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ChemistryEdu Logo Thermodynamics | Reversible and Irreversible Processes#

Irreversible Process#

  • An irreversible process is a type of process in which the driving force is much greater than the opposing force.

  • It is a very fast process and takes a very little time to complete.

Irreversible Process

  • In case of irreversible expansion, Pgas is much greater than Pext (Pgas >> Pext).

  • In case of irreversible compression, Pext is much greater than Pgas (Pext >> Pgas).

  • Work done in an irreversible process is given by:

\[W_{irr} = - \int\limits_{V_1}^{V_2} P_{ext}dV\]

Since, irreversible processes take place against constant external pressure.

\[W_{irr} = -P_{ext}(V_2 - V_1)\]
\[W_{irr} = -P_{ext}ΔV\]

Reversible Process#

  • In a reversible process, the driving force is infinitesimally greater than the opposing force.

  • If the opposing force is increased by an infinitesimally small amount, the process returns back to its previous state. Therefore, it is called reversible.

  • Reversible processes are generally very slow processes.

Reversible Process

  • For a reversible expansion, Pgas is infinitesimally greater than Pext.

  • For a reversible compression, Pext is infinitesimally greater than Pgas.

  • In general, we can write: Pext ≈ Pgas

  • Work done in a reversible process is given by:

\[W_{rev} = - \int\limits_{V_1}^{V_2} P_{ext}dV\]

Since, Pext ≈ Pgas, we can write:

\[W_{rev} = - \int\limits_{V_1}^{V_2} P_{gas}dV\]