Introduction to Acid-Base Reactions - AP Chemistry

1. In the gas-phase reaction $\mathrm{HBr(g) + NH_3(g) \rightarrow NH_4^+(s) + Br^-(s)}$, which statement correctly describes the direction of proton transfer in the forward reaction?
   Answer: A proton is transferred from $\mathrm{HBr}$ to $\mathrm{NH_3}$ This question tests understanding of proton transfer in gas-phase acid-base reactions. In the reaction HBr(g) + NH₃(g) → NH₄⁺(s) + Br⁻(s), HBr donates a proton (H⁺) to NH₃, resulting in the formation of NH₄⁺ and Br⁻. The proton transfer occurs from the acid (HBr) to the base (NH₃), even in the gas phase where no water is present. This demonstrates that Brønsted-Lowry acid-base reactions don't require water or OH⁻ ions. A common misconception is thinking no proton transfer occurs without OH⁻ present (choice D), confusing Brønsted-Lowry with Arrhenius definitions. Remember that Brønsted-Lowry acid-base reactions involve proton transfer between any suitable donor and acceptor, regardless of the phase or presence of water.
2. In the reaction $\mathrm{C_2H_5NH_2(aq) + H_2O(l) \rightarrow C_2H_5NH_3^+(aq) + OH^-(aq)}$, which species is the conjugate acid of $\mathrm{C_2H_5NH_2}$?
   Answer: $\mathrm{C_2H_5NH_3^+}$ This question tests the ability to identify the conjugate acid of a base. In the reaction C₂H₅NH₂(aq) + H₂O(l) → C₂H₅NH₃⁺(aq) + OH⁻(aq), the conjugate acid forms by proton acceptance. C₂H₅NH₂ accepts H⁺, becoming C₂H₅NH₃⁺, its conjugate acid. Brønsted-Lowry conjugates involve proton gain. A tempting distractor is H₃O⁺, but it is not present, from assuming hydronium always forms. Add H⁺ to the base to identify its conjugate acid in products.
3. Consider the Brønsted–Lowry reaction: $\mathrm{HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq)}$. Which choice correctly identifies the Brønsted–Lowry acid and base in the forward reaction?
   Answer: Acid: $\mathrm{HCl}$; Base: $\mathrm{H_2O}$ This question tests the ability to identify Brønsted-Lowry acids and bases in a reaction. In the reaction HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq), HCl donates a proton (H⁺) to water, making HCl the Brønsted-Lowry acid. Water accepts this proton to form H₃O⁺, making H₂O the Brønsted-Lowry base. The products are H₃O⁺ (the conjugate acid of water) and Cl⁻ (the conjugate base of HCl). A common misconception is thinking that H₃O⁺ and Cl⁻ are the acid and base (choice B), but these are the products, not the reactants that undergo proton transfer. To identify Brønsted-Lowry acids and bases, always look for which species donates a proton (acid) and which accepts it (base) in the forward reaction.
4. In the reaction $\mathrm{HF(aq) + NH_3(aq) \rightarrow F^-(aq) + NH_4^+(aq)}$, which species is the Brønsted–Lowry acid?
   Answer: $\mathrm{HF}$ This question tests the ability to identify the Brønsted-Lowry acid in a non-aqueous context. In the reaction HF(aq) + NH₃(aq) → F⁻(aq) + NH₄⁺(aq), the acid donates a proton. HF donates H⁺ to NH₃, forming F⁻ and NH₄⁺, so HF is the acid. Brønsted-Lowry acids are proton donors regardless of solvent. A tempting distractor is NH₄⁺, but it is incorrect as it is the conjugate acid, not the initial donor, from the misconception of identifying products as acids. Identify the acid by finding the reactant that loses a proton in the reaction.
5. For the reaction $\mathrm{HNO_2(aq) + CN^-(aq) \rightarrow NO_2^-(aq) + HCN(aq)}$, which statement about proton transfer is correct?
   Answer: $\mathrm{HNO_2}$ donates $\mathrm{H^+}$ to $\mathrm{CN^-}$ This question tests understanding of proton transfer in acid-base reactions. In the reaction HNO₂(aq) + CN⁻(aq) → NO₂⁻(aq) + HCN(aq), we must identify the correct proton transfer. HNO₂ loses a proton to become NO₂⁻, while CN⁻ gains that proton to become HCN, so HNO₂ donates H⁺ to CN⁻. Students often choose option E thinking negative ions must be acids, but charge doesn't determine acid-base behavior—proton transfer does, and negative species like CN⁻ often act as bases by accepting protons. To determine proton transfer direction, compare reactants to products to see which species lost H⁺ (acid) and which gained H⁺ (base).
