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We've been studying electrochemistry. In electrochemistry, we take advantage of redox reactions. We can manipulate the electrons to flow through an external circuit. What's the result? We can do electrical work. So, we can power our phones or computers.
So far, we've studied voltaic cells and their applications. We saw that voltaic cells are the basis of disposable and rechargeable batteries. They take advantage of spontaneous reactions to generate a current. We also studied electrolytic cells. They use an external voltage to start a nonspontaneous chemical reaction.
How did we deduce the voltages for either type of cell? We used the standard electrode potentials. But, there's a problem! We have to remember that chemistry is an experimental science. So, there is a difference between pure and applied chemistry. In electrochemistry, the standard potentials don't match the experimental results. We call the difference the overvoltage or overpotential.
What is Overvoltage?
How did we calculate a cell's voltage? We used the standard reduction potentials. The standard reduction potentials give us the theoretical value. But, what happens when we go into the lab? Let's say we made a galvanic cell. We were very meticulous! We made sure everything was under standard conditions. The concentrations are one molar. The temperature is . If there's a gas, its pressure equals one atmosphere. What would we observe? The voltage would be less than we predicted. The difference between the theoretical and experimental values is called the overvoltage.
What happens if we make an electrolytic cell. Again, we would use the standard electrode potentials to find our required voltage. Let's say we were performing the electrolysis of water. We would calculate our voltage to be . So, we'd set our external voltage source to . What would happen? Absolutely nothing! To start the reaction, you would need a larger voltage. Again, the difference between the expected and the actual voltage is the overvoltage.
What Causes Overvoltage?
But what's the cause of overvoltages? Like most things in chemistry, it's the difference between thermodynamics and kinetics. Do you remember how we defined the voltage? It was the difference between the electrons' potential energies. The electrons want to be more stable. So, they flow from high potential energy to low potential energy. Using the standard electrode potentials is like subtracting the internal energies.
To understand overvoltage, we need to remember kinetics. Kinetics is the study of reaction rates. But, we also talked about how chemical reactions happen. Every chemical reaction has an activation energy. That's the energy required to convert the reactants to the transition state. The transition state sits at higher potential energy than the products. The voltage we calculated was the difference between the reactants' and products' energies. But, we didn't account for the transition state.
But what is the transition state? In chemical reactions, we have to break the bonds. But, most of the electrodes we've seen are elemental metals. Remember, metals experience metallic bonding. The nuclei's protons attract the electrons. So, to transfer an electron, we need to free it from the valence shell. Remember, that's the ionization energy. So, the transition state is the excitation of the electron. Once it's free of the nuclei's electrostatic attraction, it can travel through the wire.
Other Sources of Overvoltage
There are other sources of overvoltage. Let's imagine we set up a voltaic cell in the lab. The electrolyte has a one molar concentration. What happens as the reaction proceeds? The anode gets oxidized. So, it loses electrons to become an ion. The ion enters the solution. At the same time, the cathode gets reduced. The ions gain electrons and become solid. What's the result? Our concentrations are changing. We studied how concentration affects voltage. So, it shouldn't surprise us that concentration changes cause overvoltage. We would also expect this if one of the products is a gas. The gas diffuses out of the solution. So, the concentration changes.
