Problem 12 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct IUPAC name of \(\mathrm{KAl}\left(\mathrm{SO}_{4}\right)_{2} .12 \mathrm{H}_{2} \mathrm{O}\) is (a) aluminium potassium sulphate-12-water (b) potassium aluminium(III) sulphate-12-water (c) potassium aluminium(III) sulphate hydrate (d) aluminium(III) potassium sulphate hydrate-12

Step-by-Step Solution

Answer

The correct IUPAC name is (b) potassium aluminium(III) sulphate-12-water.

Step 1: Analyze the Chemical Formula

The compound given is KAl(SO₄)₂ · 12H₂O, which is a type of ionic compound known as an alum. Essentially, it consists of a double salt with potassium (K), aluminium (Al), and sulfate (SO₄). Also, the 12 molecules of water indicate it is a hydrated compound.

Step 2: Assign Oxidation States and Identify the Cations

Potassium (K) has an oxidation state of +1. Aluminium is here in the form of Al³⁺ indicating an oxidation state of +3. Thus, the name must reflect the oxidation state of the aluminium which is +3.

Step 3: Understand Sulphate Group Contribution

Each sulphate group (SO₄²⁻) contributes a -2 charge. The compound has two sulphate ions in the formula balancing the charge of 2 Al³⁺ and 1 K⁺ ions, confirming the composition suits Al in the +3 state.

Step 4: Recognize the Role of Water

The '· 12H₂O' part of the formula indicates there are 12 water molecules of hydration, which needs to be reflected in the name. This is a key factor as it specifies the hydration state of the compound.

Step 5: Determine the Correct IUPAC Name

Considering all components: the main metal cation (potassium), the secondary metal with oxidation state (aluminium(III)), the anion (sulphate), and the water of crystallization (12-water), the correct IUPAC name is developed.

Key Concepts

Oxidation States

Understanding oxidation states is key to determining the proper name of compounds. An oxidation state indicates the degree of oxidation (loss of electrons) of an element in a compound. Each element can have multiple oxidation states, representing different ion charges in chemical reactions.
In KAl(SO₄)₂ · 12H₂O, we find potassium (K) and aluminium (Al) exhibiting distinct oxidation states. Potassium is a metal that typically has a +1 oxidation state, meaning it loses one electron to form a positively charged ion (K⁺).
Aluminium in this compound is in the +3 oxidation state, represented as Al³⁺. This means aluminium gives up three electrons, which is indicated in its name as aluminium(III). Recognizing these states helps us in understanding how elements combine and form stable compounds.

Hydration in Compounds

Hydrated compounds incorporate water into their crystalline structure. The formula KAl(SO₄)₂ · 12H₂O tells us that 12 water molecules are associated with each formula unit of the compound.
This word 'hydrate' refers specifically to the presence of these water molecules. The water in the compound is not just trapped but integrated structurally. Often, these water molecules are crucial for the stability or properties of the compound, affecting characteristics like solubility.
When naming such compounds, we represent this feature in its name. For example, in the compound discussed, this is denoted by phrases like '12-water' or 'dodecahydrate' in the IUPAC nomenclature.

Ionic Compounds

Ionic compounds consist of cations and anions held together by ionic bonds. These ions are formed when metals lose electrons (to form cations) and non-metals gain electrons (to form anions).
In the case of KAl(SO₄)₂ · 12H₂O, we see potassium (K⁺) and aluminium (Al³⁺) as the cations. The sulphate ion (SO₄²⁻) acts as the stable anion balancing the overall charge of the compound.
This type of compound generally forms a solid crystal lattice, making them distinct in terms of physical properties. These features, including charge balance, are reflected in how the compound is named and understood through IUPAC nomenclature.

Double Salts

Double salts are ionic compounds comprising two different metal cations. They differ from simple salts due to this complexity in their structure.
The compound KAl(SO₄)₂ · 12H₂O is a double salt because it contains two types of metal cations, potassium (K) and aluminium (Al).
Double salts like this are interesting because they exhibit properties that are distinct and sometimes enhanced when compared to their constituent simple salts. Their behavior often changes when dissolved in water, where they dissociate into their individual ions, exhibiting unique chemical properties distinct from single salts.