Science, Tech, Math › Science Entropy of Reaction Example Problem How to Calculate the Molar Entropy Change of a Reaction Share Flipboard Email Print Entropy is a measure of disorder in a system. gremlin / Getty Images Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Todd Helmenstine Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. our editorial process Todd Helmenstine Updated December 05, 2018 This example problem demonstrates how to find the entropy of reaction from standard molar entropy data on the reactants and products. Entropy is calculated as a change in the level of entropy before and after a chemical reaction. Essentially, it reflects whether the amount of disorder or randomness in the system increased or decreased as a consequence of the reaction. Standard Molar Entropy Change Problem What is the standard molar entropy change of the following reaction? 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(g)Given:S°NH3 = 193 J/K·molS°O2 = 205 J/K·molS°NO = 211 J/K·molS°H2O = 189 J/K·mol (Note, in this type of problem you'll either be given the molar entropy values of the reactants and products or you'll need to look them up in a table.)SolutionThe change in the standard molar entropy of a reaction can be found by the difference between the sum of the molar entropies of the products and the sum of the molar entropies of the reactants.ΔS°reaction = ΣnpS°products - ΣnrS°reactantsΔS°reaction = (4 S°NO + 6 S°H2O) - (4 S°NH3 + 5 S°O2)ΔS°reaction = (4(211 J/K·K) + 6(189 J/K·mol)) - (4(193 J/K·mol) + 5(205 J/K·mol))ΔS°reaction = (844 J/K·K + 1134 J/K·mol) - (772 J/K·mol + 1025 J/K·mol)ΔS°reaction = 1978 J/K·mol - 1797 J/K·mol)ΔS°reaction = 181 J/K·molWe can check our work using the techniques introduced in this example problem. The reaction involves all gasses and the number of moles of products is greater than the number of moles of reactants so the expected change in entropy should be positive. Answer The standard molar entropy change of the reaction is 181 J/K·mol.