Voltammetry - Online Tutor, Practice Problems & Exam Prep

In a galvanic or voltaic cell, spontaneous oxidation and reduction reactions occur at the anode and cathode, respectively. The anode, where oxidation happens, is negatively charged, while the cathode, where reduction occurs, is positively charged. A salt bridge facilitates ion movement, maintaining charge balance. The standard cell potential (E_cell) indicates the likelihood of reduction, with higher values signifying stronger oxidizing agents. Over time, the anode loses mass, and the cathode gains mass through plating out, illustrating the dynamic nature of electrochemical cells.

Voltammetry

Hey everyone. So when we talk about voltammetry, it's important to start with an example of a galvanic or voltaic cell. These represent a spontaneous cell that produces or discharges electricity. Essentially, it's a battery, and what's important to note is if we fully discharge all the electricity, it becomes a dead battery. In a typical galvanic or voltaic cell, we have two jars; within them, we submerge our electrodes, and they're connected together by a salt bridge.

The salt bridge is this portion right here. Below this, we have two half-reactions being shown: one at the cathode and one at the anode. For a galvanic voltaic cell, the positive electrode is the cathode, the site of reduction. Here we have three copper(II) ions, basically gaining six electrons to become three copper solids. As mentioned, the cathode is a positive electrode, and here copper solid is our electrode.

The anode is the site of oxidation, where electrons are lost. So two chromium solids release electrons to become two chromium(III) ions, and then our six electrons that were lost. This represents our chromium solid electrode. In a galvanic or voltaic cell, our anode is the negative electrode. Again, we stated that anodes are where oxidation happens, so they're losing electrons, while the cathode gains electrons. This process causes the electrons to move in this direction towards the positive cathode electrode.

Regarding electrochemical cells, it is vital to discuss closing the circuit or completing the circuit. We have our anode and our cathode. It doesn't matter if we're discussing spontaneous cells like galvanic voltaic cells or nonspontaneous cells such as electrolytic cells. Oxidation always occurs at the anode, and reduction always occurs at the cathode.

Electrons always leave the anode and move towards the cathode. To complete the circuit, similar charges need to move in the opposite direction. That's where our salt bridge comes into play. For our salt bridge, we're going to have negative ions traveling from the cathode side towards the anode side, thereby completing the circuit and allowing this process to act as a battery. Typically, we have sodium ions and, for anions, we have chloride ions or nitrate ions in our salt bridge.

As the chromium electrode loses electrons, it produces chromium(III) ions, causing a buildup of these positive ions within the solution. This buildup attracts negative charges towards those positive ions. We need these negative ions to neutralize these positive charges to prevent too much buildup, as this would inhibit the electrons from leaving the anode compartment and moving towards the cathode compartment.

To produce higher voltage, we want the anode concentration of positive ions to be low. Conversely, we want the concentration of copper(II) ions to be high, as this will attract the negative electrons towards the cathode compartment. Over time as electrons are lost at the anode, its mass will decrease, causing the anode to dissolve away. As more electrons embed themselves on the surface of the cathode, the surface becomes more negatively charged, attracting positive ions dissolved in the solution towards the cathode. Over time, these ions encrust themselves on the surface of the cathode, a process called plating out.

So, the anode gets smaller, and the cathode gets bigger. If we want to lose electrons from the anode side easily, we need low ionization energy. The cathode compartment, wanting to attract these electrons, should have high electron affinity. These dynamics work in tandem to allow the galvanic voltaic cell to function as a battery. When discussing these half-reactions, they are typically written as reductions with electrons as reactants, indicating the process of reduction. The higher the standard cell potential, the more likely reduction will occur, making you a stronger oxidizing agent. Conversely, the lower the standard cell potential, the more likely oxidation will happen, making you a stronger reducing agent.

Keep all this in mind when discussing a typical galvanic or voltaic cell. You must consider the periodic trends of ionization energy and electron affinity, and how these play a role in anodes losing electrons for cathodes to gain those electrons. The use of a salt bridge to complete the circuit also contributes to its function as a battery. However, if we fully discharge our electricity, the battery will be a dead battery and reach equilibrium.

Here’s what students ask on this topic:

What is the difference between a galvanic cell and an electrolytic cell?

A galvanic cell, also known as a voltaic cell, generates electrical energy from spontaneous redox reactions. The anode is negatively charged, and the cathode is positively charged. In contrast, an electrolytic cell requires an external power source to drive non-spontaneous redox reactions. Here, the anode is positively charged, and the cathode is negatively charged. In both cells, oxidation occurs at the anode and reduction at the cathode, but the direction of electron flow and the nature of the reactions differ.

How does a salt bridge work in a galvanic cell?

A salt bridge in a galvanic cell maintains electrical neutrality by allowing the flow of ions between the two half-cells. It typically contains a neutral salt like KCl or NaNO₃. Negative ions from the salt bridge migrate towards the anode to balance the positive charge buildup, while positive ions move towards the cathode to neutralize the negative charge buildup. This ion movement completes the circuit, allowing the redox reactions to continue and the cell to function as a battery.

What is the role of the standard cell potential (E_cell) in a galvanic cell?

The standard cell potential (E_cell) measures the voltage or electrical potential difference between the two half-cells in a galvanic cell under standard conditions. It indicates the cell's ability to drive spontaneous redox reactions. A higher E_cell value means a greater tendency for reduction to occur at the cathode, making the substance a stronger oxidizing agent. Conversely, a lower E_cell value indicates a stronger reducing agent at the anode.

Why does the anode lose mass and the cathode gain mass in a galvanic cell?

In a galvanic cell, the anode loses mass because it undergoes oxidation, releasing electrons and forming positive ions that dissolve into the solution. For example, Cr(s) → Cr³⁺(aq) + 3e^-. The cathode gains mass due to reduction, where positive ions in the solution gain electrons and deposit as solid metal on the cathode surface. For instance, Cu²⁺(aq) + 2e^- → Cu(s). This process is known as plating out.

How do ionization energy and electron affinity affect the performance of a galvanic cell?

Ionization energy and electron affinity are crucial for the performance of a galvanic cell. Low ionization energy at the anode facilitates the loss of electrons, making oxidation easier. High electron affinity at the cathode enhances the attraction of electrons, promoting reduction. Together, these properties ensure efficient electron flow from the anode to the cathode, maximizing the cell's voltage and overall performance.