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

Question

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

Quizplus · Accepted Answer

The standard entropy change for the reaction is given as ΔS° = 80.3 J/K. We can use the equation ΔS° = ΣS°(products) - ΣS°(reactants) to find the standard molar entropy of boron trichloride.

ΔS° = S°(B) + 3S°(HCl) - S°(BCl3) - 1.5S°(H2)

We know that the standard molar entropies of elements in their most stable form at 1 atm and 298 K are zero, so S°(B) = 0. The standard molar entropy of hydrogen gas is 130.7 J/K·mol, and the standard molar entropy of hydrochloric acid gas is 186.9 J/K·mol. We can look up these values in a table of thermodynamic data.

ΔS° = 0 + 3(186.9 J/K·mol) - S°(BCl3) - 1.5(130.7 J/K·mol)
80.3 = 560.7 - S°(BCl3) - 196.1
S°(BCl3) = 290.1 J/K·mol

Therefore, the best choice is C, which gives the correct value for S°(BCl3).

Elemental Boron Can Be Formed by Reaction of Boron Trichloride