Equimolar counterdiffusion is characterized by which molar flux relation?

• A. NA = NB
• B. NA = 0
• C. NA = NB
• D. NA = -NB

Answer: (D)

Equimolar counterdiffusion means the two species diffuse toward each other with equal rates, so the total molar flux of the mixture is zero. This symmetry leads to NA = -NB, meaning the flux of one component is equal in magnitude but opposite in direction to the flux of the other. In a binary system with no net bulk flow, the sum NA + NB = 0, which directly gives NA = -NB.

Why this is the signature relation: each species follows Fick’s law based on its own concentration gradient, and in the case where the gradients are such that the two fluxes balance, their magnitudes must be equal and opposite. This is the situation described by equimolar counterdiffusion.

So, NA = -NB captures the idea that they diffuse in opposite directions with the same strength, whereas NA = NB would imply both move the same way, and NA = 0 or NB = 0 would imply one component isn’t diffusing at all, not matching the equimolar diffusion scenario.