Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

Molten sodium chloride conducts electricity due to the presence of (a) free ions (b) free molecules (c) free electrons (d) atoms of sodium and chlorine

Step-by-Step Solution

Answer

Molten sodium chloride conducts electricity due to the presence of (a) free ions.

Step 1: Understanding the Question

The question asks us to identify why molten sodium chloride is able to conduct electricity. This implies knowing how the structure and composition of sodium chloride changes in its molten state.

Step 2: Concept of Ionic Compounds

In solid state, ionic compounds like sodium chloride have ions held in a rigid lattice. This means the ions cannot move freely, which is why solid sodium chloride does not conduct electricity.

Step 3: Changes in Molten State

When sodium chloride is heated until it melts, the rigid ionic lattice breaks down. The ions become free to move, which allows the substance to conduct electricity. In the molten state, sodium chloride dissociates into Na⁺ and Cl⁻ ions.

Step 4: Identifying the Conducting Particles

Since molten sodium chloride conducts electricity due to the movement of charged particles, we identify that the free-moving particles should be ions. Therefore, the presence of free ions (Na⁺ and Cl⁻) is what allows the molten compound to conduct electricity.

Key Concepts

Ionic Compounds

Ionic compounds are a type of chemical compound where ions are held together by ionic bonds. These bonds are formed by the attraction between positively charged ions and negatively charged ions. This electrostatic attraction is very strong and usually results in a solid with a high melting point. Sodium chloride, or common table salt, is a classic example of an ionic compound. In its solid form, the structure consists of a repeating pattern of sodium ions (Na⁺) and chloride ions (Cl⁻), which form a lattice-like structure. This lattice holds the ions in a fixed position, restricting their movement. Because of this immobility, solid ionic compounds generally do not conduct electricity. It is only when they are in a molten or dissolved state that their conductivity is activated.

Molten Sodium Chloride

When sodium chloride is heated to its melting point, it undergoes a transformation from a solid to a liquid. This molten state allows the rigid crystal lattice of the solid form to break apart. In the molten form, the ions that were once tightly bound in a rigid structure are now free to move around. This freedom of movement allows the Na⁺ ions and Cl⁻ ions in molten sodium chloride to drift freely. This movement of charged ions is what facilitates the conduction of electricity. The ability to conduct electricity in this state is a characteristic property of ionic compounds when they are molten. As they no longer exist in a rigid lattice, the ions form a liquid that behaves like a conductor.

Free Ions in Molten State

In the molten state, ionic compounds like sodium chloride create an environment in which ions are free. These free-moving ions are crucial for electricity conduction. As the compound melts, energy from the heat causes the ions to break free from the solid lattice. The Na⁺ and Cl⁻ ions disperse throughout the molten liquid. This forms something like a sea of mobile ions.

• The "free ions" facilitate electrical conduction because they are charged particles that can carry electrons through the liquid.
• When an electrical potential is applied, the positive ions (Na⁺) move toward the cathode, while the negative ions (Cl⁻) drift toward the anode.
• This movement of ions in response to an electric field is what allows the molten salt to conduct electricity.

Thus, the presence of free ions in a molten state transforms the ionic compound into a conductor, differentiating it from its non-conductive solid state.