Which of the following reactions has ∆Ssys> 0? (a) N2(g) + 3H2(g) -> 2NH3(g) (b) Ag+(aq) + Cl-(aq) -> AgCl(s) (c) 2H2O2(aq) -> 2H2O(l) + O2(g) (d) 2I(g) -> I2(g)

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Identify the change in the number of moles of gas in each reaction, as this often indicates a change in entropy (∆S).

For reaction (a), N2(g) + 3H2(g) -> 2NH3(g), note the decrease in the number of moles of gas from 4 to 2, suggesting ∆Ssys < 0.

For reaction (b), Ag+(aq) + Cl-(aq) -> AgCl(s), observe the formation of a solid from aqueous ions, indicating a decrease in disorder, so ∆Ssys < 0.

For reaction (c), 2H2O2(aq) -> 2H2O(l) + O2(g), note the production of a gas (O2) from a liquid, which increases disorder, suggesting ∆Ssys > 0.

For reaction (d), 2I(g) -> I2(g), note the decrease in the number of moles of gas from 2 to 1, indicating ∆Ssys < 0.

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Key Concepts

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

Entropy (∆S)

Entropy, denoted as ∆S, is a measure of the disorder or randomness in a system. A positive change in entropy (∆S > 0) indicates an increase in disorder, often associated with the production of gas from solids or liquids, or the mixing of different substances. Understanding how the states of matter change during a reaction is crucial for predicting the sign of ∆S.

States of Matter

The states of matter—solid, liquid, and gas—play a significant role in determining the entropy of a system. Gases have higher entropy than liquids, which in turn have higher entropy than solids. When analyzing reactions, recognizing the changes in states of the reactants and products helps in assessing whether the overall entropy increases or decreases.

Thermodynamic Reactions

Thermodynamic reactions involve the study of energy changes and the direction of chemical processes. In this context, reactions can be evaluated based on their enthalpy (∆H) and entropy (∆S) changes. Understanding how these factors influence the spontaneity and equilibrium of reactions is essential for determining which reactions result in a positive change in entropy.

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What is the change in entropy (∆S) when 1.32 g of propane (C3H8) at 0.100 atm pressure is compressed by a factor of five at a constant temperature at 20°C? Assume that propane behaves as an ideal gas. (a) ∆S = +13 J/K (b) ∆S = -13 J/K (c) ∆S = - 0.40 J/K (d) ∆S = + 0.40 J/K

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