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 voltometry, it's first to talk about as an example a galvanic or voltaic cell. Now, these represent a spontaneous cell that produces or discharges electricity. It's basically a battery. And what's important here is if we fully discharge all the electricity then it becomes a dead battery. For a typical galvanic or voltaic cell, we have two jars here. In them, we submerge our electrodes and they're connected together by a salt bridge. So our salt bridge is this portion right here. Below this, we have two half reactions being shown: one in the cathode, one in the anode. For a galvanic voltaic cell, the positive electrode is the cathode, the site of reduction. So here we have 3 copper Cu2+ ions basically gaining 6 electrons to become 3 copper solids. So again, we said the cathode is a positive electrode and here would be copper solid as our electrode. The anode is the site of oxidation where we lose electrons. So 2 chromium solids are releasing electrons to become 2 chromium Cr3+ ions and then our 6 electrons that were lost. This will represent our chromium solid electrode. And here in a galvanic or voltaic cell our anode is the negative electrode. Again, we said that anodes is where oxidation happens so they're losing electrons, cathode gaining electrons. This is why we see electrons moving in this direction. They're moving towards the positive cathode electrode. Now when it comes to electrochemical cells, we have to talk about closing the circuit or completing the circuit. Here we have our anode. Here we have our cathode. Now it doesn't matter if we're talking about spontaneous cells in the form of galvanic or voltaic cells or if we're talking about non spontaneous cells such as electrolytic cells. Oxidation always happens at the anode, reduction always happens at the cathode. Electrons always leave the anode and go towards the cathode. To complete the circuit, we need similar charges 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 and thereby complete the circuit and hope and allow this process to act as a battery. Here we have within our salt bridge typically we have neutral ions. Neutral meaning they are not acidic or basic in nature. Typically we have sodium ions and then for anions, we have chloride ions or nitrate ions. So these negative ions travel towards the anode, and then this positive ion travels more towards the cathode side. Now, why exactly is this occurring? Well, the chromium electrode is losing electrons over time. As it's losing electrons, it's basically producing chromium 3 ions. This causes a buildup of these positive ions within the solution. As a result of this, we're basically attracting negative charges towards those positive ions. We don't want our electrons to stay near the anode chamber. We want them to keep going towards the cathode chamber. So what has to come in their place? These negative ions here. They basically come in and they neutralize these positive ions and help them from building up too much. Because if they build up too much, the electrons won't want to leave the anode compartment and go towards the cathode compartment. Okay? So here we're gonna say to produce higher voltage we want the anode concentration of positive ions to be low. On the other side, we have sodium, we have copper Cu2+ ions. We want this concentration to be high because if it's high enough it'll attract these negative electrons towards the surf, towards the cathode compartment. Now, what's happening over time is we're losing electrons we're gonna have the anode. Electrons do add up if you keep losing them over time and over time we're gonna lose mass from this anode. So eventually the anode will basically dissolve away. As more and more electrons embed themselves on the surface of this cathode, the surface will become more negatively charged. This will in turn attract some of these positive ions dissolved in solution towards the surface of the cathode. Positive charges negative charges combine together neutralizing each other. Over time, they're going to encrust themselves on the surface of this cathode, which we call plating out, so cathode plates out. So over time the anode gets smaller, the cathode gets bigger. Now, if we want to lose electrons from the anode side, losing electrons deals with ionization. If you want to lose those electrons very easily, we want low ionization energy. Electron affinity deals with attracting electrons. The cathode compartment wants to attract these electrons, so we want electron affinity to be high. So it's all of these things working in tandem which helps to have the galvanic or voltaic cell behave as a battery. Now if we come down here, we talked about these half reactions. With these half reactions, they're typically written as a reduction. That's why all of them have their electrons as reactants here. And associated with these half reactions, we have our standard cell potential. Remember the higher your cell potential, your standard cell potential, the more likely reduction will occur. And remember, the more likely reduction occurs, the stronger you are as an oxidizing agent. And then down here the smaller your standard cell potential, then the more likely oxidation happens. And then if oxidation is occurring, that makes you the stronger reducing agent. So keep that in mind when we're talking about a typical galvanic / voltaic cell. These are all the things that are occurring at the same time. I have to take into consideration your periodic trends of ionization energy and electron affinity, and how that plays a role in anodes losing electrons for cathodes to gain those electrons, and then the salt bridge used to complete the circuit. All of this helps it to become a battery. But if we fully discharge our electricity, that battery will will be a dead battery and it'll 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.