Criterion for spontaneity

For a spontaneous process, the general thermodynamic criterion is:

• For an isolated system: $\Delta S_{\text{universe}} > 0$ Since an isolated system does not exchange matter or energy with surroundings, this becomes: $\Delta S_{\text{system}} > 0$

So, for a spontaneous process in an isolated system, entropy must increase.

Evaluate each option

Option A: For a spontaneous process in an isolated system, the change in entropy is positive

This is exactly the thermodynamic condition for spontaneity in an isolated system.

Hence, A is correct.

Option B: Endothermic processes are never spontaneous

This is false. Some endothermic processes are spontaneous if the entropy increase is sufficiently large. Example: melting of ice above $0^\circ$C, dissolution of some salts.

Hence, B is incorrect.

Option C: Exothermic processes are always spontaneous

This is false. Even if a process is exothermic ($\Delta H < 0$), it may be non-spontaneous if entropy decreases enough that: $\Delta G = \Delta H - T\Delta S > 0$

Hence, C is incorrect.

Option D: Lowering of energy in the reaction process is the only criterion for spontaneity

This is false. Spontaneity depends on both enthalpy and entropy, not only energy decrease. For constant $T$ and $P$, the criterion is: $\Delta G < 0$

Hence, D is incorrect.

Final answer

The correct statement is: $\boxed{\text{A}}$

Comparison with stored answer

Stored correct answer: A

My derived answer: A

So, the derived answer agrees with the stored correct answer.