What Is the Nernst Equation?

Nernst equation refers to the equation expression used to quantitatively describe the diffusion potential of an ion formed between A and B systems. In electrochemistry, the Nernst equation is used to calculate the equilibrium voltage of a given redox pair at an electrode relative to a standard potential. The Nernst equation is only meaningful if two substances are present in the redox pair. This equation relates chemical energy to galvanic cell electrode potentials, and has made a significant contribution to electrochemistry, so it was named after its discoverer, the German chemist Nernst, who won the Nobel Prize in Chemistry in 1920 for this. ^[1]

Chemical reactions often often occur in non-

Through the derivation of thermodynamic theory, we can find the quantitative relationship between the ion concentration ratio and the electrode potential shown in the above experimental results. For the following redox reactions:

E = E (standard)-(RT) / (nF) ln ([Zn ²⁺ ] / [Cu ²⁺ ])

For any battery reaction:

aA + bB = cC + dD

E = E (standard)-(RT) / (nF) ln (([C] ^c · [D] ^d ) / ([A] ^a · [B] ^b ))

This equation is called

The following is an example to illustrate the specific writing of the Nernst equation:

It is known that Fe ³⁺ + e- = Fe ²⁺ , (standard) = 0.770V

= (standard) + (0.0592 / 1) lg ([Fe ³⁺ ] / [Fe ²⁺ ])

= 0.770 + (0.0592 / 1) lg ([Fe ³⁺ ] / [Fe ²⁺ ])

Known Br2 (l) + 2e- = 2Br-, (standard) = 1.065V

= 1.065 + (0.0592 / 2) lg (1 / [Br-] 2)

Known MnO ₂ + 4H ⁺ + 2e- = Mn ²⁺ + 2H ₂ O, (standard) = 1.228V

= 1.228 + (0.0592 / 2) lg ([H +] ⁴ / [Mn ²⁺ ])

Known O ₂ + 4H ⁺ + 4e-= 2H ₂ O, (standard) = 1.229V

= 1.229 + (0.0592 / 4) lg ((p (O ₂ ) · [H +] ⁴ ) / 1)

First, the change in electrode potential when the ion concentration changes

According to the Nernst equation, the value of the electrode potential can be obtained when the ion concentration is changed.

Second, the effect of changing ion concentration on the direction of the redox reaction

In the non-standard state, it is not enough to judge the reaction direction only by using the standard potential for two pairs that are close to each other. The influence of the ion concentration on the reaction direction should be considered.

Effect of medium acidity on redox reaction and pH potential diagram