Use data from Appendix IIB to calculate ΔS°_rxn for each of the reactions. In each case, try to rationalize the sign of ΔS°_rxn . a. 3 NO₂(g) + H₂O(l) → 2 HNO₃(aq) + NO(g)

Key Concepts

Entropy (ΔS)

Entropy is a measure of the disorder or randomness in a system. In chemical reactions, changes in entropy (ΔS) indicate how the distribution of energy and matter changes as reactants transform into products. A positive ΔS suggests an increase in disorder, while a negative ΔS indicates a decrease in disorder.

Standard Entropy Values

Standard entropy values (S°) are tabulated values that represent the absolute entropy of a substance at standard conditions (1 atm and 25°C). These values are essential for calculating the change in entropy for a reaction (ΔS°rxn) by using the formula ΔS°rxn = ΣS°products - ΣS°reactants, where the sum is taken over all products and reactants.

Rationalizing ΔS°rxn Sign

Rationalizing the sign of ΔS°rxn involves analyzing the physical states and the number of moles of reactants and products. Generally, reactions that produce more gas molecules than they consume tend to have a positive ΔS, while those that produce fewer gas molecules or involve a transition from gas to liquid or solid typically have a negative ΔS. Understanding these trends helps predict the entropy change in a reaction.