Question

Molten sodium chloride conducts electricity due to the presence of : (a) Free electron (b) Free ions (c) Free molecules (d) Atoms of sodium and chlorine

Short answer

Molten sodium chloride conducts electricity due to the presence of (b) Free ions.

Step-by-step solution

Understanding the Concept of Electrical Conductivity in Molten Ionic Compounds

In molten ionic compounds, the ionic lattice breaks down and the ions become free to move. Electric current is conducted by the movement of these charged particles, which are either positive ions (cations) or negative ions (anions). The movement allows electric current to pass through the molten substance.

Analyzing the Options Given in the Exercise

Conductivity in molten ionic compounds isn’t due to free electrons, free molecules, or atoms, as suggested by choices (a), (c), and (d). Instead, it is due to the presence of ions that are freed once the ionic compound, such as sodium chloride, melts.

Identifying the Correct Answer

Based on our understanding that mobile ions in molten states conduct electricity, the correct option is (b) Free ions, as sodium chloride in its molten state dissociates into free sodium (Na+) and chloride (Cl-) ions that can carry electric current.

Key concepts

Ionic Lattice

An ionic lattice is a highly ordered structure characteristic of crystalline ionic compounds, such as sodium chloride in its solid state. In this arrangement, each ion occupies a specific position within the lattice, with cations and anions alternating to maximize attractive interactions and minimize repulsive ones. This rigid framework provides stability to the compound. However, it does not allow for the movement of ions within the solid, hence they cannot conduct an electric current. Only when melted or dissolved does this lattice break down, releasing ions that are free to move and hence conduct electricity.

To visualize an ionic lattice, imagine a repeating three-dimensional pattern of ions, much like a 3D chess board filled with alternating positively charged pieces (cations) and negatively charged ones (anions). This structure remains until the compound is subjected to enough energy, typically in the form of heat, to break these ionic bonds, leading to a molten state where the ions are no longer fixed in place.

Cations and Anions

Cations and anions are the positively and negatively charged particles, respectively, that make up ionic compounds. In a solid ionic lattice, these ions are held together by strong electrostatic forces of attraction in a well-defined geometric array. When the compound is heated and becomes a liquid (molten), these ions are no longer held in position by the lattice structure.

Let's look at sodium chloride as an example. Sodium ions (Na+) are the cations, and chloride ions (Cl-) are the anions. These ions have opposite charges and thus are attracted to one another, forming the compound NaCl. Upon melting, the cations and anions are released from the confines of the lattice, and they can freely move throughout the liquid. This freedom of movement is a key characteristic enabling the molten substance to conduct electricity.

Electric Current Conduction

Electric current conduction in substances is essentially the flow of charges. In metals, this is typically done by free electrons, but in ionic compounds like molten sodium chloride, it’s carried out by cations and anions. When the ionic compound is molten, the ions are free to move and can carry charge from one place to another. This movement is induced by applying a voltage, making the cations migrate towards the negative electrode (cathode) and the anions towards the positive electrode (anode).

It’s important to note that in the molten state, ions are in constant random motion, but the application of an electric potential directs their movement, allowing them to transfer charge and thus conduct an electric current through the liquid. Consequently, electrical conductivity in molten ionic compounds is directly proportional to the number of ions present and their mobility.

Dissociation of Sodium Chloride

What Happens When Sodium Chloride Melts?

When sodium chloride melts, it undergoes dissociation. This means the solid crystalline lattice breaks down, and the individual sodium (Na+) cations and chloride (Cl-) anions are released from their fixed positions. In this molten state, they are free to move about, making them capable of conducting electricity. Dissociation is a critical process that transforms a non-conductive solid into a conductive liquid.

Electrolysis of Molten Sodium Chloride

Furthermore, when an electric current is passed through molten sodium chloride during electrolysis, chemical reactions occur at the electrodes. At the cathode, sodium cations receive electrons (get reduced) to form metallic sodium. At the anode, chloride anions lose electrons (are oxidized) to form chlorine gas. This process is fundamental in the large-scale production of both sodium metal and chlorine gas.