反应性 2.3 · 远变？化学变化的限度

What would explain this observation?

• A reversible reaction 可逆反应 can continue in a closed vessel while measured concentrations stay constant. Constant composition does not mean particles have stopped reacting.
• Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.

Build the model

• Dynamic equilibrium occurs in a closed system when forward and reverse rates are equal. Reactant and product concentrations are constant, but they need not be equal.
• equilibrium: A state with equal forward and reverse reaction rates; reversible reaction: A reaction that can proceed in both directions.

Choose evidence that can test it

• A concentration or pressure change disturbs the balance. The system responds toward a new equilibrium. Temperature changes can also change the equilibrium constant; a catalyst changes how quickly equilibrium is reached.
• State the balanced equation and whether the forward reaction is exothermic before predicting a temperature effect. Count gas coefficients when considering pressure; pressure has no composition effect when gaseous amounts are equal on both sides.

Work from known quantities

• State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
• Known: in A ⇌ B, equilibrium concentrations are [A] = 0.20 and [B] = 0.60 in the same concentration unit. For this stated expression, K = [B]/[A]. K = 0.60/0.20 = 3.0. Equal rates do not imply K = 1.

Example:

For A ⇌ B, calculate [B]/[A] when [B]=0.8 and [A]=0.2. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.

Check the conclusion and its limits

• Do not use a catalyst to claim a larger equilibrium yield. For heterogeneous equilibria, pure solids are omitted from the usual equilibrium expression.
• Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.

Warn:

A catalyst changes the equilibrium constant at a fixed temperature. This claim is false: Do not use a catalyst to claim a larger equilibrium yield. For heterogeneous equilibria, pure solids are omitted from the usual equilibrium expression.

Key:

Equilibrium and changing conditions: A concentration or pressure change disturbs the balance. The system responds toward a new equilibrium. Temperature changes can also change the equilibrium constant; a catalyst changes how quickly equilibrium is reached.