Chapter 12: Electrochemistry Flashcards
What is an electrochemical cell? What are the three types? Briefly describe them.
An electrochemical cells are contained systems in which redox reactions occur. The three types studied in the book are galvanic cells (voltaic cells), electrolytic cells, and concentration cells. There are also cells such as Ni-Cd batteries. That comes later.
Galvanic (voltaic cells) and concentration cells house spontaneous reactions.
Electrolytic cells contain non spontaneous reactions.
What are electrodes? What are the two kinds of electrodes? Which is the site of oxidation? The side of reduction?
Electrodes are where oxidation and reduction take place. All the types of cells contain electrodes.
The two kinds of electrodes are the
anode (-) and cathode (+).
The anode is the site of oxidation, the cathode is the site of reduction.
AN OX and a RED CAT. ANode is the site of OXidation, REDuction in the site of the CAThode.
What is the electromotive force? What happens if emf is positive? Negative?
The emf corresponds to the voltage or electrical potential difference of the cell.
If emf is positive, the cell is able to release energy (deltaG<0), the reaction is spontaneous.
If emf is negative, the cell must absorb energy (deltaG>0), the reaction is non spontaneous.
The free energy change and emf always have opposite signs.
In which way do electrons move? In which way does current (I) move?
For all electrochemical cells, electrons move from anode (+, site of reduction) to cathode (-, site of reduction).
Current (I) moves from cathode (+, site of reduction) to anode (-, site of oxidation).
In physics, it is typical to state that current is the direction of flow of a positive charges through a circuit.
The current and flow of electrons are always equal in magnitude but opposite in direction.
Are batteries influenced by temperature?
Yep.
What kind of electrochemical cells are nonrechargeable batteries? There are two names for this.
Nonrechargeable batteries are galvanic cells, also called voltaic cells.
What are voltaic cells? Describe their emf and deltaG. Describe their inner workings.
Voltaic cells are used to supply energy and are commonly used as batteries.
Their electromotive force is positive, delta G is negative and voltaic cells are therefore spontaneous. Recall that the free energy change and emf have opposite signs.
They have two electrodes of distinct chemical identity placed in separate compartments called half cells. The electrodes are connected to each other by a conductive material, such as a copper wire. Surrounding each of the electrodes is an aqueous electrolyte solution composed of cations and anions. Also connecting the two solutions is a structure called a salt bridge which consist of an inert salt.
How does a voltaic cell do work? (Hint for specific answer: redox reaction)
Electrons flow from the anode to the cathode through the wire connecting the two.
By separating the reduction and oxidation half reactions into two compartments, we are able to harness this energy and use it to do work by connecting various electrical devices into the circuit between the two electrodes.
What is a Daniell cell? Talk about the redox reaction equation between Zn and Cu. Which one is being oxidized/reduced? Which one is the reducing/oxidizing agent? Which element is which electrode? What is the direction of flow of the electrons? Direction of flow of current (I)?
A Daniell cell is a typical voltaic, or galvanic, cell where the cations in the half cell solutions are the same element as the respective metal electrode.
What is a salt bridge and why is it important to have a salt bridge in galvanic (voltaic) cells? In other words, what purpose does a salt bridge perform? The picture says 1000 words. Use Daniell cell as example (Zn+ anode and Cu+ cathode)
Excess positive charge would build up on the anode and an excess negative charge was buildup on the cathode if there were no ions to replace ions created by the anode or cathode (Zn and Cu, respectively). The successive charge accumulation would provide a counter voltage large enough to prevent the oxidation reduction reaction from taking place, and the current would cease.
This charge gradient is dissipated by the presence of a salt bridge which permits the exchange of cations and anions. The salt bridge contains an inert electrolyte, such as KCL or NH4NO3. Anions (like Cl-) from the salt bridge diffuse into the solution on the anode side to balance out the charge of newly created Zn 2+ ions, the cations of the salt bridge (like K+) will balance the charge of the sulfate ions left in solution when Cu2+ ions are reduced to Cu and precipitate into the electrode.
What is plating or galvanizing? Use a Daniell cell as an example.
The precipitation process onto a cathode caused by the redox reaction in a voltaic (galvanic) cell.
