9.6
Células de Concentração
Uma célula de concentração é uma célula eletroquímica na qual a fem surge de uma diferença na concentração de uma espécie entre duas meias-células. Ao…
Em uma célula de concentração, a fem surge da diferença de concentrações entre duas meias-células, o que cria potenciais de eletrodo desiguais que impulsionam um fluxo espontâneo de elétrons e íons.
As células de concentração são divididas em células de concentração de eletrodos e células de concentração de eletrólitos.
Em uma célula de concentração de eletrodos, os eletrodos são idênticos, mas a concentração da espécie eletroativa difere em cada eletrodo.
Um exemplo padrão é o eletrodo de hidrogênio, onde eletrodos de platina idênticos são expostos ao gás hidrogênio em diferentes pressões na mesma solução de íons de hidrogênio. De acordo com a equação de Nernst, a célula opera espontaneamente e produz um emf positivo quando p₂ é menor que p₁.
Por outro lado, as células de concentração de eletrólitos consistem em eletrodos idênticos imersos no mesmo eletrólito em concentrações diferentes. Essas células operam sem transferência nem com transferência.
Por exemplo, uma célula de concentração de zinco utiliza eletrodos de zinco em soluções com diferentes concentrações de íons Zn²⁺. De acordo com a equação de Nernst, a fem é positiva, e o processo é espontâneo quando a₂ excede a₁.
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Q1: What is the source of emf in a concentration cell?
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.
Q2: How do electrode concentration cells differ from electrolyte concentration cells?
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.
Q3: What does the Nernst equation predict for hydrogen electrode concentration cells?
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.
Q4: How does a zinc concentration cell generate emf?
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.
Q5: What is the difference between concentration cells with and without transference?
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.
Q6: What is an example of an electrode concentration cell using amalgams?
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.
Q7: Why is the overall process in a concentration cell considered physical rather than chemical?
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.