Electrochemistry (E4) Flashcards
Oxidation/Reduction, Cathode, Anode, Battery, Salt Bridge, Potential Energy and Voltage and their dependence on Temperature (increase or decrease depending on exo/endo?)
In electrochemical cells, how do electrons from the oxidation reaction reach the reduction reaction?
run through external circuit (wire)
In electrochemical cells, what leaves the reduction side and how (through what)? Why is this necessary?
spectator ions move through salt bridge. this is necessary because electrons are flowing in so the spectator ions need to go to the oxidation side to maintain a balanced charge on both sides
what connects the wire to each half of the electrochemical cell?
electrode
where does the chemistry take place in the electrochemical cell?
on the surface of the electrode
what is the electrode on the oxidation side called?
anode
what is the electrode on the reduction side called?
cathode
what is another name for the reduction side?
cathode side
what is another name for the oxidation side?
anode side
a chemical reaction can be spontaneous in _____ direction(s)
one
what is voltaic cell? what are other names for “voltaic cell”?
a cell in electrochemistry where the reaction occurs spontaneously in one direction. oxidation will occur spontaneously at the anode and the reduction will occur spontaneously at the cathode. i.e. e- pushed from anodic (oxidation) side across wire to cathodic (reduction) side
other names: galvanic cell or battery
what is an electrolytic cell?
requires an external power source to drive the chemistry in a non-spontaneous direction
how can we change the direction of a chemical reaction?
use external voltage to adjust free energy (lowering free energy in products or reactants)
what kind of reaction occurs in an electrochemical cell?
redox reaction
oxidation reactions _______ electrons
lose/”produce”
reduction reactions _______ electrons
gain/”consume”
are electrodes parts of the reaction?
sometimes, but sometimes they simply serve as a means to “deliver/collect” electrons
Which way do ions flow across the salt bridge?
depends. positive ions can flow from the anode side to the cathode side or negative ions can flow from the cathode side to the anode side
what is specially required for reactions with no metal reactants and no metal products in an electrochemical cell
an inert (non-reactive) metal electrode
are ions in the salt bridge part of the chemical reaction?
it depends. they can be, but it could be any strong electrolyte. they don’t have to participate in the chemistry
how do you write the short hand notation for an electrochemical cell?
atomic symbol for solid anode | aqueous components of anodic side || reactant of cathodic side | product of cathodic side | cathode (refer to notes)
What is SHE? What are the values?
standard hydrogen electrode
2H+(aq) + 2e- —> H2(g)
H+ = 1 M
H2 = 1 atm
For half reactions, how do we measure their potential (free) energy? What do we compare it to?
We use the hydrogen electrode as the standard, so the potential energy of a half reaction can be greater or less than SHE. The hydrogen electrode’s standard energy is 0??
We put the hydrogen electrode on one side of the cell and put the other half reaction on the other side of the cell to determine the unknown’s potential energy. We then measure the oxidation/reduction and then determine the reduction potential for the unknown half reaction.
Half reactions with positive potential (free energy) are strong ______?
Oxidizing agents -> They are reduced and gain electrons. The more positive the potential, the more they want to take electrons.
Half reactions with negative potential (free energy) are strong _______?
Reducing agents -> They are oxidized and lose electrons. The more negative the potential, the more forceful they are in giving electrons.
The strongest oxidizing agent is
F2 (Fluorine)
The strong reducing agent is
Li (Lithium)
Why did we choose the hydrogen electrode as the standard?
I don’t know. We could choose whatever we want, but this is what most people chose. Remember the standard COULD be anything, but then everything must be related to that standard. Eg. Since SHE is often used, F2 will always be +3___ and Li -#___ because hydrogen is used as the “standard” where H+ (aq) is 1M and H2 (g) is 1 atm.
For this course, the standard potentials on the table are? The LHS of the equation _____ in oxidizing agent strength as you go _____. The RHS of the equation ______ in reducing agent strength as you go ______.
