Question

Describe an experiment that would enable you to determine which is the cathode and which is the anode in a galvanic cell using copper and zinc electrodes.

Short answer

The cathode is copper, and the anode is zinc.

Step-by-step solution

Understand the Basis

A galvanic cell generates electrical energy from a spontaneous redox reaction. It consists of two half-cells, each containing an electrode and an electrolyte. The anode undergoes oxidation, losing electrons, while the cathode undergoes reduction, gaining electrons.

Set Up the Experiment

To determine the cathode and anode, set up a simple galvanic cell. Place a copper rod in a copper sulfate solution and a zinc rod in a zinc sulfate solution. Connect the solutions with a salt bridge to complete the circuit, allowing ions to flow between the half-cells.

Connect the Electrodes

Connect the copper and zinc electrodes externally using a wire and include a voltmeter in the circuit. The voltmeter will measure any potential difference generated by the cell, indicating the flow of electrons from one electrode to the other.

Analyze Electron Flow

Observe the voltmeter reading. If the voltmeter shows a positive voltage, electrons flow from zinc to copper. Thus, zinc is the anode (undergoing oxidation), and copper is the cathode (receiving electrons and undergoing reduction).

Verify the Results

Corroborate your findings by observing any visible reactions at the electrodes. At the anode (zinc), you should see the dissolution of zinc into the solution. At the cathode (copper), you might see deposition of copper ions from the solution.

Key concepts

Anode and Cathode Identification

In a galvanic cell, identifying the anode and cathode is crucial. The anode is the electrode where oxidation occurs, meaning it loses electrons, while the cathode is where reduction happens, indicating it gains electrons.

To differentiate between these two, an experiment can be set up using zinc and copper electrodes. By connecting these metals in separate solutions (zinc sulfate for zinc and copper sulfate for copper) and using a voltmeter between them, the flow of electrons can be examined.

You'll observe the direction of electron flow through the wire. If the voltmeter displays a positive reading, electrons are moving from zinc to copper. Here, zinc is the anode, and copper is the cathode.

• Oxidation (electron loss) occurs at the anode (zinc).
• Reduction (electron gain) takes place at the cathode (copper).

This setup helps visually identify which electrode serves as the anode and which as the cathode through both electron flow and observable chemical changes on the electrodes themselves.

Redox Reaction

A galvanic cell relies on a redox reaction, which stands for reduction-oxidation reaction. It's a chemical process where one substance loses electrons (oxidation) and another gains those electrons (reduction). These reactions are coupled, meaning oxidation and reduction always occur together.

In our copper and zinc example, zinc goes through oxidation as it loses electrons: \[ \text{Zn} \rightarrow \text{Zn}^{2+} + 2e^- \] Meanwhile, copper ions go through reduction, as they gain those electrons:\[ \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu} \]

This transfer of electrons is what generates electrical energy. The spontaneous movement of electrons from the anode (zinc) to the cathode (copper) creates a current, powering any device connected to the circuit.

• Oxidation at the anode releases electrons into the wire.
• Reduction at the cathode collects electrons from the wire.

Understanding the redox reaction provides insight into how a galvanic cell works and generates electricity.

Electrode Potential

Electrode potential measures the tendency of an electrode to gain or lose electrons. This potential difference is crucial in a galvanic cell, as it determines the direction of electron flow and the overall voltage produced.

Zinc and copper have different standard electrode potentials, influencing their roles as either anode or cathode. Zinc, with a more negative electrode potential, readily loses electrons, thus becoming the anode. Copper, with a more positive electrode potential, more easily gains electrons, making it the cathode.

The potential difference between these two electrodes is measured with a voltmeter, indicating the cell's voltage. Generally, a higher potential difference means a greater ability to produce electricity.

• Electrode potential influences which metal acts as the anode or cathode.
• The greater the difference in electrode potential, the larger the voltage generated.

Understanding electrode potential helps predict the behavior of metal electrodes in a galvanic cell and assess the efficiency of the cell.

Salt Bridge

A salt bridge is an essential component of a galvanic cell, acting as a connector between two half-cells. It ensures that ions can flow freely between solutions, completing the electrical circuit and allowing continuous electron flow.

The salt bridge typically contains an inert salt solution like KCl or NaNO₃. It serves two primary functions:

• Maintaining electrical neutrality by allowing positive and negative ions to move between the half-cells.
• Preventing the solutions from mixing directly, which could lead to unwanted reactions.

Without a salt bridge, the flow of electrons would soon stop, as the build-up of charge would no longer allow the redox reaction to proceed smoothly.

By facilitating ion exchange, the salt bridge plays a critical role in sustaining the cell's operation over time, helping maintain balance in charge across the circuit.