Which of the following changes has a decrease in entropy? . A.hg N2O4 (g) 2NO2 (g) B.3O2 (g) + 2KCl (s) 2KClO3 (s) C.C2H5OH (l) + 3O2 (g) 2CO2 (g) + 3H2O (g) D.H2O (l) H2O (g)

The answer is B. 3O2 (g) + 2KCl (s) ---> 2KClO3 (s) This is a cut in entropy since you go from 5 mol of reactant to 2 mol of product. Likewise, one of the reactants is in gas stage, implying that entropy is actually high equated to the product, which is in solid phase.