Problem 40 - Chemistry Principles and Practice

Question

A voltaic cell is based on the reaction $$ \mathrm{Zn}(\mathrm{s})+\mathrm{Ni}^{2+}(\mathrm{aq}) \rightarrow \mathrm{Zn}^{2+}(\mathrm{aq})+\mathrm{Ni}(\mathrm{s}) $$ Voltage measurements show that the Ni electrode is positive. Sketch the cell, and label the anode and cathode, the positive and negative electrodes, the direction of electron flow in the external circuit, and the direction of flow of cations and anions through the salt bridge. Write the halfreaction that occurs at each electrode.

Step-by-Step Solution

Answer

The anode is Zn (negative), and the cathode is Ni (positive). Electrons flow from Zn to Ni, cations flow to Ni, and anions flow to Zn.

Step 1: Identify Anode and Cathode

The reaction shows that zinc (Zn) is oxidized into Zn²⁺ and nickel (Ni²⁺) is reduced to Ni. Since oxidation occurs at the anode and reduction occurs at the cathode, Zn is the anode and Ni is the cathode.

Step 2: Identify Positive and Negative Electrodes

In a voltaic cell, the anode is the negative electrode because it releases electrons, while the cathode is the positive electrode because it gains electrons. Thus, the Zn electrode is negative, and the Ni electrode is positive.

Step 3: Determine Electron Flow Direction

Electrons flow from the anode to the cathode in the external circuit. Hence, electrons move from the Zn electrode (anode) to the Ni electrode (cathode).

Step 4: Determine Ion Flow in Salt Bridge

To balance the charge in each half-cell, cations move towards the cathode, and anions move towards the anode through the salt bridge. Thus, positive ions (cations) flow towards the Ni electrode, and negative ions (anions) flow towards the Zn electrode.

Step 5: Write Half-Reactions

At the Zn electrode (anode): \( \text{Zn} \rightarrow \text{Zn}^{2+} + 2\text{e}^- \). At the Ni electrode (cathode): \( \text{Ni}^{2+} + 2\text{e}^- \rightarrow \text{Ni} \).

Key Concepts

Anode and Cathode Identification

In the world of voltaic cells, identifying the anode and cathode is crucial for understanding how the cell operates. The anode is the site of oxidation, which means it loses electrons during the chemical reaction. According to the given reaction, zinc (Zn) is oxidized to zinc ions (\(\text{Zn}^{2+}\)), making Zn the anode of the cell. On the other hand, the cathode is where reduction takes place, meaning it gains electrons. Thus, nickel ions (\(\text{Ni}^{2+}\)) are reduced to solid nickel (Ni), identifying Ni as the cathode of the cell.

It's important to remember that:

• The anode is typically labeled as the negative side because it is where electrons are released.
• The cathode is labeled as the positive side, as it is where electrons are accepted.

Electron Flow Direction

Understand the flow of electrons to him understand the workings of a voltaic cell. Electrons are always moving from the anode to the cathode in the external circuit.

This is a fundamental principle of voltaic cells:

• Electrons are generated at the anode, travel through an external wire, and reach the cathode.
• The zinc electrode (anode) releases electrons, which then travel towards the nickel electrode (cathode).

This flow is driven by the chemical potential difference between the electrodes.

Remember, electron flow in the circuit is opposite to conventional current flow, which is depicted from positive to negative.

Ion Flow in Salt Bridge

The salt bridge in a voltaic cell plays a vital role in maintaining electrical neutrality by allowing the flow of ions. As the cell operates, cations and anions move across the salt bridge to balance the charges in each half-cell.

Here's how it works in this reaction:

• Cations, such as Ni²⁺ ions, move towards the cathode (Ni electrode) because they are attracted to the negative charge created by electron accumulation at the cathode.
• Anions move towards the anode (Zn electrode) because they are attracted to the positive charge that forms due to the loss of electrons at the anode.

This ion movement prevents the buildup of charge, ensuring the voltaic cell continues to operate efficiently.

Half-Reactions in Electrochemistry

Half-reactions are the key to understanding the redox process in a voltaic cell. Each half-reaction represents either the oxidation or reduction that occurs at the respective electrode.

For our specific reaction:

• At the anode, zinc undergoes oxidation: \(\text{Zn}\rightarrow \text{Zn}^{2+} + 2\text{e}^-\). This shows zinc losing electrons to become zinc ions.
• At the cathode, nickel ions undergo reduction: \(\text{Ni}^{2+} + 2\text{e}^- \rightarrow \text{Ni}\). This indicates that nickel ions are gaining electrons to form solid nickel.

Understanding these half-reactions helps in visualizing the chemical changes and electron transfers happening within the cell.