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-)
i. electronegativity Cl > Se
ii. more O atoms in the material means stronger acid. That makes [SO3]^2- a stronger acid making [NO2]^- a weaker acid (stronger base).
i) To understand the observation that HClO4 is a stronger oxyacid than H2SeO4, we need to consider the molecular structure and the electronegativity of the central atom in each acid.
HClO4 and H2SeO4 are both oxyacids, meaning they contain oxygen and hydrogen bonded to another element. In both cases, the oxygen atoms are bonded to the central atoms, which are chlorine (Cl) in HClO4 and selenium (Se) in H2SeO4.
The strength of an acid is primarily determined by its ability to donate a proton (H+ ion) in solution. In oxyacids, this proton is usually attached to an oxygen atom.
The electronegativity of an element plays a crucial role in determining the acidity of oxyacids. Electronegativity refers to an atom's attraction for electrons in a chemical bond. The higher the electronegativity of an atom, the stronger it pulls electrons towards itself.
In this case, chlorine (Cl) has a higher electronegativity than selenium (Se). As a result, the oxygen atoms bonded to chlorine in HClO4 are more polar, meaning they have a higher partial negative charge. This partial negative charge makes it easier for the oxygen atom to release a proton, as it experiences stronger electrostatic repulsion from the chlorine atom.
On the other hand, the oxygen atoms bonded to selenium in H2SeO4 are less polar because of the lower electronegativity of selenium. Consequently, the oxygen atoms in H2SeO4 are less likely to donate a proton, making H2SeO4 a weaker acid compared to HClO4.
ii) Moving to the observation that nitrite (NO2-) is a stronger base than sulphite (SO32-), we need to consider the molecular structure and the availability of electrons for the acceptance of a proton.
In both nitrite and sulphite, the central atom is a non-metal (nitrogen in NO2- and sulfur in SO32-). Both ions have lone pairs of electrons available for accepting a proton, making them potential bases.
However, the difference in their strengths as bases is related to the electronegativity and resonance stabilization of the central atom.
Nitrogen (N) is more electronegative than sulfur (S), meaning it has a greater affinity for electrons. This results in nitrite (NO2-) having a more electron-dense central atom compared to sulphite (SO32-).
Additionally, nitrite undergoes resonance stabilization due to the presence of a double bond between nitrogen and one of the oxygen atoms. This resonance stabilization delocalizes the negative charge, making it more stable and increasing its basicity.
In contrast, sulphite lacks such resonance stabilization, which leads to a less stable negative charge distribution.
Thus, the combination of nitrogen's higher electronegativity and resonance stabilization in nitrite enhances its ability to accept a proton, making it a stronger base compared to sulphite.