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Equilibrium and Acids Flashcards

This topic covers chemical equilibrium, the equilibrium constant, Le Chatelier's principle, and acid-base chemistry. It includes pH calculations, strong and weak acid/base behavior, buffer systems, and titration curves essential for the AP Chemistry exam.

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What is chemical equilibrium?

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The state where the forward and reverse reaction rates are equal, so the concentrations of reactants and products remain constant over time.

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What does the equilibrium constant (Keq) expression represent?

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The ratio of product concentrations to reactant concentrations, each raised to the power of their stoichiometric coefficients, at equilibrium.

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What does a large Keq value indicate?

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The equilibrium lies far to the right, meaning products are heavily favored at equilibrium.

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State Le Chatelier's principle.

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When a system at equilibrium is disturbed, it shifts in the direction that partially counteracts the change and restores a new equilibrium.

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How does adding more reactant affect equilibrium?

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The system shifts toward products (right) to consume the added reactant and re-establish equilibrium.

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How does a catalyst affect equilibrium?

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A catalyst speeds up both the forward and reverse reactions equally. It does not change the equilibrium position or the value of Keq.

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What is the relationship between Kp and Kc?

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Kp = Kc(RT)^delta_n, where delta_n is the change in moles of gas (moles gaseous products minus moles gaseous reactants).

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What is the reaction quotient (Q) used for?

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Q is calculated the same way as Keq but at non-equilibrium conditions. If Q < K, the reaction shifts right; if Q > K, it shifts left.

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What defines a Bronsted-Lowry acid?

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A proton (H+) donor. Its conjugate base is the species formed after it donates a proton.

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What is the pH of a solution?

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pH = -log[H+]. A pH below 7 is acidic, 7 is neutral, and above 7 is basic at 25 degrees C.

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What is the relationship between Ka and Kb for a conjugate acid-base pair?

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Ka x Kb = Kw = 1.0 x 10^-14 at 25 degrees C. A stronger acid has a weaker conjugate base.

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How do you calculate the pH of a strong acid solution?

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Strong acids dissociate completely, so [H+] equals the initial acid concentration. pH = -log[H+] directly.

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What is a buffer solution?

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A solution containing a weak acid and its conjugate base (or weak base and its conjugate acid) that resists changes in pH upon addition of small amounts of acid or base.

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What is the Henderson-Hasselbalch equation?

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pH = pKa + log([A-]/[HA]). It is used to calculate the pH of buffer solutions.

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What happens at the equivalence point of a strong acid-strong base titration?

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All the acid has been neutralized by the base. The solution is neutral (pH = 7) because the conjugate species do not hydrolyze.

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Why is the equivalence point pH above 7 in a weak acid-strong base titration?

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At the equivalence point, only the conjugate base of the weak acid remains in solution. It hydrolyzes water, producing OH- and raising the pH above 7.

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What is the half-equivalence point in a weak acid titration?

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The point where half the acid has been neutralized, so [HA] = [A-] and pH = pKa. This is used to determine the Ka of the weak acid.

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Study Tips for Equilibrium and Acids

1

Practice ICE table problems (Initial, Change, Equilibrium) until they are second nature, as they appear frequently on the AP exam for both equilibrium and acid-base calculations.

2

Memorize the six strong acids (HCl, HBr, HI, HNO3, H2SO4, HClO4) and the strong bases (Group 1 and heavy Group 2 hydroxides) to quickly identify complete dissociation problems.

3

Always check whether Q vs. K analysis or Le Chatelier reasoning gives you the same prediction for equilibrium shifts; this builds deeper understanding of both approaches.

4

Sketch titration curves for strong/strong, weak/strong, and strong/weak combinations, labeling the buffer region, half-equivalence point, and equivalence point on each.

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