Question

Write equations for the half-reactions that occur in the electrolysis of a mixture of molten potassium bromide and molten lithium bromide.

Short answer

At the cathode: \( Li⁺ + e⁻ \rightarrow Li \). At the anode: \( 2 Br⁻ \rightarrow Br_2 + 2 e⁻ \). Lithium is reduced instead of potassium due to its lower reduction potential.

Step-by-step solution

Identify the Cations and Anions

In the mixture of molten potassium bromide (KBr) and lithium bromide (LiBr), potassium (K⁺) and lithium (Li⁺) are the cations, and bromide (Br⁻) is the common anion. During electrolysis, cations migrate to the cathode to gain electrons (reduction), while anions migrate to the anode to lose electrons (oxidation).

Write the Reduction Half-Reactions at the Cathode

At the cathode, both K⁺ and Li⁺ will gain electrons to form their respective neutral atoms. The reduction half-reactions for potassium and lithium are as follows:For Potassium:\( K⁺ + e⁻ \rightarrow K \)For Lithium:\( Li⁺ + e⁻ \rightarrow Li \)

Determine the More Favorable Reduction

Since both cations cannot be reduced at the same time, we need to determine which reduction is more favorable. Lithium has a lower reduction potential than potassium, which means that under standard conditions, lithium gets reduced preferentially at the cathode. The preferred reduction half-reaction is:\( Li⁺ + e⁻ \rightarrow Li \)

Write the Oxidation Half-Reaction at the Anode

At the anode, the bromide ion will lose electrons to form bromine atoms, which will pair to form diatomic bromine molecules. The oxidation half-reaction for bromide is:\( 2 Br⁻ \rightarrow Br_2 + 2 e⁻ \)

Key concepts

Molten Salt Electrolysis

Molten salt electrolysis is a fascinating process primarily used in the extraction of metals from their salts, which are in a molten state. The key feature of this process is that it involves the passage of an electric current through the molten salt, which subsequently results in the decomposition of the compound into its constituent elements.

Considering the exercise presented, in which a mixture of molten potassium bromide (KBr) and lithium bromide (LiBr) undergoes electrolysis, we focus on the melting of the salts to create an ionic liquid. This ionic liquid facilitates the free movement of ions, allowing them to migrate towards the electrodes when a voltage is applied. The cations (K⁺ and Li⁺) move towards the cathode, where they gain electrons and are reduced, while the anions (Br⁻) move to the anode to lose electrons and are oxidized, ultimately forming bromine molecules.

Reduction Potential

Reduction potential, also known as electrode potential, is a measure of the tendency of a chemical species to acquire electrons and thus be reduced. It is a critical factor in determining which species will be reduced in the presence of another. Each element has a standard reduction potential associated with it, stated under standard conditions, which is 25°C, a 1M concentration for each ion participating in the reaction, a partial pressure of 1 atm for any gases involved, and a pH of 7.

In the given exercise, where we have two potential cations (Li⁺ and K⁺) competing to be reduced, the reduction potential tells us that lithium, with its lower reduction potential, is more likely to gain electrons before potassium. This can be thought of as lithium having a stronger 'desire' to reach a neutral state compared to potassium, leading to its preferential reduction during the electrochemical process of the molten salt electrolysis.

Oxido-Reduction Reactions

Oxido-reduction reactions, commonly known as redox reactions, are chemical processes that involve the transfer of electrons between two species. These reactions encompass two half-reactions: oxidation, where an element loses electrons, and reduction, where an element gains electrons. The two processes always occur simultaneously; when one species is reduced, another is oxidized.

In the context of the exercise, bromide ions (Br⁻) being oxidized at the anode form bromine molecules, and this is an oxidation half-reaction. Simultaneously, lithium ions (Li⁺) are reduced at the cathode to form lithium atoms, and this is a reduction half-reaction. Learning to balance redox reactions is crucial because it ensures that the number of electrons lost in oxidation equals the number gained in reduction, thereby satisfying the law of conservation of charge.