Problem 67 - Chemistry: An Atoms-Focused Approach

Question

The salts obtained from the evaporation of seawater can act as a source of halogens, principally \(\mathrm{Cl}_{2}\) and \(\mathrm{Br}_{2}\), through the electrolysis of the molten alkali metal halides. As the potential of the anode in an electrolytic cell is increased, which of these two halogens forms first?

Step-by-Step Solution

Answer

Answer: Chlorine (Cl2) forms first.

Step 1: Recall the Standard Electrode Potentials

To determine which halogen will form first, we must consider their standard electrode potentials. The electrode potential for a given half-reaction indicates the likelihood of the reaction to occur; a higher potential means the reaction is more likely to take place. For halogens, the half-reaction for the oxidation process at the anode is: \(2 \mathrm{X}^{-} \rightarrow \mathrm{X}_{2} + 2 \mathrm{e}^{-}\).

Step 2: Compare the Standard Electrode Potentials of Chlorine and Bromine

The standard electrode potentials for chlorine (Cl2) and bromine (Br2) are as follows: Cl2: \(2 \mathrm{Cl}^{-} \rightarrow \mathrm{Cl}_{2} + 2 \mathrm{e}^{-}\), E° = +1.36 V Br2: \(2 \mathrm{Br}^{-} \rightarrow \mathrm{Br}_{2} + 2 \mathrm{e}^{-}\), E° = +1.07 V

Step 3: Determine which Halogen is produced first

From the given standard electrode potentials, we can see that the value for chlorine (Cl2) is higher than that of bromine (Br2). This means that chlorine (Cl2) has a higher likelihood to be produced first at the anode as the potential increases compared to bromine (Br2). Therefore, as the potential of the anode in an electrolytic cell is increased, chlorine (Cl2) forms first before bromine (Br2).

Key Concepts

Halogens

Halogens are a group of elements found in Group 17 of the periodic table. They include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements are known for being highly reactive, particularly with alkali metals and alkaline earth metals.
In industry, halogens are often extracted from salts, such as those found in seawater. Chlorine and bromine are commonly obtained through the process of electrolysis of molten alkali metal halides.
One important property of halogens is their ability to undergo oxidation and reduction reactions. This makes them vital in many chemical processes, including the production of household bleach (sodium hypochlorite) and in the purification of drinking water through chlorination.

• Fluorine is the most reactive halogen and is used in toothpaste to prevent dental cavities.
• Chlorine is used in disinfectants and PVC plastics.
• Bromine finds uses in flame retardants.
• Iodine is essential for thyroid hormone production in humans.

Electrode Potentials

Electrode potential is a measure of the potential difference between an electrode and its surrounding solution. It quantifies the ability of a half-cell to either lose or gain electrons during a redox reaction. In simpler terms, it's how likely a reaction is to occur.
In electrolysis, standard electrode potentials help predict which elements will be oxidized or reduced at the electrodes.

• These potentials are measured in volts (V).
• Standard conditions assume solutes at 1 M concentration, gases at 1 atmosphere pressure, and pure solids and liquids.

The standard electrode potentials for the half-reactions of chlorine and bromine are +1.36 V and +1.07 V, respectively. Chlorine has a higher electrode potential, meaning it is more predisposed to undergo oxidation at the anode compared to bromine. This priority in oxidation often dictates which element gets produced first during electrolysis processes.
Understanding electrode potentials allows chemists to predict the feasibility of reactions and is fundamental in designing electrochemical cells and batteries.

Anode Reactions

During electrolysis, reactions at the anode involve oxidation processes where electrons are lost by the anions in the solution. The type of reaction and element produced is primarily determined by the anode material and the species present in the solution.
In the case of halogen extraction from seawater, when the electrolytic cell's anode potential is increased, the halide ions such as chloride (Cl^-) and bromide (Br^-) are oxidized to their respective diatomic halogen molecules, Cl₂ and Br₂.

• Anode reactions are essential for extracting elements such as chlorine in purified form.
• Oxidation at the anode is associated with loss of electrons, turning anions into neutral molecules or elements.

By increasing the anode potential, it becomes possible to control and harness specific reactions, thus producing desired products first. In our case, with chlorine having a higher electrode potential, it forms first at the anode as the potential is increased, before bromine starts to form.