Electrochemistry Lecture 1 Flashcards
What is the definition of potential in electrochemistry? (kind of tricky but important to understand)
Electric potential is a measure of electric energy per unit charge at a specific point in a circuit. However, it’s important to note that potential is always defined relative to a reference point, often called the ground or zero potential.
Think of it like measuring height: You can’t say how high a mountain is without comparing it to sea level. Similarly, you can’t measure electric potential without choosing a reference point.
There is no such thing as absolute potential—only potential differences, which are also called voltages. This is because adding or subtracting a constant value from all potentials does not change the difference between them.
The potential difference (voltage) between two points is what causes electric charges to move in a circuit. It’s like the difference in height between two points that drives the flow of water downhill.
In summary:
- Potential is a measure of electric energy relative to a reference point.
- Voltage (potential difference) is what we actually measure—how much energy a charge gains or loses between two points.
- Potential differences drive electric currents, just as height differences drive water flow.
What are the different Potential terms we use in this course (imp)?
- Ecell = voltage of the electrochemical cell (U)
- E0cell = voltage of the electrochemical cell
under standard conditions (EMF) - E0 = standard potential of the half-reaction
- Enernst =potential in non-standard conditions
this is important when it comes to an exam to understand exactly what they are asking of you ;)
What causes the generation of a current of a current or current density?
The movement of multiple types of charged species causes the generation of a current or current density.
What is a current, current density, and current intensity?
- Current is the rate at which electric charge flows through a conductor:
𝑖 = ∫∫ 𝐣 ⋅ 𝐧 𝑑𝑆 = 𝑑𝑄/𝑑𝑡
It measures the total flow of charges passing through a cross-section of a conductor per unit of time. Unit is Ampere (A) = Coulombs per second (C/s). It is a scalar quantity
- Current density is the electric current per unit area of cross-section. It describes how the current is distributed over an area:
𝐣 = ∑ 𝐣𝑖
it is a vector quantity and its units are ampere per square meter (A/m²)
- Current intensity is another term for electric current (I), often used in older literature or certain regions. It emphasizes the strength of the current but means the same thing as the current.
What is the convection of the current density and why do we need it?
The current density is composed of an anodic and a cathodic current. Under all operating conditions, the direction of current flow throughout the entire system, or the orientation of the current density vector at each point, is always unambiguously defined (the direction of the current density in a galvanic cell or electrolytic cell). (This is taking about how we already defined that teh current flow in both cells goes from the anode to the cathode)
It is only necessary to establish a sign
convention for the current or current density, to distinguish between the current density of the anode and the current density of the cathode. Such a convention also helps in presenting experimental data in such a way that it is easy to distinguish between an anodic and cathodic
current density (a positive/negative peek will refer to a specific electrode). The following convention will be adopted:
Anode: 𝑗 > 0
Cathode: 𝑗 < 0
Where does the convention of charge denisty come from?
The convention is based on the definition of current through a surface, where the normal vector to the interface is oriented from the metal towards the electrolyte (shown in the reader. This approach aligns
with a widely-used thermodynamic sign convention: quantities leaving the system (or provided by the system) are counted negatively, while quantities entering the system (or supplied to the system) are counted positively. The quantities in this case are electrons
Why can the convection set be valid for the entire cell?
In an electrochemical cell, all conducting regions maintain electroneutrality, meaning there is no net accumulation of charge within the medium. Because of this, the electric current remains constant throughout the entire system, regardless of where it is measured.
This principle holds even in systems with multiple interfaces, such as the boundaries between electrodes and the electrolyte. Although different reactions occur at the anode and cathode, the total current flowing into one interface must equal the total current flowing out of the other. Specifically, the current passing through the anodic interface has the same absolute value as the current passing through the cathodic interface.
The currents at the anode and cathode are equal in magnitude but opposite in direction, ensuring that the overall system remains electrically neutral. This fundamental relationship between current and electroneutrality is essential for understanding charge balance and current conservation in electrochemical systems, showing us how the convection is valid throughout the cell.
Break moment
Okay so we just made some (VERY CONFUSING and LONG) definitions of important terms in electrochemistry. Now sadly we will start talking about the relation of the electrochemical cell to thermodynamics and we will derive the nearest equation from a thermodynamically standpoint (warning you will get triggered)
What is the relation between Gibb’s free energy and electrical work done in a system? What is its derivation?
The relation is:
Δ𝐺𝑇,𝑃 = 𝑤𝑒𝑙𝑒𝑐
Derivation is found in written notes.
What is the equation that defines electrical work? What is its derivation?
The equation is 𝑤𝑒𝑙𝑒𝑐 = −𝜈𝑒𝐹𝐸
Derivation:
Consider an electrochemical cell
which has two terminals across which there is an electric potential difference 𝐸. The two terminals are connected by wires to an external load, represented by a resistance 𝑅 (This is shown in written notes).
When a charge 𝑄 is moved through a potential difference 𝐸, the work done on the surroundings is 𝐸𝑄. The charge passing through the circuit is the product of the number of charge carriers and the charge per charge carrier. All charge carriers being electrons, we can define: (look at written notes)
What is a more accurate way to represent the equilibrium constant of a system?
Up to this point when working with the Nernst equation, we have simply used the molar concentrations to define our K. Molar concentrations, however, can only be used as an approximation at low concentrations
(< 1 M). To give a more accurate representation we actually need to use activities. The definition (y3ani formulas) of activities are highlighted in the written notes
How can we derive the Nernest Equation from the rules of standard thermodynamics and the relation between the change in Gibbs free energy and potential?
Very straightforward derivation, found in the reader. Note that for the DIAMS we of course don’t need to know any derivation by heart but practicing them and kind of memorizing them is still very very important.
