Identify the conjugate acid–base pairs in the following reactions. (a) HNO3 + N2H4 ? NO3- + N2H5+ (b) NH3 + N2H5+ ? NH4+ + N2H4 (c) H2PO4- + CO3^2- ? HPO4^2- + HCO3- (d) HIO3 + HC2O4- ? IO3- + H2C2O4
The Correct Answer and Explanation is:
(a)
Reaction:
HNO₃ + N₂H₄ → NO₃⁻ + N₂H₅⁺
- HNO₃ loses a proton (H⁺) and becomes NO₃⁻.
- N₂H₄ gains a proton and becomes N₂H₅⁺.
Conjugate acid-base pairs:
- HNO₃ / NO₃⁻ (acid/base)
- N₂H₄ / N₂H₅⁺ (base/acid)
(b)
Reaction:
NH₃ + N₂H₅⁺ → NH₄⁺ + N₂H₄
- NH₃ gains a proton to become NH₄⁺.
- N₂H₅⁺ loses a proton to become N₂H₄.
Conjugate acid-base pairs:
- NH₃ / NH₄⁺ (base/acid)
- N₂H₅⁺ / N₂H₄ (acid/base)
(c)
Reaction:
H₂PO₄⁻ + CO₃²⁻ → HPO₄²⁻ + HCO₃⁻
- H₂PO₄⁻ loses a proton and becomes HPO₄²⁻.
- CO₃²⁻ gains a proton and becomes HCO₃⁻.
Conjugate acid-base pairs:
- H₂PO₄⁻ / HPO₄²⁻ (acid/base)
- CO₃²⁻ / HCO₃⁻ (base/acid)
(d)
Reaction:
HIO₃ + HC₂O₄⁻ → IO₃⁻ + H₂C₂O₄
- HIO₃ loses a proton and becomes IO₃⁻.
- HC₂O₄⁻ gains a proton and becomes H₂C₂O₄.
Conjugate acid-base pairs:
- HIO₃ / IO₃⁻ (acid/base)
- HC₂O₄⁻ / H₂C₂O₄ (base/acid)
In acid-base chemistry, conjugate acid-base pairs are two species that differ by one proton (H⁺). The acid donates a proton, and the base accepts that proton. After donation, the acid becomes its conjugate base, and after acceptance, the base becomes its conjugate acid.
For each reaction, identify which species loses a proton (acid → conjugate base) and which gains a proton (base → conjugate acid). This change defines the pairs.
- In reaction (a), nitric acid (HNO₃) acts as an acid by donating a proton to hydrazine (N₂H₄), which acts as a base by accepting the proton. After donation, HNO₃ becomes NO₃⁻ (its conjugate base), and N₂H₄ becomes N₂H₅⁺ (its conjugate acid).
- In reaction (b), ammonia (NH₃) acts as a base accepting a proton from N₂H₅⁺, which is the acid here. Their conjugates are NH₄⁺ (acid form of ammonia) and N₂H₄ (base form of hydrazine).
- In reaction (c), dihydrogen phosphate (H₂PO₄⁻) donates a proton, making it an acid, and carbonate ion (CO₃²⁻) accepts a proton, acting as a base. Their conjugate pairs are H₂PO₄⁻ / HPO₄²⁻ and CO₃²⁻ / HCO₃⁻.
- In reaction (d), iodic acid (HIO₃) donates a proton, becoming IO₃⁻, and hydrogen oxalate (HC₂O₄⁻) accepts a proton, forming oxalic acid (H₂C₂O₄). These species form the conjugate acid-base pairs.
Recognizing these pairs helps understand proton transfer reactions, which is central in acid-base chemistry and many biological and chemical processes.
