Question

Identify each reaction as reversible or irreversible.

Detonation of TNT ["irreversible", "reversible"]      

If you have a gas in a container connected to an evacuated container, and you open the stop-cock between them, the gas will expand without doing any work. ["reversible", "irreversible"]      

Electrolysis ["irreversible", "reversible"]      

Diffusion ["irreversible", "reversible"]         

Answer 1

Almost everything that happens around us every day can be an example of an irreversible process.

• A hot cup of tea left on its own to cool down.
• An egg falling on the floor and cracking open, spilling its contents in the process.

Basically, anything whose time-reversed version cannot happen spontaneously is irreversible. A cup of tea, when left alone, will never get heated up all by itself. Similarly, a broken egg will never reassemble itself.

A reversible process then is one which can spontaneously reverse its direction such that its time-reversed version is indistinguishable from the forward time version.

Detonation of TNT is an irreversible process as the reaction is following

2 C₇H₅N₃O₆ → 3 N₂ + 5 H₂O + 7 CO + 7 C

2 C₇H₅N₃O₆ → 3 N₂ + 5 H₂ + 12 CO + 2 C

The reaction is exothermic but has a high activation energy in the gas phase (~62 kcal/mol) and the product can never produce TNT again.

If you have a gas in a container connected to an evacuated container, and you open the stop-cock between them, the gas will expand without doing any work. This process is said to be irreversible. As the expanded gas will never go back to cylinder without external pressure application.

But if the gas is in a cylinder fitted with a piston and the expansion takes place such the external pressure is always nearly equal to the internal pressure, the gas will do the maximal amount of work during the expansion. It is said to be reversible because the slightest increase in external pressure will cause the piston to go down and compress the gas.

In electrolysis, certain electrode reactions where the cell is operating away from its equilibrium (reversible) potentials determined from Thermodynamics are very fast and depart very little from the equilibrium potential. Such reactions are frequently referred to as reversible (see Fig). Other electrode reactions are inherently slow and require a potential, E, significantly greater in magnitude than the equilibrium potential to achieve a reasonable current density. This potential is called the overpotential, η (=E–E_e), and the electrode is the said to be polarized. Such reactions are referred to as irreversible (fig). As overpotential is increased in magnitude (more negative for cathodic processes, more positive for anodic processes) current density increases, typically exponentially at high overpotentials. The relationship between current density and electrode potential is the subject of electrode kinetics.

Diffusion is a natural process. Gases tend to mix automatically without any mediation in diffusion. Thus, some entropy is generated which can’t be reversed