Question about chemical potential

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Discussion Overview

The discussion revolves around the concept of chemical potential in a gas composed of two different kinds of particles. Participants explore whether there are distinct chemical potentials for each species and how this relates to various systems, including electrolytic solutions and ideal gases.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Exploratory

Main Points Raised

  • Some participants propose that there are indeed two different chemical potentials, one for each species of particle in a gas.
  • One participant explains that chemical potential is an abstract mathematical quantity that reflects the change in energy when a particle is added, indicating that different particles will have different chemical potentials due to their distinct contributions to energy changes.
  • A later reply provides an example from biological systems, noting that in an electrolytic solution, different ions (e.g., sodium, potassium, calcium) have different chemical potentials due to varying concentration gradients across cell membranes.
  • Another participant mentions the Nernst equation, which relates the chemical potential difference across a membrane to voltage, highlighting its relevance in measuring cellular function.
  • Additionally, a participant introduces the formula for the chemical potential of ideal gases, indicating that it is proportional to the logarithm of the partial pressure of each species.

Areas of Agreement / Disagreement

Participants generally agree that different species in a gas can have distinct chemical potentials, but the discussion includes various examples and contexts that illustrate this concept, leading to multiple perspectives on its implications.

Contextual Notes

The discussion touches on the abstract nature of chemical potential and its dependence on specific conditions, such as concentration gradients and the ideal gas approximation. However, the implications of these conditions remain unresolved.

Einj
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It is probably a silly question, but here it is. Suppose we have a gas composed by two different kinds of particles. There will be two different kinds of chemical potential? One for each specie?
 
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Chemical potential is not some substance that can "be there in the container". It is rather an abstract mathematical quantity that gives the change of energy of the container when one particle is added, while the entropy, volume and numbers of other particles are kept at the same value.

As the change of energy depends on the kind of particle one is adding, there is different chemical potential to each different kind of particle.
 
Einj said:
It is probably a silly question, but here it is. Suppose we have a gas composed by two different kinds of particles. There will be two different kinds of chemical potential? One for each specie?

Yes- a good example is an electrolytic solution, such as found within biological systems. The cytosol of your cells has high potassium, low sodium, and *very* low calcium concentrations, while the outside (extracelluar space) has high sodium, low potassium and low calcium concentrations. Thus, there are different driving forces on the ions corresponding to the different concentration gradients across the cell membrane. For sodium and calcium, the electrochemical potential difference is always strongly negative- sodium and calcium ions experience a strong driving force into the cell. Potassium's electrochemical difference is close to zero or slightly positive, so potassium ions experience a weak driving force out of the cells.

The Nernst equation shows that in equilibrium, the chemical potential difference across a membrane is equivalent to a voltage. In my lab, we measure this voltage as an electrophysiological readout of cellular function.
 
Perfect! Thank you very much!
 
Just wanted to add that in case of ideal gasses the chemical potential of each species is [itex]\mu_i=\mu_0+RT \ln p_i/p_0[/itex], i.e. the chemical potential is proportional to the logarithm of partial pressure of each species.
 

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