Nernst Equation (at 25 degrees C)

The Nernst Equation calculator computes the cell electric potential (E) at 25 degrees C using the Nernst Equation (E = E₀ – 0.0592/n • logQ).

INSTRUCTIONS: Choose the preferred units and enter the following:

• (E₀) Initial cell electrical potential.
• (n)  Number of electrons (e.g. a Zn²⁺ ion has a charge of +2, making n=2)
• (Q) Thermodynamic reaction quotient of balanced chemical equation

Cell Electric Potential (E): The calculator returns the potential in volts.  However, this can be automatically changed to other electrical potential units via the pull-down menu.

The Math / Science

The Nernst Equation (at 25 degrees C)^[1], E = E₀ – 0.0592/n • logQ, calculates cell potential under non-standard conditions. Cell potential is a method of measuring the amount of voltage that exists between two half cells of a battery, and will be measured in Volts. This equation relates cell potential to the reaction quotient (Q = [products]^y/[reactants]^x) and provides an accurate method of measuring equilibrium constants.

The inputs are:

• E₀ = initial cell potential (must be entered in Volts)
• n = number of electrons (example: a Zn²⁺ ion has a charge of +2, making n=2)
• Q = thermodynamic reaction quotient of balanced chemical equation

Description

Substituting the above values into the Nernst Equation at 25⁰ C will result in an electrode potential under nonstandard conditions. This equation is commonly used in electrochemistry and was even involved in the discovery of cold fusion. 

The thermodynamic reaction quotient, Q, will most likely need to be calculated before entering it into this equation (unless it is explicitly given in a problem). In order to find Q, you could enter your data into the Reaction Quotient Equation in vCalc, or, you could follow the instructions below. To find Q, you first need to write the balanced chemical reaction:

Example:

aA + bB = dD + eE

Next, put the concentration of the products over the reactants, and solve for Q, as shown below:

Q= [products] / [reactants]

Q = [E]^e [D]^d / [B]^b [A]^a

Supplemental Material

Khan Academy: Cell Potentials Under Non-standard Conditions

ChemWiki: Nernst Equation

Reaction Quotient (Q) (for the Spanish site click here)

References

Whitten, et al. "Chemistry" 10th Edition. Pp. 828

ChemWiki

[1] For the Spanish site click here