Question

Explain the following observations: (a) HCl is a stronger acid than H2S; (b) H3PO4 is a stronger acid than H3AsO4; (c) HBrO3 is a stronger acid than HBrO2; (d) H2C2O4 is a stronger acid than HC2O4–; (e) benzoic acid (C6H5COOH) is a stronger acid than phenol (C6H5OH).

Answer 1

(a) HCl is a stronger acid than H2S because the H-Cl bond is more polar than the H-S bond due to the electronegativity difference between Cl and H being larger than S and H. This greater polarity of the H-Cl bond results in a weaker bond and a stronger acid.

(b) H3PO4 is a stronger acid than H3AsO4 because the P-O bond is stronger than the As-O bond. As a result, it is more difficult to break the P-O bond and donate the proton, making H3PO4 a stronger acid.

(c) HBrO3 is a stronger acid than HBrO2 because the electronegativity of the Br atom in HBrO3 is greater than that in HBrO2, causing the O-H bond in HBrO3 to be more polar and easier to break, resulting in a stronger acid.

(d) H2C2O4 is a stronger acid than HC2O4– because the H atom in H2C2O4 is attached to a carbon atom, which is less electronegative than the oxygen atom to which the H atom is attached in HC2O4–. This results in a weaker bond between H and C in H2C2O4, making it easier to break and donate the proton.

(e) Benzoic acid (C6H5COOH) is a stronger acid than phenol (C6H5OH) because the carboxyl group (-COOH) in benzoic acid is more acidic than the hydroxyl group (-OH) in phenol. This is because the carboxyl group contains two electronegative oxygen atoms that can stabilize the negative charge on the conjugate base by resonance, whereas the hydroxyl group only contains one electronegative oxygen atom. Therefore, benzoic acid is more acidic than phenol.