4C - Electrochemistry Flashcards
While electrochemical cells share lots of similarities in electric quantities, electrolytic cells differ in their spontaneity. Explain the underlying reason for this difference. What are these cells typically used for?
Typically with these cells, an external voltage/source has to be applied to the cell in order for the cell to function. This idea can be counterintuitive because the classic definition of a battery is to provide a source of electrical voltage. However in this case, this cell requires voltage, but the inner workings of the cell mirrors a battery In a single compartment, 2 electrodes exist and different Redox reactions still take place.
These types of batteries are important in driving a reaction that is nonspontaneous!
From general chemistry, you understand that the reaction of Cl2(g) and Na(l) into NaCl is a very spontaneous reaction as a salt is very stable and Na and Cl are very reactive compounds. If you wanted to use a battery to drive the reverse reaction, which one would you use?
The process in which you are attempting to use is electrolysis: a redox reaction driven by an external source, an electrolytic cell is what you would want to use. These use the energy of another voltage source and allow non spontaneous reactions to occur. Therefore these types of cells really house thermodynamically unfavorable reactions only while it does not output any energy
Why is there no need to have to separate compartments in electrolytic cells?
Compartments as seen in galvanic and concentration cells exist because of the need to control and separate spontaneous reactions. Because the reaction desired electrolytic cells is nonspontaneous, there is no concern about having the reaction run when it is not desired and there is no need for tight control
Hydrogen fuel has been argued as the future of fuel, but the problem of this is that H2 is never found naturally due to how inert and unresponsive it is to other elements. Due to this, H2 is not found naturally in nature and therefore not naturally thought of as a fuel. How does electrolytic cells solve this issue in big industries?
Large companies can utilize electrolytic cells and make H2 fuel through something that we have readily at human disposal: H2O. The splitting of O and H in water allows a source of H fuel readily for use when it is stored properly
How much current is required to produce 0.23kg Na from a molten NaCl electrolytic cell
that runs for 30 hours? Assume the cell is 100% efficient
Moles of metal: It/nF
I = (molnF)/t
230g Na (mol Na/23gNa) = 10 mol Na
Hr -> sec; 30h (3600s/1h) = 108,000 seconds
n = the number of electrons transferred = 1 electron
I = [(10mol)(1 e) (10^5C/e)] / 108,000 s =~10C/s = 10A
An electrolytic cell is filled with water. Which of the following will move toward the cathode of such a cell? I. H+ ions II. O2- ions III. Electrons A. I only B. II only C. I and III only
C. Electrons ALWAYS flow from anode to cathode for in all electrochemical cell types, therefore option III is also correct.
Option II is incorrect is incorrect for 2 reasons because (1) it is unlikely that the anions in any cell would be O2- rather than OH- and (2) anions would flow to the anode, NOT the cathode!
Mnemonic: An Ox and a RED CAT
Oxidation - Anode
Reduction - Cathode
This is seen in ALL Electrochemical cells. Both are electrodes and all electrochemical cells have these electrodes in order to function
Zn (s) is converted to Zn2+ in an electrochemical cell. The compartment at which this occurs A. Cathode B. Salt bridge C. Anode D. Reduction
C. Anode - Oxidation occurs at the anode of all electrochemical cells.This particular example is classic of galvanic cells
In a battery, Cl- moves to an electrode and is converted to Cl2(g). The reaction at this electrode A. Base reaction B. Oxidation C. Reduction D. Acid reaction
Oxidation reaction. Cl- loses an electron and creates a bond with another to form Cl2(g). The name of the electrode is named based on the reaction process that occurs. Because this is an oxidation, this electrode is an anode
Note: this example is classic of an electrolytic cell.
Describe how the cathode of a rechargeable cell differs from a concentration cell
Cathodes of electrochemical cells are all reduction processes therefore the molecules of this electrode gains electrons
Concentration cells - the electrode in the more concentrated solution becomes the cathode. This is where reduction occurs
In rechargeable cells - these anodes are the negative electrode where oxidation occurs. These electrodes are negative because this side of the cell is where the source of electrons
Often times, there are charges assigned to different electrodes of an electrochemical cell. Explain when this difference appears.
The signs of an electrode is changes based on the battery type.