How does the relation between the change in Gibbs free energy and potential help us in determining other important thermodynamic equilibriums?
Thermodynamic quantities can be derived from electrochemical measurements now that we have a relation between the potential difference and the change in Gibbs free energy. Examples are shown in written notes AND THEY ARE INCREDIBLY IMPORTANT
What is the difficulty in determining the electrical potential of a half-cell, and how to we get past it?
Unfortunately, it is not possible to directly measure the potential difference between an electrode and the solution, meaning single electrode potentials cannot be uniquely defined. However, because this quantity is not directly measurable, we can arbitrarily assign a standard. potential to one-half cell, which will serve as a reference. This effectively establishes a potential of zero, against which all other half-cell potentials can be measured. Typically chemists choose the standard hydrogen electrode but more about that at the end of the lecture ;).
break moment
okay, so we defined important terms, showed how thermodynamic quantities can be calculated by electrochemical cell measurements, derived the Nernest equation using thermodynamics, and showed how we can use a standard electrode to measure the electric potential of half cells. All this and we are still not done ;(. Now we deep diver into exactly how we measure the electrochemical potential, and (trigger warning) how chemical potential is related and how we can derive the Nernest equation from said chemical potentials.
How do we exactly measure potential differences in an electrochemical cell?
Devices designed to measure potential
differences (e.g., potentiometers, voltmeters, or electrometers) are typically calibrated to register potential differences only between TWO phases of the SAME composition, such as the two metal contacts found on most instruments.
To better explain this we will look at an example:
Consider a cell with a Zn anode and Zn2+ in
the electrolyte, a silver/silver chloride cathode and copper contacts (Shown in the reader). The potential difference between the copper contacts will include the potential difference across each interface: The Cu-Zn interface, the Zn-electrolyte interface, the electrolyte-Ag interface and the Ag-Cu’ interface (arrows are used in the reader to depict this).
Is it possible to focus on a singular interfacial potential difference?
Yes, for example, the Zn-electrolyte
interface. If we can keep the interfacial potentials at all other junctions in the cell constant, then any change in the potential difference must be entirely due to a change in the potential difference at the Zn/electrolyte interface.
What is the chemical potential of a species?
The chemical potential of a species is a measure of its tendency to undergo change, such as participating in a reaction or moving from one phase to another. It represents how the Gibbs free energy of a system changes when the number of particles of that species changes, holding temperature, pressure, and the number of other species constant:
𝜇𝑖 = (𝜕𝐺𝑖/𝜕𝑛𝑖)𝑛𝑗≠𝑖,𝑇,𝑃
𝐺 = ∑ 𝑛𝑖𝜇𝑖
What is the relation between chemical energy and activity?
The chemical potential depends on the species’ activist in a mixture. It is given by:
𝜇𝑖 = 𝜇𝑖 0 + 𝑅𝑇 ln 𝑎𝑖
This equation shows that as the composition of the system changes (reflected in the activity), the chemical potential changes accordingly, Which indicates how Gibbs function will change.
Why is chemical potential a powerful quantity?
A system naturally evolves to minimize its total Gibbs free energy. With the use of, chemical potential we can indicate which direction a reaction will proceed—from regions of high chemical potential to low chemical potential—to reach equilibrium.
What is electrochemical potential?
Before we define it we need some foreplay ;).
In electrochemical systems, chemical potential alone is not enough to describe the behaviour of charged species. This is because charged particles (like ions) are also influenced by electric fields. To account for this, we introduce the Galvani potential ϕ): The Galvani potential is defined as the electric potential difference between the bulk of a conductor and a vacuum at an infinite distance (Shown in the reader).
To fully describe an ion’s tendency to move or react, we add the electrical contribution (𝐹𝜙z i) to the chemical potential, resulting in the electrochemical potential:
𝜇̅ 𝑖 = 𝜇𝑖 0 + 𝑅𝑇 ln 𝑎𝑖 + 𝑧𝑖𝐹𝜙 = 𝜇𝑖 + 𝑧𝑖𝐹𝜙
With 𝜇̅𝑖 the electrochemical potential, 𝑧𝑖 the charge of species 𝑖 and 𝜙 the Galvani potential.
It is common to denote the phase in which the (electro)chemical potential is considered in the superscript.
What are the properties of the electrochemical potential?
note that 𝛼 is in the superscript position. and 𝑎 is the activity as it is different than 𝛼
- For an uncharged species: 𝜇̅𝑖(𝛼) = 𝜇𝑖(𝛼)
- For any substance: 𝜇(𝛼) = 𝜇0(𝛼) + 𝑅𝑇 ln 𝑎𝑖(𝛼)
- For a pure phase at unit activity: 𝜇̅𝑖(𝛼) = 𝜇𝑖(0𝛼)
- For electrons in a metal (𝑧 = -1): 𝜇̅𝑒(𝛼) = 𝜇𝑒(0𝛼) − 𝐹𝜙(𝛼). Activity effects can be disregarded because the electron concentration never changes appreciably.
- For equilibrium of species 𝑖 between phases 𝛼 and 𝛽: 𝜇̅𝑖(𝛼) = 𝜇̅𝑖(𝛽)
What is the derivation for the Nernest equation using electrochemical potential?
Okay the derivation is found in the reader, again we don’t need to memorize it but understanding it is important, don’t be scared of it it is just algebra and applying the rules stated in the previous FC, note however that I think some signs were written wrong (or just switched around)
break moment
Okay now we will just talk about the different reference electrodes, she said we just need to understand and not memorize them since we are doing DIAMS but well see if she means it or is just lying to us
