In the kinetic theory of gases, transport phenomena such as viscosity, thermal conduction and diffusion occur only in which thermodynamic state of a gas, and are these processes reversible or irreversible?

Introduction / Context:
Transport phenomena in gases and liquids, such as viscosity, heat conduction and mass diffusion, are central topics in kinetic theory, fluid mechanics and thermodynamics. They describe how momentum, energy and particles move through a medium when gradients exist. For example, viscosity relates to momentum transport between layers of fluid moving at different velocities, while thermal conduction involves heat flow from hotter regions to colder regions. This question asks about the thermodynamic state under which such transport phenomena occur and whether they are reversible or irreversible processes in nature.

Given Data / Assumptions:
• We are considering a gas exhibiting viscosity, heat conduction and diffusion. • Transport phenomena require gradients of velocity, temperature or concentration. • Equilibrium implies no macroscopic gradients and no net flows. • Irreversibility is associated with entropy production in real processes.

Concept / Approach:
In thermodynamic equilibrium, all macroscopic properties of a system such as temperature, pressure and composition are uniform in space and remain constant in time. There are no net flows of heat, mass or momentum, and therefore transport phenomena vanish at equilibrium. Transport phenomena arise only when the system is in a non equilibrium state where gradients exist, such as temperature differences or velocity shear. These processes tend to reduce the gradients and drive the system toward equilibrium. They are inherently irreversible because they involve entropy production; heat spontaneously flows from hot to cold, and mixed gases do not spontaneously unmix. Thus, transport phenomena occur only in non equilibrium conditions and are irreversible in the thermodynamic sense.

Step-by-Step Solution:
Step 1: Recall that in equilibrium, temperature, pressure and composition are uniform, so there are no driving forces for heat flow, diffusion or viscous momentum transfer. Step 2: Recognise that viscosity requires velocity gradients between fluid layers; thermal conduction requires temperature gradients; diffusion requires concentration gradients. Step 3: Understand that these gradients define non equilibrium states because different parts of the system have different properties. Step 4: Note that as transport phenomena act, they smooth out the gradients and move the system toward equilibrium. Step 5: Identify that these processes produce entropy, making them irreversible; they do not spontaneously reverse direction without external work. Step 6: Conclude that the correct description is non equilibrium state with irreversible processes.

Verification / Alternative check:
Thermodynamics distinguishes between reversible and irreversible processes. Reversible processes are idealised, infinitely slow changes where the system passes through a continuous sequence of equilibrium states and produce no net entropy. Transport phenomena, however, are driven by finite gradients and involve finite rates of heat, mass and momentum transfer. These are textbook examples of irreversible processes that generate entropy. Kinetic theory also shows that collisions between molecules lead to relaxation toward equilibrium distributions, and the associated flows cannot be exactly reversed without detailed control of every microscopic motion. This theoretical context reinforces that transport phenomena occur in non equilibrium regimes and are irreversible.

Why Other Options Are Wrong:
Option B suggests that transport processes in non equilibrium states are reversible, which contradicts the fact that spontaneous transport increases entropy and cannot simply run backward without external intervention. Option C states that transport occurs in equilibrium states, which is incorrect because in equilibrium all net flows vanish. Option D combines equilibrium with reversible processes, but while reversible processes are indeed related to equilibrium paths, they do not describe actual transport phenomena like viscous flow, conduction and diffusion, which require non equilibrium conditions. Therefore, none of these alternatives correctly describes the nature of transport phenomena in gases.

Common Pitfalls:
Students sometimes confuse the idea that equations for transport coefficients may be derived assuming local equilibrium with the idea that transport itself happens at equilibrium. Local equilibrium means that small regions behave as if at equilibrium while overall gradients still exist. Another pitfall is to think that if a system eventually reaches equilibrium, the path it took must have been reversible, which is not true. Remember that real heat conduction, diffusion and viscous flow are always associated with entropy production and are inherently irreversible. They exist only when the system is out of equilibrium.

Final Answer:
The correct choice is Non equilibrium state, irreversible processes, because transport phenomena like viscosity, heat conduction and diffusion occur only when gradients are present in a non equilibrium gas, and they are intrinsically irreversible processes that generate entropy as the system relaxes toward equilibrium.