Chemistry · Thermodynamics · NEET
NCERT defines it clearly: spontaneity means 'having the potential to proceed without the assistance of an external agency.' In simple words, a spontaneous process CAN move forward on its own, once it is set up. You do not need to keep pushing it with an outside energy source. Water flowing downhill, a gas expanding to fill a container, and iron rusting are all spontaneous. The key word is POTENTIAL, not certainty of a fast result.
Yes. This is the most tested trap. A slow reaction can still be spontaneous. NCERT gives the exact example: hydrogen and oxygen gas can be mixed at room temperature and left for many years with no visible change, yet the reaction is STILL called spontaneous. The reaction has the potential to proceed; it just proceeds at an extremely slow rate. Spontaneity tells you the DIRECTION is allowed, not how fast it goes.
No. Many students wrongly think 'spontaneous = starts immediately when reactants meet.' NCERT specifically corrects this. Spontaneous only means the process can go without outside help. The speed depends on a separate idea called reaction rate (which involves activation energy). A spontaneous reaction can be instant, slow, or so slow you never see it finish in a lifetime.
No. A spontaneous process is an irreversible process. It moves in one direction on its own and cannot reverse itself without help. For example, heat flows from a hot body to a cold body on its own, but it will never flow back from cold to hot without an outside agency (like a refrigerator using electricity). Gas spreads out on its own but never squeezes back into one corner by itself.
Not on its own. Early scientists thought only exothermic reactions (negative delta H) are spontaneous, like a stone falling and losing potential energy. But this is wrong because some endothermic reactions are also spontaneous (like ice melting above 0 degrees C, or many salts dissolving with cooling). The full, correct test uses Gibbs energy: delta G = delta H minus T times delta S. A process is spontaneous only when delta G is negative.
A spontaneous process happens on its own once started and has delta G less than 0. A non-spontaneous process cannot happen on its own; it needs continuous outside help (energy input) and has delta G greater than 0. Example: water flowing downhill is spontaneous; pumping water uphill is non-spontaneous. Importantly, the reverse of a spontaneous process is always non-spontaneous.
The correct thermodynamic conditions for the spontaneous reaction at all temperatures is:
For a given reaction, ΔH = 35.5 kJ/mol and ΔS = 83.6 J/K/mol. The reaction is spontaneous at (assume ΔH and ΔS do not vary with temperature):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Spontaneous means the process can proceed on its own. Speed is a separate idea called rate. A spontaneous reaction can be very slow, like hydrogen plus oxygen staying unchanged for years.
For a spontaneous process, delta G (Gibbs free energy change) is negative (delta G < 0). If delta G = 0 the system is at equilibrium, and if delta G > 0 the process is non-spontaneous.
No. Most exothermic reactions are spontaneous, but not all. Some endothermic processes (like ice melting above 0 C) are also spontaneous. The complete test is delta G = delta H minus T delta S, not enthalpy alone.
A spontaneous process is irreversible. It goes one way on its own and can only be reversed by an outside agency, such as heat flowing from hot to cold but never back on its own.
Yes. Because delta G = delta H minus T delta S depends on temperature, some reactions are spontaneous only above or only below a certain temperature (see the NEET 2017 PYQ where T > 425 K is required).
