Question

nitrogen monoxide gas spontaneously decomposes into dinitrogen oxide and nitrogen dioxide gases. what is the standard entropy change for the decomposition of 3.0 mol of nitrogen monoxide

Answer 1

Values ​​tabulated standard molar entropy can be used to calculate the entropy of a chemical reaction, similar to the use of enthalpies of formation to determine the enthalpy change, seen in a previous section.
Unlike what happens in the use of entropy standard molar, when we calculate the entropy of a reaction if we put the Greek letter delta, Δ, ie, ΔS, because we speak of entropy from an initial state to a final state (product of reagents):
ΔSro = Sproductos - Sreactivos
For a generic reaction:
aA -> cC + dD
Total entropy of products: c · Smo (C) + d · Blessed (D)
Total entropy of the reactants: a · Smo (A)
(We multiply the entropies of each species by the stoichiometric coefficient because as molars are for 1 mole of compound, so it should consider the total moles present).
Thus, the entropy of the reaction will be:
ΔSro = total entropy of products - total entropy of the reactants
ΔSro = c · Smo (C) + d · Blessed (D) - [a · Smo (A)]

ΔSro = (2*(240.5+216.2)- [3*210.6]

=913.4-631.8=281.6 J/K