6. For the reaction $\mathrm{HSO_4^-(aq) + H_2O(l) \rightarrow SO_4^{2-}(aq) + H_3O^+(aq)}$, which species is the Brønsted–Lowry acid in the forward direction?
   Answer: $\mathrm{HSO_4^-}$ This question tests identification of Brønsted-Lowry acids, which are proton donors. In the reaction HSO₄⁻(aq) + H₂O(l) → SO₄²⁻(aq) + H₃O⁺(aq), we must identify which species donates a proton in the forward direction. HSO₄⁻ loses a proton to become SO₄²⁻, while H₂O gains that proton to become H₃O⁺, making HSO₄⁻ the acid. Students often incorrectly choose H₃O⁺ (option B) because it's a common acid, but H₃O⁺ is a product here, not a reactant that can donate protons in the forward reaction. To identify acids, look for the species that loses H⁺ going from reactants to products.
7. In the reaction $\mathrm{NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)}$, which pair is a conjugate acid–base pair?
   Answer: $\mathrm{NH_3}$ and $\mathrm{NH_4^+}$ This question tests the ability to identify a conjugate acid-base pair in an equilibrium reaction. In the reaction NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq), a conjugate acid-base pair consists of species differing by one proton. NH₃ accepts a proton to become NH₄⁺, making NH₃ the base and NH₄⁺ its conjugate acid. The Brønsted-Lowry principle defines conjugates as pairs where the acid has donated a proton to form the base. A tempting distractor is NH₃ and OH⁻, but it is incorrect because they are not conjugates of each other, stemming from the misconception of pairing non-related bases instead of focusing on proton difference. When identifying conjugate pairs, always check which species differ by exactly one H⁺ ion.
8. For the reaction $\mathrm{HBr(aq) + CO_3^{2-}(aq) \rightarrow HCO_3^-(aq) + Br^-(aq)}$, which species is the Brønsted–Lowry base?
   Answer: $\mathrm{CO_3^{2-}}$ This question tests the ability to identify the Brønsted-Lowry base. In the reaction HBr(aq) + CO₃²⁻(aq) → HCO₃⁻(aq) + Br⁻(aq), the base accepts a proton. CO₃²⁻ accepts H⁺ from HBr, becoming HCO₃⁻, so it is the base. Brønsted-Lowry bases gain protons. A tempting distractor is HCO₃⁻, but it is the conjugate acid, not the base, due to product confusion. Look for the reactant that gains H⁺ to form a product with one more proton.
9. In the reaction $\mathrm{HCO_3^-(aq) + H_2O(l) \rightarrow H_2CO_3(aq) + OH^-(aq)}$, water acts as a Brønsted–Lowry ____ in the forward reaction.
   Answer: acid (proton donor) This question tests understanding of water's amphoteric behavior in acid-base reactions. In the reaction $\mathrm{HCO_3^{-}(aq) + H_2O(l) \rightarrow H_2CO_3(aq) + OH^{-}(aq)}$, we must determine water's role. Looking at the products, HCO₃⁻ gains a proton to become H₂CO₃, and OH⁻ is produced, meaning water must have donated the proton to HCO₃⁻ while becoming OH⁻, making water the acid (proton donor). Students often choose option B thinking water accepts protons because it commonly acts as a base, but in this reaction water donates a proton. To determine if water acts as an acid or base, check whether it becomes $\mathrm{H_3O^{+}}$ (acted as base) or $\mathrm{OH^{-}}$ (acted as acid).
10. In the reaction $\mathrm{H_3O^+(aq) + NO_2^-(aq) \rightarrow HNO_2(aq) + H_2O(l)}$, which species is the conjugate base of $\mathrm{H_3O^+}$?
    Answer: $\mathrm{H_2O}$ This question tests the ability to identify the conjugate base of H₃O⁺. In the reaction H₃O⁺(aq) + NO₂⁻(aq) → HNO₂(aq) + H₂O(l), the conjugate base forms after donation. H₃O⁺ donates H⁺, leaving H₂O as conjugate base. Conjugates differ by H⁺. A tempting distractor is NO₂⁻, but it is the base, not conjugate of H₃O⁺, from mixing pairs. Remove H⁺ from the acid to find its conjugate base.
