JEE Main 2020 · 6 September · morning · Single correct · +4 / −1

For the reaction
Fe₂N(s) + 3/2H₂(g) ⇌ 2Fe(s) + NH₃(g)

  1. (a) K C = Kₚ(RT) 1/2
  2. (b) K C = Kₚ(RT) -1/2
  3. (c) K C = Kₚ(RT)
  4. (d) K C = Kₚ(RT) 3/2
Answer and solution

Answer (a) K C = Kₚ(RT) 1/2

Solution

Chemistry ma’am · 3 steps

Leave the solids out, count only gas moles, and Δ n_g = 1 - 3/2 tells you everything.

  1. Only gases count. Solids like Fe₂N and Fe have constant concentration, so they do not appear in K_c or Kₚ. Count moles of gas only.
  2. Find Δn. Gaseous product: 1 mol NH₃. Gaseous reactant: 3/2 mol H₂. So Δ n_g = 1 - 3/2 = -1/2.

    Kₚ = K_c(RT)^(Δ n_g) = K_c(RT)^(-1/2)

  3. So. Multiply both sides by (RT)^(1/2) to get K_c = Kₚ(RT)^(1/2), which is option (a).

The idea to keep. Always start from Kₚ = K_c(RT)^(Δ n_g), with Δ n_g = gas moles of products minus gas moles of reactants. Solids and liquids never enter the count.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Chemical Equilibrium

Asked about 90 times in JEE Main since 2020. This question is from 2020.