Multiple choice

The Kp for the formation of ammonia at 250 0C is 1.42x10-5 atm. Find the Kc for this reaction.

  1. 0.057

  2. 0.0057

  3. 0.57

  4. 0.66
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A Correct answer
Explanation

The equation for the above reaction is

N2  + 3H2 -----------------> 2NH3   (Δ n = 2 - (1+3)  = -2)

Kp = 1.42x10-5

T = 273 + 500

     = 773 K

R = 0.0821 L.atm/mol.K

By substituting the values in the following expression, we have

Kc = Kp/ (RT) Δ n

1.42x10-5/ (0.0821 x 773)-2 = 0.057

AI explanation

For N2(g) + 3H2(g) ⇌ 2NH3(g), Δn = 2 − 4 = −2, and Kp = Kc(RT)^Δn, so Kc = Kp·(RT)^2. Using R = 0.0821 L·atm·mol⁻¹·K⁻¹ and T = 500 °C = 773 K, RT = 63.5 and (RT)^2 ≈ 4027, giving Kc = 1.42×10⁻⁵ × 4027 ≈ 0.057. Because Δn is negative, Kc is much larger than Kp.

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