U7.8 · Representations of EquilibriumPractice · 平衡的粒子图表示
Lessons covered:U7.1 Introduction to EquilibriumU7.4 Calculating the Equilibrium ConstantU7.8 Representations of Equilibrium
- 1Multiple choice
At some temperature, K = 5.0 for 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). The diagram shows the molecules in a 1.0 L container at some moment; each molecule represents 0.10 mol. What is the state of the system?
OSOOSOOSOOSOOOOOOSOOOSOOOSOOOSOOExplanationThere are 4 SO₂, 2 O₂, and 4 SO₃, so the concentrations are 0.40, 0.20, and 0.40 M. Q = 0.40² ÷ (0.40² × 0.20) = 5.0 = K, so it is at equilibrium: both reactions still run, at equal rates.
- 2Numeric
CO(g) + Cl₂(g) ⇌ COCl₂(g) takes place in a 1.0 L container. The diagram shows the molecules at some moment; each represents 0.10 mol. What is the reaction quotient Q?
COCOClClClClClClClClCOClClCOClClCOClClCOClClCOClClCOClClExplanationThere are 2 CO, 4 Cl₂, and 6 COCl₂, so the concentrations are 0.20, 0.40, and 0.60 M. Q = [COCl₂] ÷ ([CO][Cl₂]) = 0.60 ÷ (0.20 × 0.40) = 7.5. Using the raw molecule counts gives 0.75: when the gas moles differ on the two sides, convert to concentrations first.
- 3Multiple choice
In a container, A₂ and B₂ react to form AB₃. The table gives the molecule counts at two times (they no longer change after t₂). What is the equation for this equilibrium?
Time A₂ B₂ AB₃ t₁ 6 9 0 t₂ 4 3 4 ExplanationA₂ falls by 2, B₂ by 6, and AB₃ rises by 4. The changes are in the ratio 2 : 6 : 4 = 1 : 3 : 2, which is the ratio of coefficients; in lowest terms, A₂ + 3B₂ ⇌ 2AB₃ (which also balances the atoms). Use the changes, not the counts at one moment.
- 4Numeric
The diagram shows the molecules in a 1.0 L container once N₂(g) + 3H₂(g) ⇌ 2NH₃(g) has reached equilibrium at some temperature; each molecule represents 0.10 mol. What is K at this temperature?
NNNNHHHHHHHNHHHNHHExplanationThere are 2 N₂, 3 H₂, and 2 NH₃, so the concentrations are 0.20, 0.30, and 0.20 M. K = [NH₃]² ÷ ([N₂][H₂]³) = 0.20² ÷ (0.20 × 0.30³) = 0.04 ÷ 0.0054 = 7.4.
Review: U7.8 Representations of EquilibriumU7.4 Calculating the Equilibrium Constant
- 5Multiple choice
On a concentration–time graph for A(g) ⇌ B(g), the curve for A falls and the curve for B rises. The two curves cross at 20 s, and from 50 s on both are flat. From when is the system at equilibrium?
ExplanationEquilibrium is marked by concentrations that stop changing, where the curves go flat, so it starts at 50 s. At 20 s the two concentrations are equal but still changing, which isn’t equilibrium; at equilibrium the concentrations are usually not equal.
Review: U7.8 Representations of EquilibriumU7.1 Introduction to Equilibrium
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