U7.1 · Introduction to EquilibriumPractice · 平衡入门
Lessons covered:U7.1 Introduction to EquilibriumU7.8 Representations of Equilibrium
- 1Multiple choice
In an unopened bottle of soda, dissolved CO₂ is at equilibrium with the CO₂ gas in the neck of the bottle: CO₂(g) ⇌ CO₂(aq). Which statement is correct?
ExplanationEquilibrium is dynamic: dissolving and escaping both continue at equal rates, so the amounts on each side stop changing. At equilibrium the concentrations are constant, not necessarily equal. Once the cap is off, CO₂ keeps escaping into the air, the system is no longer closed, and the equilibrium is lost: the soda goes flat.
Review: U7.1 Introduction to Equilibrium
- 2Numeric
At constant temperature, A and B are put into a sealed 1.00 L container, each at 0.300 M, and A(g) + B(g) ⇌ C(g) takes place. The table shows [C] over time. What is [A] at equilibrium?
t (s) [C] (M) 0 0.000 10 0.070 20 0.105 30 0.120 40 0.120 50 0.120 MExplanationFrom 30 s on, [C] stops changing, so the system is at equilibrium with [C] = 0.120 M. Each C formed uses one A, so [A] = 0.300 − 0.120 = 0.180 M. At equilibrium [A] is larger than [C]: the concentrations need not be equal, only constant.
Review: U7.1 Introduction to Equilibrium
- 3Multiple choice
Which observation best shows that a system has reached chemical equilibrium?
ExplanationColor depends on the concentration of the colored substance, so a constant color means constant concentrations, the sign of equilibrium. In an open beaker the gas escapes, and no more bubbles usually means a reactant ran out, not equilibrium. Concentrations need not be equal at equilibrium, and the forward rate is largest at the start, falling until it matches the reverse rate.
Review: U7.1 Introduction to Equilibrium
- 4Multiple choice
H₂(g) + I₂(g) ⇌ 2HI(g) is at equilibrium in a sealed container. A little deuterium gas (D₂, an isotope of hydrogen) is added, and some time later HD and DI molecules are found in the container. What does this show?
ExplanationD atoms ending up in HD and DI means molecules keep breaking apart and recombining: neither reaction has stopped, they just run at equal rates. That is dynamic equilibrium. Isotopes behave almost identically in reactions, so they are often used to label atoms and track where they go.
Review: U7.1 Introduction to Equilibrium
- 5Multiple choice
Heating limestone in a sealed container lets CaCO₃(s) ⇌ CaO(s) + CO₂(g) reach equilibrium, but heated in an open container the limestone can decompose completely. Why?
ExplanationEquilibrium needs both directions to be possible. In a sealed container CO₂ builds up, so the reverse reaction CaO + CO₂ → CaCO₃ speeds up until the rates match. In an open container the CO₂ escapes, there is almost no reverse reaction, and the CaCO₃ decomposes completely. A pure solid’s concentration is constant; it can’t be “larger”.
Review: U7.1 Introduction to Equilibrium
- 6Multiple choice
Only is placed in a sealed container, and occurs. The graph shows both concentrations against time. From about when is the system at equilibrium, and what is true from then on?
ExplanationEquilibrium is reached when the concentrations stop changing: both curves flatten after about 40 s, with NO₂ at 0.020 M and N₂O₄ at 0.040 M. The equilibrium is dynamic: the forward and reverse reactions keep going at equal rates. The curves cross at about 11 s, which only means the concentrations are equal at that instant; they are still changing, and equilibrium concentrations are usually not equal. The forward rate is greatest at the very start. Check: K = 0.04 ÷ 0.02² = 100.
Review: U7.1 Introduction to EquilibriumU7.8 Representations of Equilibrium
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