Problem 101 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement is not correct regarding the properties of ionic compounds? (a) ionic compounds have high melting and boiling points (b) their reaction velocity in aqueous medium is very high. (c) ionic compounds in their molten and aqueous solutions do not conduct electricity. (d) they are highly soluble in polar solvents.

Step-by-Step Solution

Answer

Statement (c) is incorrect.

Step 1: Understand Ionic Compounds

Ionic compounds are formed from the electrostatic attraction between positively and negatively charged ions. Each ion gains or loses electrons to achieve a full valence shell, typically resulting in a solid structure at room temperature with distinct properties.

Step 2: Analyze High Melting and Boiling Points

Ionic compounds generally have high melting and boiling points due to the strong electrostatic forces between the ions (option a). This is a correct property of ionic compounds.

Step 3: Reaction Velocity in Aqueous Medium

Ionic compounds dissociate into ions when dissolved in water, which can increase reaction rates because the ions are free to move and react (option b). This is also a correct statement about ionic compounds.

Step 4: Conductivity of Ionic Compounds

Ionic compounds in their solid state do not conduct electricity; however, in their molten form or when dissolved in an aqueous solution, they can conduct electricity due to the mobility of the ions (option c). Thus, this statement is incorrect.

Step 5: Solubility in Polar Solvents

Typically, ionic compounds are soluble in polar solvents such as water because of the solvent's ability to stabilize the separated ions (option d). This is another correct property of ionic compounds.

Key Concepts

High Melting and Boiling Points

Ionic compounds are known for their exceptionally high melting and boiling points. This feature is primarily due to the strong electrostatic forces, known as ionic bonds, which hold the oppositely charged ions together in a lattice structure. These bonds require a significant amount of energy to break.

For example, common table salt (sodium chloride) is an ionic compound. To convert it from a solid to a liquid, enough energy must be supplied to overcome the strong attractions between sodium ions (\( ext{Na}^+ \)) and chloride ions (\( ext{Cl}^- \)). This results in a relatively high melting point of around 801°C.

• Strong bonds mean higher energy is needed.
• Lattice structure contributes to the strength.
• Ionic substances remain stable at room temperatures due to these bonds.

Conductivity of Ionic Compounds

In their solid state, ionic compounds do not conduct electricity. This is because the ions are locked in place within the lattice structure, unable to move freely. However, once an ionic compound is melted or dissolved in a liquid (like water), the ions become mobile, allowing them to carry an electrical current.

This conductivity change is crucial for applications that rely on ionic conductance. For instance, when salt is dissolved in water, it becomes an excellent conductor of electricity.

• Solid state: No ion movement, no conductivity.
• Molten or dissolved: Ions are free, conducts electricity.
• Useful in electrochemistry and various technologies.

Solubility in Polar Solvents

Ionic compounds are generally soluble in polar solvents, such as water. The solubility phenomenon occurs because polar solvents have molecules with a slight electrical charge that can interact with the charged ions in the ionic compounds, effectively breaking them apart.

For example, when table salt (sodium chloride) is added to water, the slightly positive hydrogen atoms in water molecules attract the negatively charged chloride ions, while the slightly negative oxygen atoms attract the positively charged sodium ions. This interaction pulls the compounds apart, resulting in dissolution.

• Polar molecules attract and stabilize ions.
• Water is a common polar solvent for ionic compounds.
• Solubility aids in chemical reactions and daily processes.