Spontaneous processes or reactions are those that occur naturally without requiring external intervention. The change in entropy of the universe (system + surroundings) for a spontaneous process is always positive or zero according to the second law of thermodynamics, which states that the entropy of an isolated system tends to increase over time.
However, it is possible for a spontaneous process to have a negative change in the entropy of the system alone while still obeying the second law when considering the total entropy change of the universe. In such cases, the surroundings experience an increase in entropy that compensates for the decrease in the system's entropy, resulting in a positive overall change in entropy.
One example of such a process is the dissolving of a solute in a solvent. When a solute dissolves, its molecular arrangement becomes more disordered, leading to an increase in the entropy of the solvent. However, the solute molecules become more tightly packed within the solvent, reducing their own entropy. Despite the negative change in entropy for the solute, the overall change in entropy of the system and surroundings combined remains positive.
Another example is the spontaneous mixing of gases. When different gases are initially separated, they have distinct molecular arrangements. As they mix, the molecular order decreases, resulting in an increase in the entropy of the system. The mixing process also increases the volume and disperses the molecules, leading to an increase in the entropy of the surroundings. Again, even though the entropy of the system decreases due to the mixing, the total change in entropy for the universe is positive.
These examples highlight how a spontaneous process can involve a negative change in the entropy of the system alone, while the total entropy change of the universe remains positive, satisfying the second law of thermodynamics.