Chemistry · Thermodynamics · NEET
A process is irreversible when it happens at a real, finite speed, so the system and surroundings are NOT in balance (not in near-equilibrium) at every moment. In an irreversible gas expansion the gas pushes against a FIXED external pressure (p_ext) that is much lower than the gas pressure. Because the two pressures are very different, the change rushes forward and cannot be undone by just a tiny change. NCERT's definition is short: any process that is not reversible is irreversible. Almost every real-life process (a gas bursting into a bigger space, ice melting in a warm room) is irreversible.
A reversible process is imaginary and infinitely slow. It moves through a long chain of equilibrium states, with the gas pressure and external pressure almost equal at every step, so a tiny change can reverse it. An irreversible process is real and finite-speed. The external pressure stays fixed and is clearly less than the gas pressure, so equilibrium does NOT hold during the change. Key result for NEET: for the SAME expansion, the reversible path gives the MAXIMUM work; the irreversible path gives less work.
Work in expansion is the area under the curve on a p-V graph. In an irreversible expansion the gas pushes only against a low, constant external pressure, so the area (work) is a small rectangle: w = -p_ext(V_f - V_i). In a reversible expansion the gas pushes against a pressure that stays almost equal to its own (much higher) pressure the whole time, so the area under the smooth curve is larger. More area = more work. That is why reversible work is the maximum possible.
Free expansion (gas expanding into vacuum) is irreversible. Because the external pressure is zero, the work done is w = -p_ext x deltaV = 0. It is a sudden, one-way process that cannot undo itself by a tiny change. Note: even though work is zero and (for an ideal gas at constant T) internal energy does not change, the total entropy of the universe still INCREASES — that increase is the fingerprint of any irreversible process.
If the question says 'against a constant external pressure' or gives you a single p_ext value, the process is IRREVERSIBLE, so use w = -p_ext(V_f - V_i). If it says 'reversible isothermal', use w = -nRT ln(V_f/V_i) = -2.303 nRT log(V_f/V_i). Reading the wording is half the marks. Watch units: 1 L bar = 100 J.
Yes, it can. Both paths can start and end at the same state, so state functions like deltaU, deltaH and deltaS(system) are the SAME for both (they depend only on start and end, not the path). But path functions like work (w) and heat (q) are DIFFERENT: the reversible path gives more work. Only the reversible path also keeps the surroundings in balance, so it is the only one where deltaS(universe) = 0.
For the irreversible expansion of an ideal gas under isothermal conditions, the correct option is:
Under isothermal condition, a gas at 300 K expands from 0.1 L to 0.25 L against a constant external pressure of 2 bar. The work done by the gas is (1 L bar = 100 J):
The correct option for free expansion of an ideal gas under adiabatic condition is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Ice melting in a warm room, a gas rushing out into a larger empty space, or a hot cup of coffee cooling down. All of these happen on their own at a finite speed and cannot undo themselves by a tiny change, so they are irreversible.
Yes. Every actual process in nature is irreversible because it happens at a finite rate and is never perfectly in equilibrium. The reversible process is an idealised limit that we use only as a benchmark for maximum work and for calculations.
Reversible expansion does the maximum work between two given states. An irreversible expansion (against a fixed lower external pressure) always does less, because the area under its p-V path is smaller.
Because they hide two easy traps: the correct work formula (w = -p_ext deltaV, not the reversible log formula) and the entropy fact (deltaS(universe) > 0). Knowing which formula the wording demands earns quick, sure marks.
