Question

Would entropy increase or decrease for changes in state in which the reactant is a gas or liquid and the product is a solid? What sign would the entropy change have?

Short answer

Entropy decreases, and the entropy change is negative.

Step-by-step solution

Understand Entropy

Entropy is a measure of the disorder or randomness in a system. Higher disorder corresponds to higher entropy and vice versa.

Change in State from Reactant to Product

Identify the states of the reactants and products. The problem states that the reactant is a gas or liquid, and the product is a solid.

Compare Entropy of Different States

Gases have the highest entropy followed by liquids, and solids have the lowest entropy. This is because the molecules in gases move most freely, followed by liquids, and then solids have the least freedom of movement.

Determine the Entropy Change

Since the reactant is either a gas or liquid and the product is a solid, the entropy will decrease because the system is moving toward a state of lower disorder.

Sign of the Entropy Change

A decrease in entropy is represented by a negative change. Therefore, the sign of the entropy change will be negative.

Key concepts

entropy

Entropy is a central concept in chemistry. It quantifies the disorder or randomness in a system. Gases, for example, have high entropy because their molecules move freely, bumping into each other constantly. This chaotic movement signifies high disorder. In contrast, solids have low entropy due to their structured and orderly arrangement of molecules. Understanding entropy helps in predicting the spontaneity of reactions and changes in state. More disorder means higher entropy, while more order means lower entropy. Always remember, the second law of thermodynamics tells us that in an isolated system, entropy will not decrease.

state changes

State changes in chemistry refer to the transformation of a substance from one phase to another: solid, liquid, or gas. Each phase has distinct properties. For example, a change from gas to solid will see a significant decrease in entropy. Gas molecules are much farther apart and move freely, making them highly disordered. When these gas molecules become a solid, they arrange themselves in a rigid and orderly structure, reducing the disorder. This transformation also means the system loses energy, hence a decrease in entropy. Understanding state changes is crucial to determine the direction and feasibility of chemical reactions.

disorder and randomness

Disorder and randomness are key concepts when discussing entropy. In a chemical context, disorder refers to the number of ways molecules can be arranged and still achieve the same energy level. More arrangements correspond to higher entropy. For instance, in gases, molecules move in all directions with high energy, leading to a high level of randomness and disorder. Conversely, solids have very limited movement, resulting in lower randomness and thus lower entropy. When analyzing reactions, always consider how the states of reactants and products contribute to disorder and randomness. More randomness translates to higher entropy.

negative entropy change

Negative entropy change indicates a decrease in disorder within a system. If a reaction results in a state with less randomness, the entropy decreases, which is a negative change. In our example where a gas or liquid transforms into a solid, the randomness drastically reduces because solid-state molecules are tightly packed in an orderly manner. The system moves to a more ordered state, resulting in a negative entropy change. This concept is critical in predicting reaction spontaneity and helps in determining whether a particular change of state or reaction is likely to occur. A negative entropy change usually means moving to a more stable, less energetic state.