Question

Standard entropies of N_2(g), H_2(g), and NH_3(g) are a₁, a₂ and a₃ J K^-1 mol^-1 respectively. What is value of ΔS° for formation of NH_3(g)?

Options

• `"a"_3 - 1/2 "a"_1 + 3/2 "a"_2`

• `"a"_3 - (1/2 "a"_1 + 3/2 "a"_2)`

• `"a"_2 - (1/2 "a"_1 + 3/2 "a"_2)`

• `"a"_1 - (1/2 "a"_3 + 3/2 "a"_2)`

Answer 1

`"a"_3 - (1/2 "a"_1 + 3/2 "a"_2)`

Explanation:

\[\ce{N_2(g) + 3H_{2(g)} -> 2NH_{3(g)}}\]

`therefore 1/2 "N"_(2("g")) + 3/2 "H"_(2("g")) -> "NH"_(3("g"))`

Standard entropies `1/2 "a"_1       3/2"a"_2     "a"_3`

ΔS° = `sum Δ"S"_"product"^circ - sum Delta "S"_"reactant"^circ`

`= "a"_3 - (1/2 "a"_1 + 3/2 "a"_3)`