How to Calculate the Standard Gibbs Free Energy Change for Cells – Part 1

Electromotive force (EMF) or electromotance is an energy transfer to an electric circuit per unit of electric charge.


An electrochemical cell is a device that either generates electrical energy from chemical reactions in a so-called galvanic or voltaic cell, or induces chemical reactions (electrolysis) by applying external electrical energy in an electrolytic cell.

Mnemonics: An Ox, Red Cat (Anode Oxidation, Reduction Cathode)

OIL RIG (Oxidation Is Loss, Reduction Is Gain)

The Gibbs free energy (or Gibbs energy) is a thermodynamic potential which may be used to calculate the maximum amount of work, other than pressure–volume work, which may be done by a thermodynamically closed system at constant temperature and pressure, and provides a required condition for processes, viz., chemical reactions that may occur under these conditions.

The term ‘spontaneous’ has historically been used to describe processes including chemical reactions for which the change in Gibbs free energy in the standard state, ΔrGѲ < 0. The phrase ‘thermodynamically favored’ is preferred instead so that common misunderstandings equating ’spontaneous’ with ‘suddenly’ or ‘without cause’ can be avoided.

Question:

In the button cells widely used in watches and other devices the following reaction takes place:

Zn(s) + Ag2O(s) + H2O(l) → Zn2+(aq) + 2Ag(s) + 2OH–(aq)
Determine EѲ & ΔrGѲ for the reaction. EѲ (Ag2O/Ag) = +0.34 V, EѲ (Zn2+/Zn) = +0.76 V

Solution:



Click/tap to enlarge the image

n = Number of electrons in the balanced half-cell equation, F = Faraday

Note that the negative sign of the change in standard Gibbs free energy of the reaction indicates that it is thermodynamically favorable.

Energized by electrochemistry❓ Let Chemaficionado know in the comments below or at mychemistryhomework@gmail.com❗

References:

(1)    Wikipedia Contributors. Electromotive force. Wikipedia. https://en.wikipedia.org/wiki/Electromotive_force.

‌(2)    Wikipedia Contributors. Electrochemical cell. Wikipedia. https://en.wikipedia.org/wiki/Electrochemical_cell.

‌(3)    Ncert. Chemistry : Textbook for Class XII - Part.I; National Council Of Educational Research And Training: New Delhi.

(4)    Brown, C.; Ford, M. Higher Level Chemistry, 2nd ed.; Pearson Education: Harlow, Essex, 2014.

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