11. For the reaction $\mathrm{HNO_3(aq) + HS^-(aq) \rightarrow NO_3^-(aq) + H_2S(aq)}$, which species is the conjugate acid of $\mathrm{HS^-}$?
    Answer: $\mathrm{H_2S}$ This question tests the ability to identify the conjugate acid of HS⁻. In the reaction HNO₃(aq) + HS⁻(aq) → NO₃⁻(aq) + H₂S(aq), the conjugate acid forms by acceptance. HS⁻ accepts H⁺, becoming H₂S, its conjugate acid. Conjugates involve proton gain. A tempting distractor is HNO₃, but it is the acid, not conjugate of HS⁻, from role swap. Add H⁺ to the base to spot its conjugate acid.
12. Consider the reaction $\mathrm{CH_3COOH(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + CH_3COO^-(aq)}$. Which species is the conjugate base of $\mathrm{CH_3COOH}$?
    Answer: $\mathrm{CH_3COO^-}$ This question tests the identification of conjugate bases in equilibrium reactions. In the reaction CH₃COOH(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CH₃COO⁻(aq), acetic acid (CH₃COOH) donates a proton to water. When CH₃COOH loses a proton, it becomes CH₃COO⁻ (acetate ion), which is the conjugate base of acetic acid. The conjugate base is always what remains after an acid donates its proton. A common misconception is thinking H₂O is the conjugate base (choice C) because it appears in the reaction, but H₂O is actually acting as a base (proton acceptor) in this reaction, not as the conjugate base of CH₃COOH. To find the conjugate base of any acid, remove one H⁺ from its formula.
13. A student mixes aqueous solutions and observes the reaction: $\mathrm{HCl(aq) + NH_3(aq) \rightarrow NH_4^+(aq) + Cl^-(aq)}$ Which choice lists the two conjugate acid–base pairs present?
    Answer: $\mathrm{HCl/Cl^-}$ and $\mathrm{NH_3/NH_4^+}$ This question tests the identification of conjugate acid-base pairs in a complete acid-base reaction. Conjugate acid-base pairs differ by exactly one proton (H⁺). In the reaction HCl(aq) + NH₃(aq) → NH₄⁺(aq) + Cl⁻(aq), HCl donates a proton to become Cl⁻, making HCl/Cl⁻ one conjugate pair. NH₃ accepts a proton to become NH₄⁺, making NH₃/NH₄⁺ the other conjugate pair. Students who choose option B incorrectly pair species from different sides that don't differ by one proton, such as HCl/NH₄⁺, showing confusion about what constitutes a conjugate pair. To identify all conjugate pairs, match each acid with what it becomes after losing H⁺, and each base with what it becomes after gaining H⁺.
14. In the reaction $\mathrm{NH_3(aq) + H_2O(l) \rightarrow NH_4^+(aq) + OH^-(aq)}$, which option correctly identifies the conjugate acid–base pairs?
    Answer: $\mathrm{NH_3/NH_4^+}$ and $\mathrm{H_2O/OH^-}$ This question tests your understanding of conjugate acid-base pairs in Brønsted-Lowry theory. In the reaction NH₃(aq) + H₂O(l) → NH₄⁺(aq) + OH⁻(aq), conjugate pairs differ by exactly one proton. NH₃ accepts a proton to become NH₄⁺, making NH₃/NH₄⁺ a conjugate base/acid pair. H₂O donates a proton to become OH⁻, making H₂O/OH⁻ a conjugate acid/base pair. A common error is pairing reactants with reactants or products with products (option D pairs NH₃/H₂O and NH₄⁺/OH⁻), but conjugate pairs must be on opposite sides of the equation. To identify conjugate pairs, find species that differ by one H⁺ with one on the reactant side and one on the product side.
15. In the reaction $\mathrm{H_2O(l) + H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)}$, which statement correctly identifies the Brønsted–Lowry roles of water?
    Answer: One $\mathrm{H_2O}$ acts as an acid and the other as a base This question tests the understanding of amphoteric behavior in water's autoionization. In the reaction H₂O(l) + H₂O(l) ⇌ H₃O⁺(aq) + OH⁻(aq), water acts both as acid and base. One H₂O donates H⁺ to another, forming H₃O⁺ and OH⁻, showing amphoterism. Brønsted-Lowry allows substances to be both depending on context. A tempting distractor is both H₂O act only as acids, but it is incorrect because one accepts the proton, from the misconception that water only donates in ionization. Recognize amphoteric species by checking if they can donate or accept protons in different reactions.
