Question

Based on formulas alone, classify each of the following oxoacids as strong or weak. (a) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (b) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (c) HClO (d) \(\mathrm{HClO}_{4}\) (e) \(\mathrm{HNO}_{3}\) (f) \(\mathrm{H}_{2} \mathrm{CO}_{3}\) (g) \(\mathrm{HNO}_{2}\)

Short answer

Strong: H₂SO₄, HClO₄, HNO₃. Weak: H₃PO₄, HClO, H₂CO₃, HNO₂.

Step-by-step solution

Understand Oxoacids

Oxoacids are acids that contain hydrogen, oxygen, and another element (the central atom). Their acidity is determined by the ability to dissociate in water to release hydrogen ions, H⁺.

General Rule for Oxoacids Acidity

For oxoacids with the same central atom, the acidity increases with the number of oxygen atoms. More oxygen atoms generally mean stronger acids due to the increasing ability to stabilize the conjugate base through resonance.

Classify H₂SO₄

Sulfuric acid ( H₂SO₄) is known to be a strong acid because it fully dissociates in aqueous solution.

Classify H₃PO₄

Phosphoric acid ( H₃PO₄) is a weak acid as it does not fully dissociate in water.

Classify HClO

Hypochlorous acid (HClO) is a weak acid due to limited dissociation and fewer oxygen atoms in its structure.

Classify HClO₄

Perchloric acid (HClO₄) is a strong acid because it has many oxygen atoms that stabilize the ion, making it capable of full dissociation.

Classify HNO₃

Nitric acid ( HNO₃) is a strong acid as it fully dissociates in solution, having three oxygen atoms providing stability.

Classify H₂CO₃

Carbonic acid ( H₂CO₃) is a weak acid as it incompletely dissociates in water.

Classify HNO₂

Nitrous acid ( HNO₂) is a weak acid with only two oxygen atoms, leading to limited dissociation.

Key concepts

Acidity of oxoacids

Oxoacids are a fascinating group of acids characterized by having a central atom bonded to at least one oxygen atom, and they also contain hydrogen. A crucial aspect of oxoacids is understanding how their structure affects their acidity.
The ability of an oxoacid to donate a proton (H⁺) is primarily what determines its acidity. This power of donation is greatly influenced by the presence of oxygen atoms.

• More oxygen atoms tend to increase acidity. This is because extra oxygen atoms can spread out or delocalize charge, stabilizing the conjugate base after dissociation.
• In a comparison of oxoacids with the same central atom, the one with more oxygen atoms is generally the stronger acid.
• The strength of an oxoacid can also vary due to the electronegativity of its central atom, which can further influence the stability of the conjugate base.

For students tackling acid strength predictions, noting these structural characteristics can provide helpful insights.

Strong acids

Strong acids, in the context of oxoacids, are those that completely dissociate in water. This means when dissolved, they release all their hydrogen ions into the solution. Here’s how it often works:

• Sulfuric acid (\(\mathrm{H}_{2} \mathrm{SO}_{4}\)) is a classic example of a strong oxoacid. It releases its protons fully, adjusting the pH of solutions significantly.
• Perchloric acid (\(\mathrm{HClO}_{4}\)) also exhibits strong acidic behavior because of its high oxygen content, allowing for full dissociation.
• Nitric acid (\(\mathrm{HNO}_{3}\)) is similarly known for being strong due to its effective dissociation and stabilization of its conjugate base.

The key takeaway is that strong acids are efficient at altering solution properties due to their total dissociation.

Weak acids

Weak acids, unlike strong acids, only partially dissociate in water. This means they do not release all of their hydrogen ions, resulting in a mixed equilibrium of dissociated and undissociated forms. When dealing with oxoacids:

• Phosphoric acid (\(\mathrm{H}_{3} \mathrm{PO}_{4}\)) exhibits weak acidic characteristics by retaining a significant amount of its protons even in solution.
• Carbonic acid (\(\mathrm{H}_{2} \mathrm{CO}_{3}\)) also demonstrates weak acid behavior due to its reversible reaction in water.
• Hypochlorous acid (\(\mathrm{HClO}\)) and Nitrous acid (\(\mathrm{HNO}_{2}\)) are other examples where limited dissociation is observed, leading to less pronounced acidity.

For weak acids, the partial dissociation stems from a balance between releasing and retaining protons.

Chemical dissociation

The process of chemical dissociation involves the breakdown of a compound into smaller constituents, such as ions. In the case of acids in aqueous solutions, dissociation is the separation of hydrogen ions from the rest of the molecule.
Understanding the dissociation helps in predicting the acid’s behavior and how it impacts its environment.

• For a strong acid, complete dissociation means releasing all of its available H⁺ ions, driving the solution to be acidic.
• In weak acids, dissociation is partial, meaning not all the hydrogen ions are separated from the molecule. This results in a weaker acidic effect.
• Acid dissociation is influenced by the acid's structure, notably the ability of oxygen atoms to stabilize the negative charge that forms in conjugate bases.

Grasping the nuances of chemical dissociation is essential for understanding why certain acids affect their environments more than others.