Chemistry · Thermodynamics · NEET
A system is in thermodynamic equilibrium when its bulk properties, mainly pressure (p), volume (V) and temperature (T), stop changing with time AND are the same at every point inside the system. There is no hot corner and no cold corner, no high-pressure spot and no low-pressure spot. Once you leave the system alone and it reaches this steady, uniform state, it is at equilibrium. NCERT describes the state of a system using exactly these macroscopic properties, and that description only makes sense when the system has settled into equilibrium.
Thermodynamic laws use state functions like p, V, T, internal energy (U) and enthalpy (H). A state function must have ONE definite value for the whole system. If a gas is still mixing, or one side is hot and the other cold, then temperature and pressure do not have a single value, so you cannot even write down T or p for the system. Without single values, the equations of thermodynamics have nothing to plug in. That is why the laws are stated for equilibrium states, where every property has one clear value.
Not for the whole system. If you suddenly push a piston and the gas is swirling, some parts are hotter and denser than others. In that moment there is no single temperature or single pressure for the gas as a whole. You can only assign one T and one p once the gas settles and becomes uniform, that is, once it reaches equilibrium. This is the key reason NEET expects you to know that state variables belong to equilibrium states.
At the level of what you can measure, yes, nothing changes: p, V, T stay constant. But at the molecular level things are still busy. In chemical equilibrium the forward and backward reactions still happen, just at equal rates, so the amounts stay constant. So equilibrium means no NET change in bulk properties, not that molecules have frozen. This is a very common NEET trap.
Chemical equilibrium is only about composition: forward and backward reaction rates are equal, so concentrations stay constant. Thermodynamic equilibrium is broader. It needs three things together: thermal equilibrium (same temperature everywhere, no net heat flow), mechanical equilibrium (same pressure, no net motion or expansion) and chemical equilibrium (composition constant). A system is in full thermodynamic equilibrium only when ALL three hold at once.
A reversible process is carried out infinitely slowly, through a long series of near-equilibrium states. Because the system is almost at equilibrium at every instant, its p, V and T each have a well-defined single value the whole way through. That is why you can apply thermodynamic relations (like w = -p dV with p_ex = p) at each step of a reversible process, but not during a fast irreversible one.
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.
Thermal equilibrium (same temperature everywhere, no net heat flow), mechanical equilibrium (same pressure, no net expansion or motion) and chemical equilibrium (composition constant, no net reaction). All three must hold together.
No. If parts of the system are at different temperatures, there is no single T for the whole system. One definite temperature exists only after the system reaches equilibrium and becomes uniform.
State functions need one definite value for the whole system. That single value exists only when p, V and T are uniform and steady, which is exactly the equilibrium condition.
Yes. A reversible process goes infinitely slowly through a chain of near-equilibrium states, so the system's properties are well-defined at every step. This is why thermodynamic equations apply throughout a reversible process.
No. Thermodynamics only compares the initial and final equilibrium states. It does not deal with the rate or the path in between. Reaction speed is the job of chemical kinetics.
