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Ch.19 - Chemical Thermodynamics
All textbooksBrown 15th EditionCh.19 - Chemical ThermodynamicsProblem 42a
Chapter 19, Problem 42a

Predict the sign of ΔSsys for each of the following processes: (a) Molten gold solidifies.

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Step 1: Understand the concept of entropy (ΔS). Entropy is a measure of the disorder or randomness in a system. A positive ΔS indicates an increase in disorder, while a negative ΔS indicates a decrease in disorder.
Step 2: Consider the process described: molten gold solidifying. This process involves a phase change from liquid to solid.
Step 3: Recall that in general, the transition from a liquid to a solid results in a decrease in entropy. This is because the particles in a solid are more ordered and have less freedom of movement compared to those in a liquid.
Step 4: Predict the sign of ΔSsys. Since the process involves a decrease in disorder (from liquid to solid), ΔSsys is expected to be negative.
Step 5: Conclude that the sign of ΔSsys for the solidification of molten gold is negative, indicating a decrease in entropy.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Entropy (ΔS)

Entropy, denoted as ΔS, is a measure of the disorder or randomness in a system. In thermodynamics, a positive ΔS indicates an increase in disorder, while a negative ΔS signifies a decrease in disorder. Understanding how entropy changes during a process is crucial for predicting the spontaneity and direction of chemical reactions.
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Phase Changes

Phase changes refer to the transitions between solid, liquid, and gas states of matter. During these transitions, energy is absorbed or released, affecting the system's entropy. For example, when a substance solidifies, it typically moves from a more disordered liquid state to a more ordered solid state, which usually results in a decrease in entropy.
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Spontaneity of Processes

The spontaneity of a process is determined by the changes in enthalpy (ΔH) and entropy (ΔS) according to the Gibbs free energy equation, ΔG = ΔH - TΔS. A process is spontaneous if ΔG is negative. In the case of molten gold solidifying, the decrease in entropy (negative ΔS) must be considered alongside the enthalpy change to assess the overall spontaneity of the process.
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