Problem 60 - Chemistry The Molecular Science

Question

Write chemical equations for the electrolysis of molten salts of three different alkali halides to produce the corresponding halogens and alkali metals.

Step-by-Step Solution

Answer

Electrolysis of molten NaCl yields Na and Cl2.

Step 1: Understand Electrolysis Basics

Electrolysis involves breaking down a compound into its elements using electricity. In molten salts, sodium chloride (NaCl), potassium bromide (KBr), and lithium iodide (LiI) will dissociate into their constituent ions due to the force of electricity.

Step 2: Write the Electrolysis Reaction for Sodium Chloride

When molten sodium chloride undergoes electrolysis, sodium ions (Na⁺) migrate to the cathode and are reduced, while chloride ions (Cl⁻) migrate to the anode and are oxidized. The chemical equations for the reactions are:\[ \text{Cathode (Reduction): } 2\text{Na}^+ + 2\text{e}^- \rightarrow 2\text{Na} \]\[ \text{Anode (Oxidation): } 2\text{Cl}^- \rightarrow \text{Cl}_2 + 2\text{e}^- \]

Key Concepts

Understanding Alkali Halides

Alkali halides are compounds that consist of an alkali metal combined with a halogen. These compounds are salts that are formed by the reaction between alkali metals, such as sodium (Na), potassium (K), and lithium (Li), and halogens like chlorine (Cl), bromine (Br), and iodine (I).
Alkaline metals are known for losing an electron to form positive ions, while halogens gain an electron to form negative ions.

• Alkali metals: Group 1 elements (e.g., Na, K, Li)
• Halogens: Group 17 elements (e.g., Cl, Br, I)

When these elements react, they form ionic compounds, which are stable and commonly found in nature as solid crystalline salts. Examples of alkali halides include:

• Sodium chloride (NaCl)
• Potassium bromide (KBr)
• Lithium iodide (LiI)

These salts dissolve in water and can also exist in a melted or molten state at high temperatures.

Writing Chemical Equations

Writing chemical equations for electrolysis involves representing the processes that occur at the anode and cathode separately. A chemical equation helps describe what reactants are involved and what products are formed. In the context of electrolysis, these equations typically reflect oxidation-reduction reactions.
There are key steps to follow when writing these equations:

• Identify the ions present in the molten salt.
• Determine which ions will migrate to the cathode and which to the anode.
• Write the reduction reaction for the cathode.
• Write the oxidation reaction for the anode.

For example, the electrolysis of molten sodium chloride (NaCl) involves the following steps:
1. NaCl dissociates into Na⁺ and Cl⁻ ions.
2. At the cathode, Na⁺ ions gain electrons and are reduced to form sodium metal:
\[ 2 ext{Na}^+ + 2 ext{e}^- \rightarrow 2 ext{Na} \]
3. At the anode, Cl⁻ ions lose electrons and are oxidized to form chlorine gas:
\[ 2 ext{Cl}^- \rightarrow ext{Cl}_2 + 2 ext{e}^- \]

Reduction and Oxidation Reactions in Electrolysis

Reduction and oxidation reactions, often termed "redox" reactions, are critical to understanding electrolysis. In these reactions:

• Reduction refers to the gain of electrons by an ion or molecule.
• Oxidation involves the loss of electrons by an ion or molecule.

In electrolysis, these reactions happen at different electrodes:

• The cathode is where reduction takes place. Ions gain electrons here, turning into neutral atoms.
• The anode is where oxidation happens. Ions lose electrons, turning into gas molecules or other products.

A helpful mnemonic is "OIL RIG": Oxidation Is Loss, Reduction Is Gain (of electrons). Understanding this can help unravel the changes occurring during electrolysis. For instance, in the electrolysis of NaCl:

• Reduction: Na⁺ ions gain electrons at the cathode, forming sodium metal.
• Oxidation: Cl⁻ ions lose electrons at the anode, forming chlorine gas.

Molten Salts in Electrolysis

Molten salts are salts that have been heated to a point where they become liquid. This is crucial for electrolysis as it allows for the free movement of ions, which are necessary for carrying charges in the electrolyte.
When salts are molten:

• The solid ionic lattice breaks apart because of the heat.
• Ions are free to move, allowing the salt to conduct electricity.

In the context of electrolysis, molten salts enable the separation of compounds into their elemental forms. Since ions can migrate freely towards respective electrodes, they facilitate reactions like:

• Reduction at the cathode forming metals.
• Oxidation at the anode producing gases.

For example, when sodium chloride is in molten form, sodium metal and chlorine gas are produced through electrolysis. This process has significant industrial applications, most notably in the extraction of metals from their ores and in the production of chlorine.