U7.9 · Introduction to Le Châtelier's PrinciplePractice · 勒夏特列原理入门
Lessons covered:U7.9 Introduction to Le Châtelier's Principle
- 1Multiple choice
Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) is at equilibrium, and the solution is red (FeSCN²⁺ is red). A little solid KSCN is added. How does the color change, and why?
ExplanationKSCN dissolves and raises [SCN⁻], so the equilibrium shifts forward to use up SCN⁻, making more FeSCN²⁺ and a deeper red. “Adding a solid has no effect” applies only to a pure solid that is part of the equilibrium; KSCN dissolves and supplies ions in solution. K⁺ is a spectator ion, and K changes only with temperature.
- 2Multiple choice
2SO₃(g) ⇌ 2SO₂(g) + O₂(g) is at equilibrium in a container with a movable piston. At constant temperature and total pressure, argon is added, so the volume increases. Which way does the equilibrium shift?
ExplanationAdding argon at constant total pressure makes the volume grow, so the partial pressures (concentrations) of SO₃, SO₂, and O₂ all drop, just like diluting or decompressing the mixture. The equilibrium shifts toward more gas molecules: 3 mol on the right versus 2 on the left, so it shifts forward. Only argon added at constant volume leaves the partial pressures unchanged and causes no shift.
- 3Multiple choice
A cobalt chloride solution contains the equilibrium [CoCl₄]²⁻(aq) (blue) + 6H₂O(l) ⇌ [Co(H₂O)₆]²⁺(aq) (pink) + 4Cl⁻(aq). In hot water the pink solution turns blue; back in ice water it turns pink again. What is the sign of ΔH for the forward reaction?
ExplanationRaising the temperature shifts an equilibrium in the endothermic direction. Turning blue on heating means a shift in reverse, so the reverse reaction is endothermic and the forward reaction is exothermic: ΔH < 0. Cooling shifts it forward, the exothermic way, back to pink. Temperature changes K, not just the rate.
- 4Select all
In a sealed container, CaCO₃(s) ⇌ CaO(s) + CO₂(g) is at equilibrium, and the forward reaction is endothermic. Which changes increase the number of moles of CO₂ at equilibrium? Select all that apply.
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ExplanationKp = PCO₂. The forward reaction is endothermic, so heating increases K and both the pressure and the moles of CO₂ at equilibrium. Increasing the volume drops the CO₂ pressure below Kp, so more CaCO₃ decomposes until the pressure is back at Kp; in the larger volume that means more moles of CO₂. Pure solids aren’t in the expression, so adding them doesn’t shift anything, and neither does a catalyst.
- 5Multiple choice
A dichromate solution contains the equilibrium 2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l) (CrO₄²⁻ is yellow, Cr₂O₇²⁻ is orange). What happens when NaOH solution is added to the orange solution?
ExplanationOH⁻ isn’t in the equation, but it neutralizes H⁺ to make water, lowering [H⁺]. The equilibrium shifts in reverse, using up Cr₂O₇²⁻ to replace H⁺, so CrO₄²⁻ builds up and the solution turns yellow. Adding acid turns it back to orange. Sodium salts are soluble, so nothing precipitates.
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