Concentration Cells

In a concentration cell, emf arises from concentration differences between two half-cells, creating unequal electrode potentials that drive spontaneous electron and ion flow. Unlike galvanic cells powered by chemical reactions, the driving force here is the transfer of matter from higher to lower concentration, making the overall process physical in nature.

Electrode concentration cells have identical electrodes with different concentrations of electroactive species, such as hydrogen gas at different pressures. Electrolyte concentration cells have identical electrodes immersed in the same electrolyte at different concentrations. Both types generate emf through concentration gradients, but the location of the difference distinguishes them.

According to the Nernst equation, a hydrogen electrode concentration cell with identical platinum electrodes exposed to hydrogen gas at different pressures operates spontaneously with positive emf when the lower pressure (p₂) is less than the higher pressure (p₁). This spontaneous flow is analogous to gas expansion from high to low pressure regions.

A zinc concentration cell consists of zinc electrodes immersed in solutions with different Zn²⁺ ion concentrations. The Nernst equation shows that emf is positive and the process is spontaneous when the ion activity at one electrode (a₂) exceeds that at the other (a₁), driving zinc transfer from higher to lower concentration regions.

Electrolyte concentration cells without transference use a salt bridge to separate solutions, allowing indirect ion transfer via the bridge. Cells with transference employ a porous membrane keeping solutions in direct contact, enabling ions to migrate across the junction. Both configurations generate emf from concentration differences but differ in ion transport pathways.

A lead amalgam cell exemplifies electrode concentration cells with different metal concentrations. The cell notation Hg–Pb(c₁) | Pb²⁺(aq) | Hg–Pb(c₂) shows two mercury-lead amalgam electrodes at different lead concentrations. Positive emf corresponds to spontaneous lead transfer from the more concentrated amalgam to the less concentrated one.

Concentration cells operate through the transfer of matter from regions of higher to lower concentration, driven by concentration gradients rather than chemical reactions. This physical process of matter redistribution, similar to gas expansion or diffusion, distinguishes concentration cells from galvanic cells where electrical energy originates from chemical transformations.