Problem 37 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Molten sodium chloride conducts electricitry due to the presence of [1981-1 Mark] (a) free electrons (b) free ions (c) free molecules (d) atoms of sodium and chlorine

Step-by-Step Solution

Answer

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

Step 1: Understanding the Question

The question asks about the reason behind the conductivity of molten sodium chloride. We are provided with four options: free electrons, free ions, free molecules, and atoms of sodium and chlorine. Our task is to determine which option correctly explains the conductivity of molten sodium chloride.

Step 2: Define Conductivity in Ionic Compounds

In ionic compounds like sodium chloride, conductivity is enabled when charged particles are free to move. The solid state of sodium chloride does not conduct electricity because ions are held in fixed positions in a lattice. Molten sodium chloride differs because it allows these charged particles to move freely.

Step 3: Eliminate Incorrect Options

\(a\) Free electrons: Common in metals, but sodium chloride is an ionic compound, so this does not apply. \(c\) Free molecules: Sodium chloride is ionic and forms a lattice rather than molecules. \(d\) Atoms of sodium and chlorine: In the molten state, sodium chloride is not composed of individual atoms but rather positive and negative ions.

Step 4: Identify the Correct Option

\(b\) Free ions: When sodium chloride is melted, the ions are free to move, allowing it to conduct electricity. The Na⁺ and Cl⁻ ions in the molten state are the charges that flow to conduct electricity.

Key Concepts

Ionic Compounds

Ionic compounds are an interesting type of chemical compound. They consist of positively and negatively charged ions. This means they are made up of elements that have lost or gained electrons to fulfill a stable electronic configuration. For example, sodium chloride (NaCl) is a classic ionic compound. Sodium (Na) gives up one electron to become Na⁺, and chlorine (Cl) accepts one electron to become Cl⁻. Because opposite charges attract, these ions stick together to form a strong bond or lattice.

• In a solid state, these ions are fixed in place within the lattice, which means they can't move freely.
• This fixed position explains why solid ionic compounds don't conduct electricity.
• However, when ionic compounds are melted or dissolved, the lattice breaks down, freeing the ions to move.

Understanding these characteristics helps in recognizing why ionic compounds like sodium chloride can conduct electricity when in molten or dissolved form.

Free Ions

Free ions are crucial for electrical conductivity in ionic substances. When an ionic compound becomes molten, the rigid crystal structure breaks apart. This process frees the ions. In molten sodium chloride, the Na⁺ and Cl⁻ ions are no longer held in place by the lattice. This freedom allows them to move through the liquid. When an electrical potential is applied, these free ions move towards either the positive or negative electrode depending on their charge. This movement creates a flow of electrical current.

• Na⁺ ions, being positively charged, move towards the negative electrode.
• Cl⁻ ions, being negatively charged, move towards the positive electrode.
• This flow of ions is what allows electricity to pass through the molten compound.

Without the presence of free ions, ionic compounds would remain non-conductive, just as they are when solid.

Molten Sodium Chloride

Molten sodium chloride is simply sodium chloride in its liquid form. This state is achieved when the solid reaches a high enough temperature to melt. During this phase change, several important properties change. Most notably, the structural lattice of the solid is disrupted. When sodium chloride is heated and becomes molten, the ions (Na⁺ and Cl⁻) are liberated from their rigid positions and can move.

• The mobility of these ions is the key to its electrical conductivity in the molten state.
• Unlike in the solid state, there is no rigid structure to restrict ion movement.
• Electrical devices can exploit this property, relying on the flow of ions to conduct electricity.

Without the ability to melt or dissolve, sodium chloride would not be able to conduct electricity. Therefore, the molten state is essential for displaying ionic conductivity.