Cu2+ are reduced to Cu(s) in a Daniell cell. The precipitation of Cu2+ onto the cathode (the reduction of Cu2+) is called plating or galvanizing.
What two depletions cause the relatively short lifespan of batteries?
Depletion of ions from the salt bridge caused by balancing charges in the cell and the limited quantity of ion in solution acting as an oxidizing agent (the side being reduced aka the cathode side).
What is a cell diagram regarding electrochemistry? What are the rules for constructing a cell diagram? What is the cell diagram for a Daniell cell?
Muy importante
A cell diagram is a shorthand notation representing the reactions in an electrochemical cell. The rules for constructing a cell diagram are as follows:
What is an electrolytic cell? They have similarities and differences to a galvanic cell. Example of NaCl electrolysis in image.
All types of electrochemical cells have a reduction reaction happening at the cathode and an oxidation reaction happening at the anode, a current flowing from the cathode to the anode, and electron flow from the anode to cathode.
Electrolytic cells are opposite galvanic cells. The reaction in an electrolytic cells is nonspontaneous, the reaction in a voltaic cell is spontaneous. Thus the emf of electrolytic cells is negative and deltaG is positive.
Electrolytic cell half reactions do not need to be separated into different compartments because the reaction is nonspontaneous.
What is a faraday unit? What is a coulomb?
A faraday unit (F) is equivalent to the amount of charge contained in one mole of electrons:
1F= 96,485 C
ROUND THIS UP TO 10^5 C/mol e-
Coulomb (C) is SI unit of charge equal to electric charge transferred by a current of 1 amp in one second.
What is the Faraday constant?
The Faraday constant is equivalent to the amount of charge contained in one mole of electrons (10^5 C/mol electrons).
What is the electrodeposition equation?
The electrodeposition equation helps determine the number of moles of element being deposited on a plate. This equation can also be used to determine the amount of gas liberated during electrolysis.
What is a good mnemonic to remember the electrodeposition equation?
Electrodeposition equation example page 443
Couple things.
t needs to be in seconds as the coulomb is Amp/s.
n is the number of electron equivalents for a specific metal ion. For example, a Daniell cell uses copper electrode on copper sulfate solution, meaning that Cu2+ reduces to Cu(s) (oxidation state of +2 to 0, gaining two electrons) therefor n=2.
What is a concentration cell?
A concentration cell is a special type of galvanic (voltaic) cell. The distinguishing characteristic of a concentration cell and its design: THE ELECTRODES ARE CHEMICALLY IDENTICAL.
Therefore, current is generated as a function of a concentration gradient established between the two solutions surrounding the electrodes. The concentration gradient results in a potential difference between the two compartments and drives the movement of electrons in the direction that results and equilibration of the ion gradient.
When the concentration of the ionic species in the half cells of a concentration cell, the current will stop.
What is the voltage, or electromotive force of a concentration cell if the concentrations are equal?
If the concentrations in a concentration cell are equal, the voltage is zero meaning current has stopped.
What is a good representation of a concentration cell in a biological system?
In a biological system, a concentration cell is best represented by the cell membrane of a neuron. Sodium and potassium, cations, and chlorine and ions, are exchanged as needed to produce an electrical potential. In this way, a resting membrane potential (Vm) can be maintained. Disturbances of the resting membrane potential, if sufficiently large, may stimulate the firing of an action potential.
Remember that the transfer of ions and electrons during an action potential produces biochemical work. The
What kind of cell would a rechargeable cell or a rechargeable battery be considered?
A rechargeable cell or rechargeable battery is one that can function as both a galvanic and electrolytic cell. Example examples being lead acid batteries, and nickel cadmium and batteries.
What is a lead acid battery? How does it work? This is a long one. Page 444.
Lad acid battery, also known as as a lead storage battery, is a specific type of rechargeable battery.
As a voltaic cell:
When fully charged, it consist of two half cells - a Pb anode and a porous PbO2 cathode, connected by a conductive material (concentrated sulfuric acid).
When fully discharged, it consists of two PbSO4 electroplated led electrodes with a dilute concentration of sulfuric acid.
When charging, the lead acid cell is part of an electrolytic circuit, requiring the input of energy to reestablish it as a voltaic cell.
