Problem 43 - Chemistry and Chemical Reactivity

Question

An aqueous solution of KBr is placed in a beaker with two inert platinum electrodes. When the cell is attached to an external source of electrical energy, electrolysis occurs. (a) Hydrogen gas and hydroxide ion form at the cathode. Write an equation for the half-reaction that occurs at this electrode. (b) Bromine is the primary product at the anode. Write an equation for its formation.

Step-by-Step Solution

Answer

(a) \( 2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq) \). (b) \( 2Br^-(aq) \rightarrow Br_2(l) + 2e^- \).

Step 1: Understand the Electrode Reactions

Electrolysis of an aqueous KBr solution involves water and ions being reduced or oxidized at the electrodes. At the cathode (negative electrode), reduction occurs, while at the anode (positive electrode), oxidation occurs.

Step 2: Identify the Cathode Reaction

At the cathode, water and potassium ions can be reduced, but water is more easily reduced than K+. Hence, water is reduced to form hydrogen gas (H2) and hydroxide ions (OH-). The half-reaction is: \[ 2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq) \]

Step 3: Identify the Anode Reaction

At the anode, bromide ions (Br-) are oxidized instead of water, as bromine is produced. The half-reaction is: \[ 2Br^-(aq) \rightarrow Br_2(l) + 2e^- \]

Step 4: Write the Overall Reaction

Combine the cathode and anode reactions to write the overall reaction for the electrolysis process: \[ 2H_2O(l) + 2Br^-(aq) \rightarrow H_2(g) + 2OH^-(aq) + Br_2(l) \]

Key Concepts

Electrode Reactions

In the process of electrolysis, electrical energy drives a non-spontaneous chemical reaction. The essence of this process lies in the reactions that occur at the electrodes. Electrodes are conductive materials that transfer electrons. During electrolysis, two electrodes are submerged in a solution: the cathode and the anode.

• The cathode is the electrode where reduction occurs. It is negatively charged, attracting positive ions or cations from the solution.
• The anode is the electrode where oxidation occurs. It is positively charged, attracting negative ions or anions.

In the electrolysis of an aqueous solution of KBr, the reactions at these electrodes are crucial for the formation of the products. For instance, at the cathode, water is reduced, while at the anode, bromide ions are oxidized.
Understanding the behavior of positive and negative ions during this process is key to predicting the products formed at each electrode.

Half-Reaction Equations

Half-reaction equations are simplified equations that show either the oxidation or reduction part of a reaction separately. In electrolysis, these help us understand what is happening at each electrode.

• At the cathode, water is reduced to form hydrogen gas and hydroxide ions. The half-reaction for the reduction is: \[ 2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq) \]
• At the anode, bromide ions are oxidized to form bromine gas, with the half-reaction being: \[ 2Br^-(aq) \rightarrow Br_2(l) + 2e^- \]

By combining these half-reactions, we can derive the overall reaction for the process of electrolysis. This combination allows us to see the movement of electrons and the transformation of reactants to products at both electrodes.
The half-reaction equations not only illustrate the specific ions or molecules involved but also help in balancing the charges and conserving mass in a reaction.

Oxidation and Reduction

Oxidation and reduction, collectively known as redox reactions, are fundamental to understanding electrolysis reactions. Each process involves the transfer of electrons between substances.
Oxidation refers to the loss of electrons, while reduction refers to the gain of electrons.

• In oxidation, an element increases its oxidation state by losing electrons. For instance, bromide ions lose electrons to form bromine gas during electrolysis.
• In reduction, an element decreases its oxidation state by gaining electrons. Water molecules gain electrons to form hydrogen gas and hydroxide ions at the cathode.

Together, oxidation and reduction reactions create a flow of electrons through the circuit, which is integral for electrolysis.
Redox reactions are essential as they determine what substances get oxidized and reduced in the process. Understanding these concepts helps us predict the outcome of the electrolysis based on electron transfer and changes in oxidation states.