Chemistry · Thermodynamics · NEET
A process is spontaneous when the Gibbs energy change is negative: ΔG < 0. If ΔG = 0 the system is at equilibrium (no net change). If ΔG > 0 the process is non-spontaneous in that direction (the reverse direction is spontaneous instead). This one sign of ΔG is the final master rule NEET tests again and again.
No. This is the biggest trap. Spontaneous only means the process CAN happen on its own without any external help. It says nothing about speed. NCERT gives the example of hydrogen and oxygen: they can sit mixed for years with no visible change, yet the reaction is still spontaneous because it has the potential to proceed. Speed is decided by kinetics (activation energy), not by thermodynamics.
Because energy release is only one part of the story. Many endothermic reactions (ΔH positive) are still spontaneous, like ice melting above 0 °C or many dissolving processes. The real driving force combines TWO things: energy (enthalpy, ΔH) and disorder (entropy, ΔS). The full test is ΔG = ΔH − TΔS. So a reaction can be spontaneous even when it absorbs heat, as long as the TΔS term wins.
Entropy (ΔS) measures how much disorder increases. Nature favours more disorder, so a positive ΔS pushes toward spontaneity. Temperature T scales how strong that push is, through the TΔS term. At high T the entropy term dominates; at low T the enthalpy term dominates. That is why some reactions are spontaneous only above a certain temperature and others only below it.
You need ΔH < 0 (exothermic, so the ΔH term is negative) AND ΔS > 0 (disorder increases, so −TΔS is also negative). With both terms negative, ΔG = ΔH − TΔS stays negative no matter what T is. This is exactly the answer to the NEET 2016 question below.
NCERT states that a spontaneous process is an irreversible process; it cannot reverse its direction on its own and can only be reversed by an external agency. So every real spontaneous change is irreversible. 'Spontaneous' describes the natural direction; 'irreversible' describes that you cannot undo it for free.
The correct thermodynamic conditions for the spontaneous reaction at all temperatures is:
For a given reaction, ΔH = 35.5 kJ mol⁻¹ and ΔS = 83.6 J K⁻¹ mol⁻¹. The reaction is spontaneous at: (Assume ΔH and ΔS do not vary with temperature.)
For 2A(g) + B(g) → 2D(g), ΔU° = −10 kJ mol⁻¹ and ΔS° = −44 J K⁻¹ at 298 K. Identify ΔG° and the spontaneity at 298 K. (R = 8.31 J mol⁻¹ K⁻¹)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. When ΔG = 0 the system is at equilibrium, so there is no net change in either direction. Spontaneous means ΔG < 0.
Yes. If the reaction increases disorder enough (large positive ΔS) so that TΔS is bigger than the positive ΔH, then ΔG = ΔH − TΔS becomes negative and the reaction is spontaneous, usually at high temperature.
Look only at the sign of ΔG. ΔG < 0 means spontaneous (forward). ΔG = 0 means equilibrium. ΔG > 0 means non-spontaneous forward (spontaneous in reverse).
Because one clean rule covers four cases: both signs favourable = spontaneous at all T; both unfavourable = never spontaneous; and the two mixed cases depend on temperature through T = ΔH/ΔS. It is a fast, repeatable question type, exactly like NEET 2016 and 2017.
Often, yes. Because ΔG = ΔH − TΔS contains T, changing temperature can flip the sign of ΔG in the two mixed cases. This is why some reactions switch from non-spontaneous to spontaneous above a crossover temperature T = ΔH/ΔS.
