Chemistry · Thermodynamics · NEET
No. This is the single biggest confusion in this topic. Spontaneous only means the reaction has the potential to proceed on its own, without any outside help pushing it forward. It says nothing about how fast or slow it goes. A spontaneous reaction can finish in a fraction of a second, or it can take thousands of years. Speed is a separate question.
When you mix hydrogen and oxygen gas at room temperature, they can sit together for many years with no visible change. Yet NCERT still calls this reaction spontaneous. Why? Because the reaction has the potential to happen (it releases a lot of energy and moves toward a more stable product, water). The reason it looks 'stuck' is that its RATE is extremely slow. It needs a spark or catalyst to speed it up, but the potential to react was always there. So it is spontaneous but slow.
The speed (rate) of a reaction is decided by chemical kinetics, NOT thermodynamics. Rate depends on things like activation energy (the energy barrier that must be crossed), temperature, and catalysts. Thermodynamics only tells you the DIRECTION a reaction can go and whether it is allowed. Kinetics tells you HOW FAST it gets there. NEET keeps these two ideas in separate chapters for exactly this reason.
NCERT defines it clearly: spontaneity means 'having the potential to proceed without the assistance of an external agency.' In plain words, the reaction can happen by itself once it starts, even if it needs a tiny push (like a spark) to get going and even if it is very slow.
You give it a small push to overcome the energy barrier. A spark, a flame, or a platinum catalyst lowers the barrier or supplies the starting energy. Once it starts, it proceeds on its own and releases a huge amount of energy (this is why it is used as rocket fuel). The spark does not make the reaction spontaneous; the reaction was always spontaneous. The spark only affects the RATE.
Speeding it up does not make it spontaneous. A catalyst or high temperature can increase the rate of a reaction, but it cannot change whether the reaction is thermodynamically allowed. Whether a reaction is spontaneous is decided only by the sign of the Gibbs energy change (delta G), which does not depend on catalysts.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Exothermicity alone is not the criterion for spontaneity. Some endothermic reactions are also spontaneous. The real test is the Gibbs energy change (delta G less than zero), which combines enthalpy and entropy. This is covered in the next concept on whether decrease in enthalpy is a criterion for spontaneity.
A process is spontaneous when the Gibbs energy change is negative: delta G = delta H minus T times delta S, and delta G must be less than zero. This single condition decides direction, and it has nothing to do with how fast the reaction goes.
Because it has the potential to proceed on its own and moves toward a lower-energy, more stable state (water). NCERT explicitly uses this example to teach that spontaneity does not depend on rate.
No. A catalyst only increases the rate by lowering the activation energy. It cannot make a non-spontaneous reaction spontaneous, because it does not change delta G.
Thermodynamics tells you the direction a reaction can go and whether it is allowed (spontaneity). Kinetics tells you the speed. A reaction can be thermodynamically spontaneous yet kinetically very slow, like H2 + O2 at room temperature.
