U7.3 · Reaction Quotient and Equilibrium ConstantPractice · 反应商 Q 与平衡常数 K
Lessons covered:U7.3 Reaction Quotient and Equilibrium Constant
- 1Multiple choice
What is the expression for for CH₄(g) + 2H₂S(g) ⇌ CS₂(g) + 4H₂(g)?
ExplanationProducts go on top and reactants on the bottom, each concentration raised to its coefficient: . All four species are gases, so all of them appear.
- 2Multiple choice
Nickel is purified by reacting it with carbon monoxide to form gaseous nickel tetracarbonyl: Ni(s) + 4CO(g) ⇌ Ni(CO)₄(g). What is the expression for ?
ExplanationNi is a pure solid, so it is left out. Ni(CO)₄ goes on top and CO on the bottom, raised to its coefficient 4: Kc = [Ni(CO)₄] ÷ [CO]⁴.
- 3Numeric
For 2NO(g) + O₂(g) ⇌ 2NO₂(g), at some moment [NO] = 0.020 M, [O₂] = 0.010 M, and [NO₂] = 0.040 M. What is the reaction quotient Q?
ExplanationQ has the same form as K, using the concentrations at this moment: Q = [NO₂]² ÷ ([NO]²[O₂]) = 0.040² ÷ (0.020² × 0.010) = 1.6 × 10−3 ÷ 4.0 × 10−6 = 400.
- 4Multiple choice
At some temperature, K = 0.045 for SO₂Cl₂(g) ⇌ SO₂(g) + Cl₂(g). At some moment [SO₂Cl₂] = 0.40 M, [SO₂] = 0.20 M, and [Cl₂] = 0.15 M. What happens next?
ExplanationQ = [SO₂][Cl₂] ÷ [SO₂Cl₂] = 0.20 × 0.15 ÷ 0.40 = 0.075 > 0.045: too much product, so the reaction runs in reverse. SO₂ and Cl₂ decrease and SO₂Cl₂ increases until Q = K.
- 5Multiple choice
At 25 °C, liquid water and water vapor are at equilibrium in a sealed container: H₂O(l) ⇌ H₂O(g). What is Kp for this equilibrium?
ExplanationThe pure liquid is left out, leaving only the partial pressure of the product, water vapor: Kp = PH₂O. At a given temperature this is a constant, the vapor pressure of water (about 0.0313 atm at 25 °C). So as long as some liquid remains, the vapor pressure holds at that value.
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