Phosgene, COCl2(g), decomposes as shown:

COCl2(g) ⇌ CO(g) + Cl2(g)

Kc for the reaction is 8.3 10–4 M at 360 °C. Select the correct statement when 5.00 mol of COCl2 decomposes at 360 °C in a 10.0 L flask.

Question

Phosgene, COCl2(g), decomposes as shown:

COCl2(g) ⇌ CO(g) + Cl2(g)

Kc for the reaction is 8.3  10–4 M at 360 °C. Select the correct statement when 5.00 mol of COCl2 decomposes at 360 °C in a 10.0 L flask.

BlackTom AI · Accepted Answer

We’re given the decomposition: COCl2(g) ⇌ CO(g) + Cl2(g) with Kc = 8.3 × 10^−4 at 360 °C. Starting with 5.00 mol of COCl2 in a 10.0 L flask gives an initial concentration of COCl2 = 5.00 mol / 10.0 L = 0.50 M, and initial concentrations of CO and Cl2 are 0 M.

Let x be the amount (in moles per liter) of COCl2 that decomposes at equilibrium. Then at equilibrium:
- [COCl2] = 0.50 − x
- [CO] = x
- [Cl2] = x
Kc = [CO][Cl2]/[COCl2] = x^2/(0.50 − x) = 8.3 × 10^−4

To gauge the position of equilibrium, estimate x by assuming x is small relative to 0.50 (since Kc is small). If x ≈ 0.020, then x^2 ≈ 4 × 10^−4 and x^2/(0.50 − x) ≈ 4 × 10^−4 / 0.50 ≈ 8 × 10^−4, which is close to the given Kc. This shows that only a small fraction of COCl2 decomposes.

Interpretation: A small Kc means the reaction lies largely to the left at equilibrium, i.e., most COCl2 remains as reactant, and only a small amount decomposes to form CO and Cl2. Nevertheless, the forward reaction must occur to some extent to establish equilibrium; starting from pure COCl2, the reaction proceeds in the forward direction until the concentrations satisfy the Kc expression. Therefore, the forward reaction does predominate in the approach to equilibrium, in the sense that it is the direction that produces products until the system reaches equilibrium, even though the overall extent is small.

Explanation of possible misinterpretations:
- If one thought the reverse reaction would dominate because Kc is small, that would misinterpret the meaning of Kc. A small Kc means the equilibrium mixture favors reactants, not that the reverse rate is greater than the forward rate at all times.
- The statement does not claim that products are abundant at equilibrium; it only states the forward reaction predominates until equilibrium is established, which aligns with the initial feed of reactant and the small extent of decomposition dictated by the small Kc.

In summary, given the small Kc and initial pure COCl2, only a small fraction decomposes to form CO and Cl2, and the forward reaction governs the approach to equilibrium, consistent with the statement provided.

Chem 1C General Chemis... Self Assessment Quiz 8

Phosgene, COCl2(g), decomposes as shown: COCl2(g) ⇌ CO(g) + Cl2(g) Kc for the reaction is 8.3 10–4 M at 360 °C. Select the correct statement when 5.00 mol of COCl2 decomposes at 360 °C in a 10.0 L flask.

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