9.6
집중 셀
농축전지는 두 반전지 간 한 종의 농도 차이에서 발생하는 전기화학 전지입니다. 갈바닉 전지와 달리, 전기 에너지는 화학 반응에서 나오지만, 여기서 동력은 농도가 높은 영역에서 낮은 농도로 물질이 이동하는 것입니다. 따라서 전체 과정은 물리적인 성격을 띠고 있습니다. 고…
농축 셀에서는 두 반전지 간 농도 차이에서 기전력이 발생하며, 이는 전자와 이온의 자발적 흐름을 유도하는 불균등한 전극 전위를 만듭니다.
농축전지는 전극 농축전과 전해질 농축전으로 나뉩니다.
전극 농축 셀에서는 전극이 동일하지만, 각 전극에서 전기활성 종의 농도가 다릅니다.
대표적인 예로는 수소 전극이 있는데, 동일한 백금 전극이 동일한 수소 이온 용액에서 서로 다른 압력의 수소 가스에 노출됩니다. 넨스트 방정식에 따르면, 셀은 자발적으로 작동하며 p₂가 p₁보다 작을 때 양의 기전력을 발생시킵니다.
반면, 전해질 농축 셀은 동일한 전극이 서로 다른 농도로 같은 전해질에 담겨 있는 형태입니다. 이 세포들은 전이 없이, 또는 전이 없이 작동합니다.
예를 들어, 아연 농축전지는 서로 다른 Zn²⁺ 이온 농도를 가진 용액의 아연 전극을 사용합니다. 넨스트 방정식에 따르면, 기전력은 양수이며, a₂가 a₁를 초과할 때 과정은 자발적입니다.
View the full transcript and gain access to JoVE Core videos
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.