Nernst equation is used to quantitatively describeIonsFormed between systems A and BDiffusion potentialThe equation expression of.In electrochemistry, Nernst equation is used to calculate the equilibrium voltage of the specified redox pair on the electrode relative to the standard potential.Nernst equation is meaningful only when two substances in the redox pair exist at the same time.This equation connects the chemical energy with the electrode potential of the primary battery and has made great contributions to electrochemistry, so it was named after its discoverer, German chemist Nernst, who was awarded the title of "Nernst" in 1920Nobel Prize in Chemistry。[1]
In fact, chemical reactions often occur in nonStandard statusAnd the ion concentration will change during the reaction.For example, laboratorychlorineOne of the preparation methods ofmanganese dioxideAndConcentrated hydrochloric acidReaction;Under heating, chlorine can occur continuously.But usingStandard electrode potentialTo judge the direction of the above reaction, the opposite conclusion will be drawn.
So the reaction cannot be spontaneous to the right.
The reason for the contradiction between the φ (standard) judgment result and the actual reaction direction is that hydrochloric acid is not 1mol/L, CltwoThe partial pressure is not necessarily 101.3kpa, and heating will also change the value of electrode potential.Because chemical reactions are often carried out under non-standard conditions, it is required to study the influence of ion concentration, temperature and other factors on electrode potential.
However, since the reaction is usually carried out at room temperature, and the temperature has a relatively small effect on the electrode potential, it is important to focus on the effect of concentration on the electrode potential when the temperature is fixed at room temperature (298K).
The effect of ion concentration change on electrode potential can be discussed through the example of Cu Zn galvanic cell.
If the battery reaction starts, Zn2+And Cu2+The concentration is 1mol/L, and the measured electromotive force of the battery should be 1.10V in the standard state.
Zn(s)+Cu2+(1mol/L)=Zn2+(1mol/L)+Cu (s) E=(standard) 1.10V
Figure 1
When the battery starts to discharge, the reaction continues to move to the right, Zn2+Concentration increases while Cu2+Concentration decreases.As the concentration ratio of reactant and product ions changes, [Zn2+]/[Cu2+]Gradually increase, and the trend of reaction to the right will gradually decrease,Battery electromotive forceThe measured value of will also decrease accordingly.As shown in Figure ⑴, the abscissa is [Zn2+]And [Cu2+]The ordinate is the electromotive force E of the battery.Zn2+Concentration increase, Cu2+When the concentration decreases, the electromotive force of the battery decreases from 1.10 until the reaction reaches equilibrium.
When the reaction reaches the equilibrium state, the battery stops discharging, and the battery electromotive force decreases to zero;[Zn2+]And [Cu2+]Is equal to the equilibrium constant K=[Zn2+]/[Cu2+]=10thirty-seven, lg K=37. When the battery electromotive force is zero, the straight line intersects the abscissa, and the abscissa value of the intersection point is about 37.
In addition to the above, [Zn2+]/[Cu2+]In addition to changes, there are also many ways to change the ion concentration ratio.If soluble zinc salt is added to the zinc half battery, dilute it with water or add S2-Make Cu2+The precipitation concentration decreases, etc.No matter how you operate it, you will find that as long as [Zn2+]/[Cu2+]The increase will reduce the battery electromotive force;On the contrary, when the ion concentration ratio decreases, the electromotive force of the battery increases.
Equation content
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The quantitative relationship between the ion concentration ratio and the electrode potential presented by the above experimental results can be found through the deduction of thermodynamic theory.For the followingRedox reaction：
⑴ This equation is calledNernst (Nernst, W.H.1864~1941) equation[2]。It points out the quantitative relationship between the electromotive force of the battery and the nature of the battery (E) and the electrolyte concentration.
When the temperature is 298K, Nernst equation is[3]：
When the temperature is 298K, the Nernst equation of Cu Zn galvanic cell reaction is:
E=E (standard) - (0.0592/n) lg ([Zn2+]/[Cu2+]）……………………⑶
The graph of the equation should be a straight line with an intercept of E=1.10V and a slope of -0.0592/2=-0.03, which is consistent with the above experimental results.The Nernst equation of an electrode can be obtained by expanding formula ⑶:
={φ (standard,+)+(0.0592/2) lg [Cu2+]}-{φ (standard, -)+(0.0592/2) lg [Zn2+]}
So φ (+)=φ (standard,+)+(0.0592/2) lg [Cu2+]
φ (-)=φ (standard, -)+(0.0592/2) lg [Zn2+]
General formula:
φ=φ (standard)+(0.0592/n) lg ([oxidized]/[reduced]).............................. ⑷
In the formula, n-the number of electron transfer in electrode reaction.
[Oxidation type]/[Reduction type] - the ratio of the product of the concentration of all substances involved in the electrode reaction to the product of the concentration of reaction products.And the power of concentration should be equal to their coefficient in electrode reaction.
The concentration of pure solid and pure liquid is a constant, and the treatment is 1.The unit of ion concentration is mol/L (activity shall be strictly used).Gas is expressed in partial pressure.
Equation writing
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The following examples illustrate the specific writing method of Nernst equation:
⑴ Known Fe3++e-=Fe2+, φ (standard)=0.770V
Φ=φ (standard)+(0.0592/1) lg ([Fe3+]/[Fe2+])
=0.770+(0.0592/1)lg([Fe3+]/[Fe2+])
⑵ Known Br2 (l)+2e -=2Br-, φ (standard)=1.065V
Φ=1.065+(0.0592/2)lg(1/[Br-]∧2)
⑶ Known MnOtwo+4H++2e-=Mn2++2HtwoO. φ (standard)=1.228V
Φ=1.228+(0.0592/2)lg([H+]four/[Mn2+])
⑷ Known Otwo+4H++4e-=2HtwoO. φ (standard)=1.229V
Φ=1.229+(0.0592/4)lg((p(Otwo)·[H+]four)/1)
Equation application
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1、 Change of electrode potential when ion concentration changes
According to Nernst equation, the change value of electrode potential can be obtained when the ion concentration changes
2、 Influence of ion concentration change on the direction of redox reaction
In non-standard state, it is not enough to use standard potential to judge the reaction direction for two electric pairs with similar potential, and the influence of ion concentration change on the reaction direction should be considered.
3、 Influence of medium acidity on redox reaction and pH potential diagram
