Molar Entropy Change of a Reaction - Example Problem

Molar Entropy Change of a Reaction Problem 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:SÂ °NH3 193 J/KÂ ·molSÂ °O2 205 J/KÂ ·molSÂ °NO 211 J/KÂ ·molSÂ °H2O 189 J/KÂ ·mol (Note, in this type of problem youll either be given the molar entropy values of the reactants and products or youll 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.ΔSÂ °reaction ÃŽ £npSÂ °products - ÃŽ £nrSÂ °reactantsΔSÂ °reaction (4 SÂ °NO 6 SÂ °H2O) - (4 SÂ °NH3 5 SÂ °O2)ΔSÂ °reaction (4(211 J/KÂ ·K) 6(189 J/KÂ ·mol)) - (4(193 J/KÂ ·mol) 5(205 J/KÂ ·mol))ΔSÂ °reaction (844 J/KÂ ·K 1134 J/KÂ ·mol) - (772 J/KÂ ·mol 1025 J/KÂ ·mol)ΔSÂ °reaction 1978 J/KÂ ·mol - 1797 J/KÂ ·mol)ΔSÂ °reaction 181 J/KÂ ·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 pos itive. Answer The standard molar entropy change of the reaction is 181 J/KÂ ·mol.