Which of the following compounds is the strongest Bronsted acid? CH2BrCH2CO2H CH2FCH2CO2H b. Which of the following compounds is the strongest Bronsted acid? CH2ClCO2H CH2ICO2H c. What is the conjugate base of H3S+? S2- HS- H4S2+ H3S+ H2S None of these choices is correct. d. What is the conjugate acid of H2S? H2S H4S2+ HS- H3S+ S2- None of these choices is correct.
The Correct Answer and Explanation is:
Correct Answers:
a. CH₂FCH₂CO₂H
b. CH₂ClCO₂H
c. H₂S
d. H₃S⁺
Explanation
Part a:
Between CH₂BrCH₂CO₂H and CH₂FCH₂CO₂H, the stronger Brønsted acid is CH₂FCH₂CO₂H. This is due to the inductive effect of fluorine, which is more electronegative than bromine. Electronegative atoms withdraw electron density through sigma bonds, stabilizing the negative charge on the conjugate base (the carboxylate anion). Since fluorine withdraws more electron density than bromine, it stabilizes the conjugate base better, increasing the acidity of the acid.
Part b:
Comparing CH₂ClCO₂H and CH₂ICOOH, the stronger Brønsted acid is CH₂ClCO₂H. Chlorine is more electronegative than iodine. As a result, it exerts a stronger electron-withdrawing effect via induction, which stabilizes the conjugate base more effectively. Greater stability of the conjugate base correlates with a stronger acid.
Part c:
The conjugate base of an acid is formed by removing one proton (H⁺). The species H₃S⁺ is a protonated form of hydrogen sulfide. Removing a proton from H₃S⁺ gives H₂S. So, H₂S is the correct conjugate base of H₃S⁺.
Part d:
The conjugate acid of a base is formed by adding one proton (H⁺). The molecule H₂S, when it gains a proton, forms H₃S⁺. Thus, H₃S⁺ is the conjugate acid of H₂S.
In acid-base chemistry, the strength of Brønsted acids is often predicted using factors such as electronegativity, inductive effects, resonance stabilization, and atom size. Electronegativity and inductive effects are especially important in carboxylic acids, as electron-withdrawing groups stabilize the negative charge of the conjugate base, enhancing acid strength. Understanding conjugate pairs also helps in determining acid-base reactions and buffer behavior.