What is a Nickel-Cadmium battery?
Ni-Cd batteries are a type of rechargeable cell. They consist of two half cells made of solid cadmium (the anode) and nickel (III) (the cathode) connected by a conductive material, typically potassium hydroxide (KOh). Some AA batteries are Ni-Cd batteries.
In a galvanic cell, is the anode negative or positive? In electrolytic cell, is the anode negative or positive?
In a galvaNic cell, the anode is Negative and the cathode is positive. Because the anode of a galvanic cell is the source of electrons, it is considered the negative electrode.
In an electrolytic cell, the anode is positive and the cathode is negative. The anode of an electrolytic cell is considered positive because it is attached to the positive pole of the external voltage source, and it attracts anions from the solution.
ANions are attracted to the ANode, CATions are attracted tit he CAThode. This is true regardless of the type of cell.
Despite all the confusing charge designations, what two things are always true about anodes and cathodes and oxidation and reduction?
Oxidation ALWAYS takes place at the anode and reduction ALWAYS takes place at the cathode.
Electrons always flow through the wire from the anode to the cathode and current flows from cathode to anode.
The cathode always attracts cations in the anode, always attracts anions.
What is isoelectric focusing?
Isoelectric focusing is a technique used to separate AA or polypeptides based on their isoelectric points. The positively charged AA (protonated at the solutions pH) is I’ll suggested toward the cathode, negatively charged amino acids (deprotonated at the something’s pH) will migrate toward the anode.
Remember: anions are attracted to the anode, cations are attracted to the cathode.
Concept check electrochemical cells 12.1 page 448 question 1
Concept check electrochemical cells 12.1 page 448 question 2
Concept check electrochemical cells 12.1 page 448 question 3
Concept check electrochemical cells 12.1 page 448 question 4
Concept check electrochemical cells 12.1 page 448 question 5
Concept check electrochemical cells 12.1 page 449 question 6
What does a positive Eored indicate? Negative?
Example page 450 tendency to oxidize or reduce based on reduction potential. It would be helpful to memorize this.
The MCAT typically uses reduction potentials. However, oxidation potentials are a thing. What is important to know about oxidation potential and reduction potentials regarding their value?
It should be noted that reduction and oxidation potentials are opposite processes. To obtain the oxidation potential of a given half reaction, both the reduction half reaction and the sign of the reduction potential are reversed.
Remember: the higher the value (the more positive) the higher potential it has to be reduced or oxidized, depending on which potential you’re talking about (Eored or Eoox).
What is the standard electromotive force (emf) of a reaction? How do you calculate emf of a reaction?
Standard reduction potentials are also used to calculate the standard electromotive force, which is the difference in potential (voltage) between two half cells under standard condition conditions. The emf of a reaction is determined by calculating the difference in reduction potential between the two half cells.
Example page 451 standard reduction potential and electromotive force.
A few notes about this one:
First. Determine the oxidation and reduction half reactions. Do this by finding out which species is reduced in which species is oxidized per the given reaction.
Second. There was no need to multiply by a common denominator balance out the electrons, if there were a need, remember to not multiply the reduction potentials by that number as this would indicate a different species.
Third. To calculate the emf, we need to know which one is the cathode and which one is the anode. Recall that oxidation occurs at the anode and reduction occurs at the cathode.
Four. We ended up with a negative emf. We need to recall that the signs of emf and deltaG are opposite. Negative emf (it required an input of energy) means positive deltaG means non-spontaneous means electrolytic.
Concept check page 452 cell potentials question 1
Example is measured by setting up a cell relative to a standard hydrogen electrode, which is given a reduction potential of 0 V by convention.
Concept check page 452 cell potentials question 2
Concept check page 452 cell potentials question 3
The first reaction is electrolytic as emf is negative, therefore positive deltaG, therefore non-spontaneous. (think of a negative EMF as it took energy to react)
The second reaction is galvanic or voltaic as emf is positive, therefore negative deltaG, therefore spontaneous. (think of a positive emf as energy was released in the reaction)
Concept check page 452 cell potentials question 4
What is the equation that relates delta G and emf? What are the units of deltaG? What is the significance of the signs?