16. In the reaction $\mathrm{NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)}$, which species is the conjugate acid of $\mathrm{NH_3}$?
    Answer: $\mathrm{NH_4^+}$ This question tests understanding of conjugate acid-base pairs in Brønsted-Lowry theory. In the reaction NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq), ammonia (NH₃) acts as a base by accepting a proton from water. When NH₃ gains a proton (H⁺), it becomes NH₄⁺, which is the conjugate acid of NH₃. The conjugate acid is always formed when a base accepts a proton, differing by exactly one H⁺. A common misconception is thinking OH⁻ is the conjugate acid (choice A), but OH⁻ is actually the conjugate base of water in this reaction. To find a conjugate acid, add one proton to the base; to find a conjugate base, remove one proton from the acid.
17. Consider the reaction: $\mathrm{NH_3(aq) + H_2O(l) \rightarrow NH_4^+(aq) + OH^-(aq)}$ Which species is the conjugate acid of $\mathrm{NH_3}$ in this reaction?
    Answer: $\mathrm{NH_4^+}$ This question tests understanding of conjugate acid-base pairs in Brønsted-Lowry theory. A conjugate acid is formed when a base accepts a proton (H⁺). In the reaction NH₃(aq) + H₂O(l) → NH₄⁺(aq) + OH⁻(aq), NH₃ acts as a base by accepting a proton from H₂O. When NH₃ gains this proton, it becomes NH₄⁺, which is therefore the conjugate acid of NH₃. Students who choose option D might confuse the original base (NH₃) with its conjugate acid, not recognizing that conjugate acids and bases differ by exactly one proton. To identify conjugate pairs, remember that a base plus one H⁺ equals its conjugate acid.
18. Consider the reaction: $\mathrm{HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq)}$. Using Brønsted–Lowry definitions, which species acts as the base in the forward reaction?
    Answer: $\mathrm{H_2O}$ This question tests your ability to identify Brønsted-Lowry bases, which are proton (H⁺) acceptors. In the reaction HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq), we need to identify which species accepts a proton. HCl donates a proton to become Cl⁻, while H₂O accepts that proton to become H₃O⁺, making water the base. A common misconception is choosing H₃O⁺ (option A) because it contains the proton, but H₃O⁺ is actually the product formed after water accepts the proton, not the base itself. To identify bases in acid-base reactions, look for the species that gains H⁺ in the forward reaction.
19. In the reaction $\mathrm{H_2SO_4(aq) + H_2O(l) \rightarrow H_3O^+(aq) + HSO_4^-(aq)}$, which pair correctly identifies the conjugate acid and conjugate base formed?
    Answer: Conjugate acid: $\mathrm{H_3O^+}$; conjugate base: $\mathrm{HSO_4^-}$ This question tests the ability to identify conjugate acid and base formed. In the reaction H₂SO₄(aq) + H₂O(l) → H₃O⁺(aq) + HSO₄⁻(aq), H₃O⁺ is conjugate acid of H₂O, and HSO₄⁻ is conjugate base of H₂SO₄. H₂SO₄ donates H⁺ to H₂O, forming these conjugates. Brønsted-Lowry identifies pairs from transfer. A tempting distractor is conjugate acid: HSO₄⁻; conjugate base: H₃O⁺, but it reverses them, from misconception of swapping roles. Label conjugates by noting the base's protonated form as conjugate acid and acid's deprotonated form as conjugate base.
20. Consider the reaction: $\mathrm{HNO_2(aq) + OH^-(aq) \rightarrow NO_2^-(aq) + H_2O(l)}$ Which choice correctly identifies the conjugate base of the acid in this reaction?
    Answer: $\mathrm{NO_2^-}$ This question tests understanding of conjugate bases in Brønsted-Lowry acid-base reactions. A conjugate base is what remains after an acid donates a proton. In the reaction HNO₂(aq) + OH⁻(aq) → NO₂⁻(aq) + H₂O(l), HNO₂ acts as the acid by donating a proton to OH⁻. When HNO₂ loses this proton, it becomes NO₂⁻, which is therefore the conjugate base of HNO₂. Students who choose option D (HNO₂) confuse the acid itself with its conjugate base, not understanding that the conjugate base is the deprotonated form of the acid. To identify a conjugate base, find the acid in the reaction and determine what it becomes after losing one H⁺.

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