Question

Nickel and copper ores are generally sulfides, whereas aluminum ore is an oxide. Explain this observation on the basis of the hard/soft acid/base concept.

Short answer

Since Al has a higher charge density, it is a stronger acid as compared to Ni and Cu.

Since O has a smaller size than the S atom, it will be a harder base. So it will create a stronger bond with the stronger acid that is Al.

Step-by-step solution

Hard/soft acid base concept

The terms hard and soft are employed for acids and bases based on the charge density. For acids, the greater the oxidation state and the smaller is the size of the metal the harder is the acid and vice-versa. For bases, the higher the electron charge density and smaller the size, the harder is the base and vice-versa.

Explanation of nickel and copper being sulfides and aluminum ore being an oxide

The general oxidation state of Al is +3 whereas for Ni and Cu is +2. Since Al has a higher charge density, it is a stronger acid as compared to Ni and Cu. A hard acid forms strong bonds with a hard base. A soft acid makes strong bonds with a soft base. Since O has a smaller size than the S atom, it will be a harder base. So it will create a stronger bond with the stronger acid that is Al.

Hence, aluminum ore is an oxide whereas Ni and Cu ore is a sulfide.