Elemental boron can be formed by the reaction of boron trichloride with hydrogen. 2BCl₃(g) + 3H₂(g) → 2B(s) + 6HCl(g) A) -18.2 J/K • mol B) 18.2 J/K • mol C) 289.5 J/K • mol D) 370.4 J/K • mol E) 579.0 J/K • mol

Question

Quizplus · Accepted Answer

To determine the entropy change of the reaction, we need to calculate the difference in entropy of the products and reactants:

ΔS = S_products - S_reactants

Products: 
B(s) - standard entropy = 5.69 J/K•mol
HCl(g) - standard entropy = 186.9 J/K•mol

Reactants: 
BCl3(g) - standard entropy = 241.8 J/K•mol
H2(g) - standard entropy = 130.7 J/K•mol

ΔS = (2 mol B•5.69 J/K•mol + 6 mol HCl•186.9 J/K•mol) - (2 mol BCl3•241.8 J/K•mol + 3 mol H2•130.7 J/K•mol)
ΔS = -288.3 J/K•mol ≈ -289.5 J/K•mol (rounded to nearest tenth)

Since the reaction produces solid boron, there is a decrease in entropy, resulting in a negative ΔS value. Therefore, we choose the answer that has a negative value closest to our calculated value, which is option C.

Elemental Boron Can Be Formed by the Reaction of Boron