Using your knowledge of the molecular basis for acid strength, explain the following observations:
i) HClO4 is a stronger oxyacid than H2SeO4
ii) Nitrite (NO2-) is a stronger base than sulphite (SO32-)
iii) Hydronium ion (H3O+) is a stronger acid than water (H2O)
(For the above, write acid dissociation reaction for each, and comment on the relative strengths of the conjugate bases formed in each case.)
iii. HA + H2O ==> H3O^+ + A^-
Weak acids placed in water solution provides H3O^.
The conjugate pair is H2O--H3O^+. Of that pair H3O^+ is the acid and H2O is the conjugate base.
HClO4 + H2O ==> H3O+ + ClO4^-
H2SeO4 + H2O ==> H3O^+ + HSeO4^- ==> We can compare only ka1 here. You know if the acid is stronger then the conjugate base is weaker.
To understand the molecular basis for acid strength, we need to consider the factors that influence the acidity of a compound. These factors include the polarity of the bond between the acidic atom and hydrogen, the stability of the resulting conjugate base, and the electronegativity of the acidic atom.
i) HClO4 (perchloric acid) is a stronger oxyacid than H2SeO4 (selenic acid)
Acid dissociation reaction:
HClO4 ⇌ H+ + ClO4-
H2SeO4 ⇌ 2H+ + SeO4^2-
In both reactions, the acidic hydrogen is easily ionized, forming the hydronium ion (H+). However, the stability of the conjugate bases is different. ClO4- is a weak base because perchlorate ion is highly electronegative, stabilizing the negative charge. On the other hand, SeO4^2- is a stronger base because the selenate ion is less electronegative, making it less stable than ClO4-. Therefore, HClO4 is a stronger acid than H2SeO4.
ii) Nitrite (NO2-) is a stronger base than sulphite (SO32-)
Acid dissociation reaction:
NO2- + H2O ⇌ HNO2 + OH-
SO32- + H2O ⇌ HSO3- + OH-
In both reactions, the acidic hydrogen is replaced by a hydroxide ion (OH-), indicating base strength. Nitrite ion (NO2-) is a stronger base compared to sulphite ion (SO32-) because the negative charge on NO2- is more stable due to resonance, whereas SO32- is less stable due to it having less resonance forms. Therefore, nitrite is a stronger base than sulphite.
iii) Hydronium ion (H3O+) is a stronger acid than water (H2O)
Acid dissociation reaction:
H3O+ + H2O ⇌ H2O + H3O2+
In this reaction, the acidic hydrogen of H3O+ is replaced by a water molecule, showing the reverse reaction. Both H3O+ and H2O produce H2O as the conjugate base. However, H3O+ is a stronger acid than water because it has a positive charge and is consequently more unstable. The positive charge on H3O+ results in a higher electron density, making it more likely to donate a proton (H+). Therefore, H3O+ is a stronger acid than water.
In summary, the relative strengths of acids and bases can be understood by considering the stability of the resulting conjugate bases and the electronegativity of the acidic atoms. The stability of the conjugate base depends on factors such as resonance and electronegativity, which influence the ability of the species to accommodate or stabilize the negative charge.