- reduction potential
- increases, up
- decreases, down
oxidizing agents want to be ______. Strong oxidizing agents will ______ electrons
- reduced
- forcefully take
reducing agents want to be ______. Strong reducing agents will ______ electrons
- oxidized
- forcefully give
lower (eg negative) standard potential energy means
the RHS of the equation is a good reducing agent. non-spontaneous??? reactions goes backwards??? the more negative, the harder it is to reduce that half reaction (remember table is written in reduction half reactions). likewise, the easier it is to oxidize.
higher (eg positive) standard potential energy means
the LHS of the equation is a good oxidizing agent. spontaneous??? reaction goes forward??? the more positive, the easier it is to reduce that half reaction (remember the table is written in reduction half reactions). likewise, the harder it is to oxidize.
The LHS of the standard reduction table are the half-reactions that are going to be _______. The RHS of the standard reduction table are the half-reactions that are going to be _____.
- reduced
- oxidized
Will Ag+ oxidize Fe? Why or why not?
Table link:
http://ch301.cm.utexas.edu/data/standard-potentials.php
Yes. reduction potential of Ag+ > reduction potential of Fe
Ag+ on LHS and Fe on RHS
Standard Potential equation for a Cell
Where do we get these values?
E standard of cell = E standard of cathode - E standard of anode
OR (since cell should be written anode || cathode | electrode? eg Pt)
E = right - left
Get values from table of standard reduction potentials
Is potential energy dependent on number of electrons?
NO. potential (free energy) is simply the energy difference between two half reactions.
the standard potential for a voltaic cell is ____
positive
how are standard potential energy and standard free energy related? what does this mean?
negative free energy = spontaneous = positive standard potential energy
likewise
positive free energy = non spontaneous = negative standard potential energy
how can we measure free energy difference in electrochemical cells?
measure the difference in electrical potential
how do electrons flow in voltaic cells? electrolytic cells?
in both, electrons flow from the anode to the cathode via a wire even though voltaic cells are spontaneous (electrons are driven through the external circuit by the free energy of the spontaneous redox reactions occuring in each half cell) and electrolytic cells are nonspontaneous
what is the sign on a cathode for a voltaic cell? why? what is the sign on a cathode for an electrolytic cell? why?
voltaic:
+
drawing in electrons to “feed” reduction, so the cathode must be positively charged
because reaction is not spontaneous, so there’s nothing there to pull/push electrons. so electrons are FORCED onto the cathode, making it negative
what is the sign on an anode for a voltaic cell? why? what is the sign on a anode for an electrolytic cell? why?
pushing electrons out, so must be a negative charge
electrolytic:
+
nonspontaneous. electrons are forcefully REMOVED from the anode, making it positive
how do we force a reaction to occur in an electrolytic cell?
attach external power supply to overcome uphill in free energy. voltage must be significant enough to overcome the negative potential of the cell itself (which is spontaneous in the reverse direction)
the standard potential for an electrolytic cell is _____. its free energy is ______
negative, positive (nonspontaneous)
other names for “electrolytic cell”?
electroplating, electrodeposition
why would an electrolytic cell be used?
to generate unstable (high free energy) compounds or elements from stable (low free energy) compounds
what must we separate in voltaic cells? why? electrolytic cells?
reactants because they could react with one another since voltaic cells are spontaneous
we don’t have to separate reactants in electrolytic cells because electrolytic cells are nonspontaneous, but we DO have to separate the products because they will spontaneously recombine
current
the amount of electrical charge that flows during a period of time (charge/time)
eg 1 Amp = 1 Coulomb/sec
Why do we use Faraday’s constant? What is it?
to convert charge to numbers of electrons
charge in Coulombs of 1 mole of electrons
= 96485 C/mole. derived from charge on one electron divided by Avogadro’s number (1.602E-19/6.022E23).
formal def’n: the charge in Coulombs carried by one mole of electrons
Faraday’s Law of Electrolysis
the amount of product formed or reactant consumed by an electric current is stoichiometrically equivalent to the amount of electrons supplied
in electrochemistry, how can the “amount of chemistry” by quantified? any special cases?
measuring current over time. current might change over long periods of time, but it will likely remain constant for short periods.
