 2e- ↔ 2F-(aq)    

Li+(aq) + e- ↔ Li(s)

5. Find the equilibrium constant (K) for the following reaction: (Hint: Find E°Cell first!)

Zn(s) + 2H+(aq) ↔ Zn2+(aq) + H2(g)

6. Find E°Cell for the given reaction at standard conditions:

Cu+(aq) + e- ↔ Cu(s)

I2(s) + 2e- ↔ 2I-(aq)

(See answer key for solutions)

Answers

  1. +0.530 V
  2. +0.137 V
  3. +0.339 V
  4. +1139.68 kJ
  5. 6.4 x 1025
  6. +0.195 V

References

  1. Brady, James E., Holum, John R. “Chemistry: The Study of Matter and Its Changes”, John Wiley & Sons Inc 1993
  2. Brown, Theodore L., LeMay, H. Eugene Jr. “Chemistry: The Central Science” Third Edition, Prentice-Hall, Inc. Englewood Cliffs, N.J. 07632 1985
  3. Brown, Theodore L., LeMay, H. Eugene Jr., Bursten, Bruce E. “Chemistry: The Central Science” Fifth Edition,  Prentice-Hall, Inc. Englewood Cliffs, N.J. 07632 1991
  4. Gesser, Hyman D. “ Descriptive Principles of Chemistry”, C.V. Mosby Company 1974
  5. Harwood, William, Herring, Geoffrey, Madura, Jeffry, and Petrucci, Ralph, General Chemistry: Principles and Modern Applications, Ninth Edition, Upper Saddle River,New Jersey, Pearson Prentice Hall, 2007.

Contributors

  • Deborah S. Gho, Shamsher Singh

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