U7.5 · Magnitude of the Equilibrium ConstantPractice · K 的大小
Lessons covered:U7.5 Magnitude of the Equilibrium ConstantU7.7 Calculating Equilibrium Concentrations (ICE Tables)
- 1Multiple choice
At 25 °C, K for 2H₂(g) + O₂(g) ⇌ 2H₂O(g) is around 1080, yet a mixture of hydrogen and oxygen at room temperature shows no change for years. Which is the best explanation?
ExplanationK is a thermodynamic quantity: it says that at equilibrium the mixture is almost all water. Whether anything visibly happens depends on the rate. At room temperature the activation energy is too high and the reaction barely runs; a spark supplies the energy and it goes violently. The mixture is far from equilibrium (Q is far below K).
- 2Multiple choice
Four reactions of the form A(g) ⇌ B(g) are run at the same temperature, each starting with only 1.0 M A. Which one has the most B at equilibrium?
ExplanationK = [B]/[A], so the larger K, the larger the share of B at equilibrium. With K = 2.0 × 10³, B is about 99.95% of the mixture; with K = 1.0 it is half; with K = 5.0 × 10⁻³ it is only about 0.5%.
- 3Numeric
At some temperature, K = 1.0 × 10−5 for A(g) ⇌ B(g) + C(g). Initially there is only A, at 0.10 M. Estimate [B] at equilibrium.
× 10MExplanationK is small, so A barely decomposes and [A] ≈ 0.10. x² ÷ 0.10 = 1.0 × 10−5, so x = √(1.0 × 10−6) = 1.0 × 10−3 M. That is only 1.0% of the starting concentration, so the shortcut holds; the exact solution is 9.95 × 10−4.
Review: U7.5 Magnitude of the Equilibrium ConstantU7.7 Calculating Equilibrium Concentrations (ICE Tables)
- 4Multiple choice
A reaction has K = 1.2 at some temperature. What is the equilibrium mixture most likely to be like?
ExplanationK near 1 means reactants and products end up in similar amounts, so both are present in significant quantities. A very large K (above about 10³) favors products and a very small one (below about 10⁻³) favors reactants. K = 1.2 doesn’t mean exactly equal concentrations; the exact ratio also depends on the coefficients and starting amounts.
- 5Multiple choice
Of these weak acids, each at 0.10 M, which solution has the highest [H⁺]? (Ka: HClO 2.9 × 10−8, HNO₂ 4.6 × 10−4, HCN 4.9 × 10−10, HCOOH 1.8 × 10−4)
ExplanationAt the same concentration, the larger Ka, the more the acid ionizes and the more H⁺ it makes. Estimating [H⁺] ≈ √(Ka × 0.10): HClO 5.4 × 10−5, HNO₂ 6.8 × 10−3, HCN 7.0 × 10−6, HCOOH 4.2 × 10−3 M. HNO₂ has the largest Ka.
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