Example page 453 free energy calculation of an electrochemical cell
First break this equation up into oxidation and reduction half reactions. Balance the electrons (no need to in this one).
Determine which is the anode, and which is the cathode based on which one is oxidized in which one is reduced. Oxidation happens at the anode, reduction happens at the cathode.
Notice that the reduction potential for chlorine is greater than iron (III) and thus this reaction is happening in the opposite from their natural tenancy, clueing you that this reaction is non-spontaneous, given the net equation.
Be careful of the signs when calculating for emf.
What is the Nernst equation (non simplified using natural log, simplified using log base10). When is the Nernst equation used?
The Nernst equation is used when conditions deviate from standard conditions in an electrochemical cell.
The simplified version uses log base10 and combines T at 298K (25°C), R, and F.
Recall standard reaction quotient. Apply Q to a Daniell cell.
Example page 455 of calculating emf of a cell under non-standard conditions.
Regarding standard and non-standard condition conditions for electrochemical cells, what happens when the concentration of the products is equal? Show your equation using the Nernst equation. The purpose of this card is to show you standard and non-standard conditions and application of the Nernst equation.
This is a good card.
It is important to realize that, even though the concentrations of the products are equal, temperature can still impact Q and therefore impact emf. Know that emf is temperature (T) and reaction quotient (Q) dependent.
How is deltaG determined using Keq? Relate this to positive and negative and 0 Keq.
Combine the two expressions that solve for standard free energy change.
The purpose of this card is to see what happens to Eocell regarding Keq.
Combine the two expressions that solve for standard free energy change.
The purpose of this card is to see what happens to Eocell regarding Keq.
What can be said about electrochemical cells (redox reactions) if:
Keq is less than one.
Keq is greater than one.
Keq is equal to one.
For redox reactions with equilibrium constant less than one (equilibrium state favors the reactants) the Eocell (emf) will be negative because the natural logarithm of any number between zero and one is negative. THESE PROPERTIES ARE CHARACTERISTIC OF AN ELECTROLYTIC CELL, WHICH HAS NON-SPONTANEOUS REDOX REACTIONS.
If the equilibrium constant for the reaction is greater than one (equilibrium state favors the products), the Eocell (emf) will be positive because the natural algorithm of any number greater than one is positive. THESE PROPERTIES ARE CHARACTERISTIC OF GALVANIC CELLS, WHICH HAS SPONTANEOUS REDOX REACTIONS.
If the equilibrium constant is equal to one (concentrations of the reactants and products are equal at equilibrium), the Eocell (emf) will be equal to zero. An easy way to remember this is that Eocell = 0V for any concentration cell with equal molar concentrations in both half cells because there is no net ionic equation (both half cells contain the same ion).
What is the equation for change in Gibbs free energy of an electrochemical cell with varying concentrations?
Concept check 12.3 page 457 emf and thermodynamics question 1
Concept check 12.3 page 457 emf and thermodynamics question 2
Note that these calculations do not assume standard conditions.
Mastery chapter 12 electrochemistry page 432 question 1
Mastery chapter 12 electrochemistry page 432 question 2
Kat is zinc because it is being reduced, the antinode is silver because it is being oxidized. You need to look at the reaction equation to determine which is being oxidized, which is being reduced.
Sidenote. emf is negative, deltaG is positive, nonspontaneous, therefor electrolytic.
Mastery chapter 12 electrochemistry page 432 question 3
Mastery chapter 12 electrochemistry page 432 question 4
Mastery chapter 12 electrochemistry page 432 question 5
Mastery chapter 12 electrochemistry page 432 question 6
Mastery chapter 12 electrochemistry page 432 question 7
Mastery chapter 12 electrochemistry page 432 question 8
Mastery chapter 12 electrochemistry page 432 question 9
Sodium can’t be the cathode. Process of elimination. Calculate the emf, get positive meaning its electrolytic. More of a process of elimination here I suppose.
Mastery chapter 12 electrochemistry page 432 question 10
Mastery chapter 12 electrochemistry page 432 question 11
Mastery chapter 12 electrochemistry page 432 question 12
Mastery chapter 12 electrochemistry page 432 question 13
Mastery chapter 12 electrochemistry page 432 question 14
Mastery chapter 12 electrochemistry page 432 question 15
