Elemental boron can be formed by reaction of boron trichloride with hydrogen. BCl₃(g) + 1.5H₂(g) → B(s) + 3HCl(g)
Substance: BCl₃(g) H₂(g) B(s) HCl(g)
S°(J/K·mol) : ? 130.6 5.87 186.8
If ΔS° = 80.3 J/K for the reaction above, what is S° for BCl₃(g) ?

Question

Quizplus · Accepted Answer

First, we can use the equation ΔS°rxn = ΣS°products - ΣS°reactants to find the entropy of elemental boron:
ΔS°rxn = S°(B) + 3S°(HCl) - S°(BCl3) - 1.5S°(H2)
Substituting in the known values, we get:
80.3 J/K = 5.87 J/K·mol + 3(186.8 J/K·mol) - S°(BCl3) - 1.5(130.6 J/K·mol)
Solving for S°(BCl3), we get:
S°(BCl3) = -290.1 J/K·mol
Note that the negative sign indicates that the entropy of BCl3 is lower than that of the products, indicating that the reactant molecules are more ordered than the product molecules.

Elemental Boron Can Be Formed by Reaction of Boron Trichloride