Electrochemistry Lecture 0 Flashcards
How do we measure the half-cell reactions?
The half-cell reactions are measured by their standard potential values E^o. Note that these values are given when the half-cell is written in the reduction form so sometimes when doing calculation we would need to switch the sign
What is the standard cell potential equation?
it is the difference between the standard potential at the cathode E◦ (cathode) and the standard potential of the anode E◦ (anode) . In other words it is E (reduction)- E (oxidation). Note reduction will always happen at the cathode (Remember the mnemonic RED-CAT)
Which species gets oxidized and which species get reduced when there are multiple species in a given half-cell?
the more oxidizing species will get reduced at the cathode, and the more reducing species will get oxidized at the anode.
What is the effect of a media on the half-cell reactions?
Half-cell reactions in which the same species are reduced/oxidized will change depending on the media in which they take place. In acidic media, the proton species produced or donated are hydrogen ions, while in alkaline media these will be hydroxide ions. When the media-specific reactions are not available (ya3ni you don’t have a pre-written, standard half-cell reaction equation specifically balanced for either acidic or alkaline conditions), one needs to balance the reaction equation for the respective media
How do we balance reaction equations in a given media? (imp will be on DIAMS)
In acidic media:
- Split the equation into half equations
- Balance elements other than oxygen or hydrogen
- Balance oxygen atoms with water
- Balance H atoms by adding H+
- Balance charge by adding e -
- Match the number of electrons
- Simplify the equations and cancel out same terms, add the equations together.
Basic media:
All the steps in acidic but with an extra step of balancing the H^+ with OH
Examples are shown in written notes
What is the Nernst Equation?
The standard potentials reported in literature, and the standard cell potentials calculated from those values, hold only at standard conditions (1 M, 1 bar, 25 °C). The potential (𝐸) at an arbitrary composition and conditions can be related to the standard potential through the Nernst equation:
𝐸 = 𝐸0 − 𝑅𝑇/𝜈𝑒𝐹 ln 𝑄
With 𝑅 the molar gas constant (8.3145 J mol−1 K−1), 𝑇 the temperature in K, 𝜈𝑒 the number (of moles) of electrons exchanged, 𝐹 the Faraday constant (96485 C mol−1) and 𝑄 as the reaction quotient.
Cell potential and spontaneity?
If 𝐸𝑐𝑒𝑙𝑙 is larger than 0, the reaction is spontaneous; if 𝐸𝑐𝑒𝑙𝑙 is smaller than zero the reaction is non-spontaneous.
What happens to the Nernst equation when equilibrium is reached?
When a reaction reaches equilibrium, i.e. the forward and backward reactions progress at the same rate, the cell potential equals 0. In this instance, the reaction quotient turns into the equilibrium constant 𝐾. Knowing that the cell potential is zero, the Nernst equation can be rewritten to the following:
ln 𝐾 = 𝜈𝑒𝐹/(𝐸0𝑐𝑒𝑙𝑙)*𝑅𝑇
Example of Nernst Equation?
Shown in the reader i printer
What is an important note to remember when it comes to redox reaction and generally the Nernest equation?
A considerable number of redox reactions involve the transfer of hydrogen or hydroxide ions. Therefore the potentials of the reactions depend on the pH in the system. Because multiple numbers of H+ or OH− ions are often involved, the potential can vary greatly with the pH (logically with the fact that the concentration of the H or OH (when included in the equation for Q) is to the power of its coefficient, we ofc can see the impact).
We can also define the potential of the cell using the pH and the number of protons as shown in written notes.
Break moment
Now we move on the the most confusing bit of the recap, the electrochemical cells.
How can we control an electric parameter?
To control an electric parameter (potential/current), like with a power supply, the two terminals of the electrochemical system must be made of electronic conducting materials. The system
must also include at least one material that conducts ions termed the electrolyte. The solution may also contain non-electroactive species that, for example, increase the conductivity, termed the supporting electrolyte
What is the formal definition of an electrode?
The term “electrode” is commonly
used in electrochemistry, with electrodes taking on many different forms. Commonly electrodes are made of metal, serving as one of the terminals in the system.
What is the electrochemical interface?
The electrochemical interface is a highly specific zone where a redox half-reaction occurs, with the zone having a typical thickness of a few nanometers.
What are the two categories of electrochemical cells? + definitions
- Galvanic cells: Galvanic cells are electrochemical cells in which spontaneous oxidation-reduction reactions produce electrical energy. The positive electrode is the cathode and the negative electrode is the anode. In galvanic cells the current flows from the positive to the negative electrode.
- Electrolytic cells: An electrolytic cell is in various senses the opposite of a galvanic cell. In an electrolytic cell the reaction is non-spontaneous, meaning that external electricity needs to be supplied to run the
reaction. In electrolytic cells the positive electrode has become the anode and the negative electrode has become the cathode.
A beautiful table represents all of this in the reader
How can we determine whether the electrodes can exist in the same compartment, sharing the electrolyte
Answer in Reader pag 9 ;)
What is the short had notation?
It is a notation used to easily donate electrochemical cells:
Anode | Anodic electrolyte + species || Cathodic electrolyte + species | Cathode
Anode at the left side
Cathode at the right side
|| indicates a salt bridge
When two or more
components exist in the same phase they are separated by a comma
indicates a phase boundary
What is a current?
The current going through an electrochemical cell is a measure of the reaction rate in the case of a single reaction occurring at the given electrode. As such the current characterizes how much
of a chemical species has been consumed or produced.
What is Faraday’s law
Faraday’s law relates the current or
charge going through the system to the number of moles produced or consumed of a chemical species.
𝑛𝑖 = (𝑄/𝐹) ( 1/𝜈𝑒) = 𝑖𝑡/𝐹𝜈𝑒
With 𝑛𝑖 the number of moles produced/consumed of species 𝑖, 𝑄 the charge in C, 𝑖 the current in A and 𝑡 the time in s.
What is Faradaic efficiency?
In certain systems it is possible for side reactions to occur due to for example side reactions occurring at the electrode. In these instances one can define the so-called Faradaic efficiency:
𝐹𝐸 = 𝑛𝑜𝑏𝑡𝑎𝑖𝑛𝑒𝑑/𝑛𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙
