Question

The formation of nitric oxide (N2(g) + O2(g) Imported Asset 2NO(g)) has a Kc value of 4.7 × 10-31. A classmate claims that this reaction must really make a lot of nitric oxide since the exponent is such a large value. Explain to the student why her reasoning is correct or flawed.

Answer 1

Answer is: her reasoning is flawed, because Kc is very small, so the concentration of nitric(II) oxide is also very small.

Balanced chemical reaction: N₂(g) + O₂(g) ⇄ 2NO(g).
The equilibrium constant (Kc) is a ratio of the concentration of the products (in this reaction nitrogen(II) oxide) to the concentration of the reactants (in this reaction nitrogen and oxygen):
Kc = [NO]² / [N₂] · [O₂].
Kc = 4.7·10⁻³¹.
If we take equilibrium concentration of oxygen and nitrogen to be 1 M:
[N₂] = [O₂] = 1 M.
[NO] = √[N₂] · [O₂] · Kc.
[NO] = 6.855·10⁻¹⁶ M; equilibrium concentration of nitric oxide.