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Discussion Response Post to: nerst equation's dimensions

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Bipin Dubey (13679)

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Some of the species which take part in electrode reactions are pure solid compounds and pure liquid compounds. In dilute aqueous solutions, water can be treated as a pure liquid because the amount of water is so much greater than the amount of any other species. For pure solid compounds or pure liquid compounds, activities are constant, so in the Nernst equation, as elsewhere in chemical thermodynamics, their values are considered to be one. The activities of gases are usually taken as their partial pressures and the activities of solutes such as ions are usually taken as their molar concentrations. Since ions are present in electrode reactions far more often than are gases or pure solids, the Nernst equation is often written with the gas and pure solid or liquid activities understood, in the form

E = E₀ - (RT/zF)ln([red]/[oxd])

It is understood, however, that the activities of pure solid compounds and pure liquid compounds are still taken as equal to one and that the activities of gases are still taken as equal to their partial pressures.

Since activities are dimensionless quantities therefore the quantity inside log is also dimensionless.

Bipin Kumar Dubey Chemical Dept. IIT Kharagpur

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