Class 12 Chemistry Electrochemistry Nernst Equation

Nernst Equation

• This equation was named after a German physicist Walther Nernst.

• The Nernst Equation empowers the assurance of cell potential under non-standard conditions and relates the measured cell potential to the reaction quotient and permits the exact measurement of equilibrium constants.
• Let us consider an electrochemical reaction of the following type:

aA +bB  --> cC + dD

• Nernst equation for this can be written as follows:

• In case of daniel cell Nerst equation is as follows:

• The above equation implis that the value of increases with the increase in the concentartion of Cu2+ ion increases and decrease in the concentration Zn2+
• Putting the values of R, F at T= 298 K. the equation becomes

• If the circuit in Daniel cell is closed:

Zn(s) + Cu2+ (aq) →Zn2+ (aq) + Cu(s)

• With time the concentration of Zn2+
• The concentration of Cu2+
• Voltage reading of the cell on the voltmeter decreases.
• After some time there is no alteration in the concentration of Cu2+ and Zn2+ ions and the voltmeter gives zero reading. At this point of time equilibrium has been reach
• The Nernst equation for the reaction is:

• But at equilibrium,

At T = 298 K.

The equation can be rewritten as

Problem:

Calculate the emf of the cell in which the following reaction takes place: Ni(s) + 2Ag+ (0.002 M) → Ni2+ (0.160 M) + 2Ag(s). Given that Eøcell = 1.05 V.

Solution:

By using Nernst equation

= 1.05 - 0.02955 log 4 × 104

= 1.05 - 0.02955 (log 10000 + log 4)

= 1.05 - 0.02955 (4 + 0.6021)

= 0.914 V

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