Question

\(\mathrm{Al}^{3+}\) is not a Bronsted acid, but \(\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}\) is Explain.

Short answer

While the \(\mathrm{Al}^{3+}\) ion cannot donate a proton (\(H^+\)), and so is not a Bronsted acid, the \(\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}\) complex ion can donate a proton from one of its attached water molecules, and thus is a Bronsted acid.

Step-by-step solution

Understand Bronsted Acid Definition

A Bronsted acid is defined as a chemical substance that can donate a proton (\(H^+\)) to another substance. In its reaction, it gives off a proton which is taken up by a Bronsted base.

Analyze the nature of \(\mathrm{Al}^{3+}\)

\(\mathrm{Al}^{3+}\) is simply the cation of aluminium. It does not have any protons that it can donate and therefore cannot act as a Bronsted acid.

Explain why \(\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}\) is a Bronsted acid

\(\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}\) is a metal ion complex where a central aluminium ion is bonded to six water molecules. The aluminium ion provides the necessary charge to polarize the water molecules. The oxygen atoms of the water molecules are attracted to the positive charge on aluminium. This results in the protons (\(H^+\)) on the water molecules being available for donation. Thus, when a base is introduced, \(\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}\) can donate a proton from one of its water molecules, acting as a Bronsted acid.

Key concepts

Proton Donation

In the realm of chemistry, a Bronsted acid is a substance capable of donating a proton, or hydrogen ion (\(H^+\)), to another species. This donation means that the substance can act as an acid, releasing the proton to be captured by a base.
When \(\mathrm{Al}^{3+}\) is alone, it doesn't contain any hydrogen atoms, so it can't donate a proton. However, when \(\mathrm{Al}^{3+}\) bonds with water, it forms the complex \(\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}\).
This complex enables proton donation because the water molecules within the metal ion complex can release a proton. By donating a proton, \(\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}\) functions as a Bronsted acid.

Metal Ion Complex

A metal ion complex involves a central metal ion surrounded by molecules or ions, which are known as ligands. These ligands are bonded to the metal ion, creating a more stable structure.
In our example, \(\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}\) is a metal ion complex. Here, the central aluminium ion is surrounded by six water molecules, forming a stable and coordinated structure.
This complex formation is essential as it allows the aluminium ion to impart its positive charge over a wider area, influencing the behavior of the ligands connected to it, such as the ability to donate a proton.

Water Polarization

Polarization in chemistry refers to the distortion of the electron cloud of a molecule, influenced by electric charge.
In the complex \(\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+}\), water molecules experience polarization because of the strong positive charge of the aluminium ion \(\mathrm{Al}^{3+}\).

• The positive charge of the aluminium draws the negatively charged oxygen in water closer, distorting the electron cloud.
• This makes the hydrogen atoms in the water molecules more positive and ready to be released as protons.

Thus, polarization is key to enabling proton donation, transforming the water-bound aluminium complex into a Bronsted acid.