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Ch.18 - Free Energy and Thermodynamics
All textbooksTro 4th EditionCh.18 - Free Energy and ThermodynamicsProblem 35a,c,d
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Chapter 18, Problem 35a,c,d

Without doing any calculations, determine the sign of ΔSsys for each chemical reaction. a. 2 KClO3(s) → 2 KCl(s) + 3 O2(g) c. Na(s) + 2 Cl2(g) → NaCl(s) d. N2(g) + 3 H2(g) → 2 NH3(g)

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1
Identify the states of matter for each substance in the reaction.
Recognize that entropy (ΔS) is a measure of disorder or randomness in a system.
Note that gases generally have higher entropy than solids due to their greater freedom of movement.
Compare the number of moles of gaseous products to the number of moles of gaseous reactants.
Conclude that if the number of moles of gas increases, ΔS_sys is likely positive, indicating an increase 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 chemical reactions, an increase in the number of gas molecules typically leads to an increase in entropy, while a decrease in the number of gas molecules can result in a decrease in entropy. Understanding how the states of matter and the number of particles change during a reaction is crucial for predicting the sign of ΔS.
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States of Matter

The states of matter—solid, liquid, and gas—play a significant role in determining the entropy of a system. Solids have a fixed structure and lower entropy, while gases have more freedom of movement and higher entropy. In the given reaction, the transition from solid KClO3 to gaseous O2 indicates a change in the state of matter that affects the overall disorder of the system.
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Chemical Reaction Stoichiometry

Chemical reaction stoichiometry involves the quantitative relationships between reactants and products in a chemical reaction. In the provided reaction, the stoichiometry shows that 2 moles of solid KClO3 produce 3 moles of gaseous O2 and 2 moles of solid KCl. This increase in the number of gaseous products relative to the reactants suggests an increase in entropy, which is essential for determining the sign of ΔS.
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Without doing any calculations, determine the sign of ΔSsys for each chemical reaction. a. Mg(s) + Cl2(g) → MgCl2(s) b. 2 H2S(g) + 3 O2(g) → 2 H2O(g) + 2 SO2(g) c. 2 O3(g) → 3 O2(g) d. HCl(g) + NH3(g) → NH4Cl(s)

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