Galvanic cell: An electrode is considered to be negative when this electrode has electrons/source of electrodes. The electrode without electrons = +
Electrolytic cell: The sign of electrodes is dependent on the electrode attachment to the pole of the external voltage source (and the charge of the pole it is attached to). Anodes = + because these electrodes are attached to the positive pole of external voltage source; anions are attracted to this pole/electrode. Cathodes = - because attach to the - pole of external voltage; cations are attracted to this pole/electrode.
Remember that while the signs change, the processes that occur at each electrode is still same in ALL electrode
Mnemonic: Anions are attracted to anode, cations are attracted to the cathode.
This is true of ALL cell types: galvanic, electrolytic or concentration cells.
Explain the role of electrolytes in batteries
Electrolytes consist of cations and anions. These solutions are sources and serves different purposes. In galvanic cells, electrolytes of anodes provide the compound a solution to deposit their oxidized molecule, and in the same cells, electrolytes provide a source for the cathode to reduce (cations in cathodes are uptaken and reduced)
Charge of electron A. 2 x10^-19 B. 1.6x10^-9 C. 3x10^-8 D. 6x10^23
A. 2 x10^-19. Precisely, the charge of an electron is equal to 1.6x10^-19 Coulomb
Faraday’s constant is often used to describe electrolysis calculations. What does it measure and how do you find it?
This is a constant describing the charge of 1 mole of electron.
Should you forget the value of it, simply use the charge of 1 electron and multiply that by Avogadro’s number
1.6x10^-19 C/electron (6x10^23 electron/1mol) = 96,485 C/mol
Use 10^5 for simple calculations on the tes
Mnemonic to remember Electrodeposition equation: Moles of Metal, It is Not Fun
mol M = It/nF M = moles of metal I = current t = time the cell was turned on for n = number of electron equivalents F = faraday’s constant; 10^5 C/mol This equation is used to determine how many moles of electrons are deposited onto a plate (at a cathode)
What mass of copper will be deposited in a Daniel cell if a current of 2A flows through the cell for 3 hours?
Goal: Find the mass with the given current and time
3h(3600s/1h) = 10800 seconds
n = Cu tends to transfer 2 electrons at a time; therefore n = 2
Ampere = C/sec
mol M = It/nF => (2C/s)(10800s) / [(2e-)(10^5 C/mol)] = 0.1 mol e- Cu
0.1mol Cu (63.5g/mol) = 6.35g
The discussion of Faraday’s law centers around electrolytic cells. What is its use? What chemical quantity is being assessed here?
Faraday’s equation and law accounts for the number of electrons transferred during the liberation of a gas/decomposition of a molecule during redox reactions. Often times normality or gram weight (chemical quantity) are the proxy measurements used to study the flow of electrons/current in a circuit
Direction of electron flow in a Electrochemical cells
When conductive materials are connected to one another, electrons will flow from a place/compartment of high [ ] to low [ ]. The place of low [ ] uses this electron and reduces the materials in the compartments.
Therefore in all electrochemical cells, electrons flow from anode to cathode and the current flows from cathode to anode.
Based on your understanding of how electrons flow from one electrode to another, how does current flow in electrochemical cells.
Electrons flow from anode to cathode (A -> C) because of ABC
Current is the opposite in charge but equal in magnitude value, current therefore flows from cathode to anode!
Upon studying for the biochemical section, you note that in isoelectric focusing (the separation technique of amino acids and proteins based on pI points) you note that positively charged molecules move to the anode and negatively charged amino acids move to the cathode. Explain how this makes sense based on your knowledge of electrochemistry.
Isoelectric focusing is the application of charge onto a gel to induce motion of amino acids and polypeptide chains (a nonspontaneous process). This means that this essentially functions much like an electrolytic cell, where the cathode is the - electrode and the anode is the + electrode (both are opposite in charges to galvanic cells)
Compare and contrast Galvanic cells from Daniel cells
Galvanic cells AKA voltaic cells are electrochemical cells which are spontaneous cells with different compartments. There are a variety of metals of electrodes
Daniel cells are galvanic cells with copper and zinc electrodes
Both are -∆G and +EMF
Explain the inner workings of voltaic cells
2 separate compartments of different electrolyte in each compartment = half cells
2 electrodes in each half cell are connected by a conductive material
Salt bridge - this consist of inert salts that can neutralize the changing electrolyte solutions in either compartments. This combats the potential changes in charges (which can hinder redox reactions)
