Chemistry · Thermodynamics · NEET
A reversible process is a change carried out so slowly that at every moment the system and surroundings are almost in balance (near-equilibrium). NCERT says: a process is reversible if it can be reversed at any moment by an infinitesimal (tiny) change. For gas expansion this means the outside pressure is only a hair smaller than the inside pressure, written as p_ex = p_in - dp. Because the difference is so small, one tiny change in pressure can push the process back the other way.
It is infinitely slow because the pressure difference driving it is infinitesimally small (only dp). A tiny push moves the piston a tiny bit, then the system settles into a new balance, then another tiny push moves it again. Since each step is almost zero, you need an infinite number of tiny steps to finish, which takes infinite time. This is why a true reversible process is an ideal (imaginary) limit and cannot actually happen in real life, but it is the standard we compare real processes to.
It means the system passes through many balance points, one after another, like a staircase with infinitely small steps. At every point the system's pressure and the outside pressure are almost equal, so the system is never far from equilibrium. In an irreversible process the outside pressure is suddenly much lower, so the system is thrown out of balance and rushes forward in one big jump. Reversible = many tiny near-equilibrium steps; irreversible = one big out-of-balance jump.
During expansion, the gas pushes out only if the outside pressure (p_ex) is a little less than the inside pressure (p_in). If that difference is just dp (an infinitesimally small amount), then adding a tiny bit of pressure back makes p_ex = p_in + dp and the gas gets compressed instead. So the same tiny change flips the direction. This is the exact condition NCERT gives for a reversible process, and it is why the system stays in near-equilibrium the whole time.
During expansion, the gas works against the outside pressure. In a reversible process the outside pressure is kept as high as possible (only dp below the inside pressure) at every instant, so the gas pushes against the largest possible opposing pressure the whole way. That means the gas does the maximum possible work. In an irreversible process the outside pressure drops suddenly to a low value, so the gas pushes against less, and does less work. For NEET, remember: reversible isothermal work w = -2.303 nRT log(V_f/V_i), and this magnitude is the biggest for that expansion.
No. A reversible process is the ideal slow near-equilibrium change. A spontaneous process is one that happens on its own, and NCERT clearly states a spontaneous process is an irreversible process. So real spontaneous changes (like heat flowing hot to cold, or free expansion) are irreversible, not reversible. Reversible is the imaginary best-case standard; irreversible is what really happens.
The work done during the reversible isothermal expansion of one mole of hydrogen gas at 25°C from a pressure of 20 atmosphere to 10 atmosphere is (Given R = 2.0 cal K⁻¹ mol⁻¹):
For the irreversible expansion of an ideal gas under isothermal conditions, the correct option is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. A true reversible process needs infinite time because each step is infinitesimally small. It is an ideal (imaginary) limit. Real processes are always irreversible, but we compare them to the reversible ideal because reversible work is the maximum possible.
The external pressure must be only infinitesimally less than the internal pressure: p_ex = p_in - dp. This keeps the system in near-equilibrium, so a tiny change can reverse it.
Maximum work. Because the gas pushes against the highest possible opposing pressure at every instant, the reversible isothermal expansion gives the largest possible work, w = -2.303 nRT log(V_f/V_i).
ΔU = 0 for both reversible and irreversible isothermal ideal-gas changes, so it cannot tell them apart. But ΔS_total = 0 only for reversible and ΔS_total > 0 for irreversible. So entropy is the property that distinguishes them — a common NEET point.
No. NCERT states a spontaneous process is an irreversible process. It can only be reversed by an outside agency, so it is never reversible on its own.
