Explain Nerst equation with examples....
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Answer:
In electrochemistry, the Nernst equation is an equation that relates the reduction potential of an electrochemical reaction to the standard electrode potential, temperature, and activities of the chemical species undergoing reduction and oxidation
Answer:
What is Nernst Equation?
The Nernst equation provides a relation between the cell potential of an electrochemical cell, the standard cell potential, temperature, and the reaction quotient. Even under non-standard conditions, the cell potentials of electrochemical cells can be determined with the help of the Nernst equation.
The Nernst equation is often used to calculate the cell potential of an electrochemical cell at any given temperature, pressure, and reactant concentration. The equation was introduced by a German chemist Walther Hermann Nernst.
Table of Content
Expression
NERNST Equation at 250C
Derivation
Determining Equilibrium Constant
Applications
Limitations
Solved Examples
Expression of Nernst Equation
Nernst equation is an equation relating the capacity of an atom/ion to take up one or more electrons (reduction potential) measured at any conditions to that measured at standard conditions (standard reduction potentials) of 298K and one molar or one atmospheric pressure.
Nernst Equation for Single Electrode Potential
Ecell = E0 – [RT/nF] ln Q
Where,
Ecell = cell potential of the cell
E0 = cell potential under standard conditions
R = universal gas constant
T = temperature
n = number of electrons transferred in the redox reaction
F = Faraday constant
Q = reaction quotient
The calculation of single electrode reduction potential (Ered) from the standard single electrode reduction potential (E°red) for an atom/ion is given by the Nernst equation.
⇒ Also Read: Redox Reactions
For a reduction reaction, Nernst equation for a single electrode reduction potential for a reduction reaction
Mn+ + ne– → nM is;
Ered = EMn+/M = EoMn+/M – [2.303RT/nF] log [1/[Mn+]]
Where,
R is the gas constant = 8.314 J/K Mole
T = absolute temperature,
n = number of mole of electron involved,
F = 96487 (≈96500) coulomb/mole = charged carried by one mole of electrons.
[Mn+] = active mass of the ions. For simplicity, it may be taken as equal to the molar concentration of the salt.
Nernst Equation at 25oC
For measurements carried out 298K, the Nernst equation can be expressed as follows.
E = E0 – 0.0592/n log10 Q
Therefore, as per the Nernst equation, the overall potential of an electrochemical cell is dependent on the reaction quotient.
Derivation of Nernst Equation
Consider a metal in contact with its own salt aqueous solution. Reactions of metal losing an electron to become an ion and the ion gaining electron to return to the atomic state are equally feasible and are in an equilibrium state
Mn+ + ne– → nM
In the reduction reaction, ‘n’ moles of an electron is taken up by the ion against a reduction potential of Ered.Change in the free energy at standard conditions of 298K and one molar /one atmospheric pressure conditions is ∆G°. From the above relation, it can be written that
∆G° = – nFE°red
Where,E°red is the reduction potential measured at standard conditions.
4. During the reaction, concentration keeps changing and the potential also will decrease with the rate of reaction.
To get the maximum work or maximum free energy change, the concentrations have to be maintained the same. This is possible only by carrying out the reaction under a reversible equilibrium condition.
For a reversible equilibrium reaction, vant Hoff isotherm says:
∆G = ∆G° + RT ln K