Identify the true statement about a spontaneous process. (a) A reaction that is nonspontaneous in the forward direction is spontaneous in the reverse direction. (b) Adding a catalyst will cause a nonspontaneous reaction to become spontaneous. (c) In a spontaneous process, the entropy of the system always decreases. (d) An endothermic reaction is always spontaneous.

Verified step by step guidance

1. Step 1: Understand the concept of spontaneity. A spontaneous process is one that occurs without needing to be driven by an external force.
2. Step 2: Analyze option (a). A reaction that is nonspontaneous in the forward direction is indeed spontaneous in the reverse direction, according to the second law of thermodynamics.
3. Step 3: Evaluate option (b). Adding a catalyst does not change the spontaneity of a reaction; it only speeds up the rate at which equilibrium is reached.
4. Step 4: Consider option (c). In a spontaneous process, the total entropy of the system and surroundings increases, but the entropy of the system itself can decrease.
5. Step 5: Review option (d). An endothermic reaction is not always spontaneous; spontaneity depends on both enthalpy and entropy changes, as described by the Gibbs free energy equation: \( \Delta G = \Delta H - T\Delta S \).

Key Concepts

Spontaneity of Reactions

A spontaneous process is one that occurs without external intervention, typically characterized by a decrease in free energy. Spontaneity is determined by the Gibbs free energy change (ΔG); if ΔG is negative, the process is spontaneous. This concept is crucial for understanding how reactions proceed in nature and the conditions under which they occur.

Entropy and the Second Law of Thermodynamics

Entropy is a measure of disorder or randomness in a system, and the Second Law of Thermodynamics states that the total entropy of an isolated system can never decrease over time. In spontaneous processes, the total entropy of the universe (system plus surroundings) increases, which often drives reactions forward. Understanding entropy helps clarify why some reactions are spontaneous while others are not.

Role of Catalysts

Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. They work by lowering the activation energy required for the reaction to occur. However, catalysts do not change the spontaneity of a reaction; they cannot make a nonspontaneous reaction spontaneous, as they do not affect the overall energy change of the reaction.