Exam Two: Electrochem Flashcards
Redox reactions are what
electron transfer reactions
A redox reaction is the sum of
two half reactions
Oxidation is the
loss of electrons
Reduction is the
gain of electrons
Chemical redox reactions occur
in solution. Oxidant and reductant are directly mixed with one another
What causes redox reactions in solution
the direct collision of atoms, ions, etcleading to a direct transfer of electrons
In an electrochemical reaction
the two half reactions are physically separated
How are electrons transferred in an electrochemical cell
- through external circuit
- In electrodes
- In solution
- Inside salt bridge
What are the anolyte and the catholyte connected by
a salt bridge
oxidation occurs at the
anode
reduction occurs at the
cathode
In electrodes and external circuits, what is electron transfer based on
mechanism of current transfer in conductive metals
In the interface of electrodes and electrolytes, what is electron transfer based on
oxidation- reductions reactions
In solution, what is electron transfer based on
the migration of charged particles (ions)
anions move towards
anode
cations move towards
cathode
How are electrons transferred in the salt bridge?
Through moving ions
What is the the salt bridge composed of
agar saturated with KNO3 or KCl
Galvanic (Voltaic) cells generate electricity from a _____ redox reaction
spontaneous
Why should the anolyte and catholyte be physically separated
chemical reaction between the oxidizing agent and reducing agent would happen
Whats an example of the galvanic cell
Daniel cell
Electrolytic cells are
non spontaneous - external energy is needed to induce the reaction
example of electrolytic cell
electrolysis of water
___ is always on the left side of the salt bridge
The anode
___ is always on the right side of the salt bridge
The cathode
what is current
(I) is the flow of electric charge through a conductor such as a wire
What is the unit on current
A- Ampere
What is an ampere
the amount of electric charge flowing per second
What is charge
q
what is the unit of q
(C) coulomb
What is the electric current in an electrochemical proportional to?
the rate of reaction
Faraday’s number is the
charge of one mole of electrons
What is electric potential
(E) is the work needed (or that can be done) when moving an electric charge from one point to another
what is the unit of E
(V) volt
what is a volt
the potential difference between two points that will impart one joule of energy per coulomb of charge that passes through it
You get E with a ____ reaction
Spontaneous
You use E with a ____ reaction
Nonspontaneous
What is free energy change?
The maximum possible electrical work that cna be done by a reversible chemical reaction at constant temperature and pressure
Delta G is
free energy change
What is Ohm’s law
(1) Current is directly proportional to the potential difference across a circuit
(2) Current is inversely proportional to resistance of a circuit
What is power
work done per unit time
What is unit on power
J/s or watt (W)
Higher Current= ____ Power
HIgher
What does cell potential depend on?
The potential of both half cells
What are relative potentials of half-cells are measured against a ______
reference electrode
what is S.H.E.
Standard Hydrogen electrode
The standard reduction potential for any other half cell can be measured by
connecting them to S.H.E
When is an electrode “standard”
when it’s activity is 1 or it’s 1 M
Can the S.H.E. be the anode or the cathode?
Both
Standard half-cells reactions are written as
reductions
If Eo is positive the reaction is taking place in the
cathode
if Eo is negative the reaction is taking place in the
anode
A larger Eo indicates
better reduction
A smaller Eo indicates
better oxidation
The effect of concentration on the electrode potential is expressed by
Nerst Equation
The standard electrode potential for a half reaction can be defined as
electrode potential of the half-reaction when reactants and products all ave unit activity
Characteristics of standard electrode potential
- relative potential (relative to SHE)
- written for reduction half-reaction\
- (+) sign when electrode acts as cathode against SHE
- (-) sign wen electrode acts as anode against SHE
- Depends on temperature
- Not changed when half reaction is multiplied by any number to balance
What is Ecell
potential of complete cell
What is E
potential of complete reaction
What is Ec
potential of cathodic (reduction) half reaction
What is Ea
potential of anodic (oxidation) half reaction
If Ecell>0 the cell is
galvantic. spontaneos
If Ecell<0 the cell is
electrolytic. non-spontaneous
If Ea > Ec, Eo is ___, therefore the equilibrium constant, K, ____
negative, decreases
What is thermodynamic standard potential?
Standard potential of an electrode at ionic strength (µ) of zero
What is concentration standard potential?
By changing the ionic strength, thermodynamic standard potential is changed to concentration or apparent standard potential which has a different value.
What is true of gamma in concentration standard potential?
gamma=1 bc [] is low
What is conditional (formal) standard potential
Standard potential value dependent on activity (concentration) of a chemical species other than Red and Ox such as H+, OH-, Cl-, I-, NH3
What is potentiometric analysis
Analysis of a two-electrode system including a reference electrode and an indicator electrode
What is reference electrode
electrode with a fixed composition and constant potential
Three reference electrodes
- Standard hydrogen electrode (SHE)
- Silver-Silver Chloride Electrode (Ag/AgCl)
- Calomel Electrode (Hg/Hg2Cl2)
Whats an indicator electrode
The electrode with a potential related to the concentration of analyte
Two indicator electrodes
- metallic indicator electrodes
2. membrane indicator electrodes
Arbitrarily, wha is E of SHE
0 V
SHE is used in all
Calculations
SHE is not appropriate for
daily use
What does the electrode potential of the silver-silver chloride electrode depend on
the concentration/ activity of Cl-
What does the electrode potential of the calomel electrode depend on
the concentration/activity of Cl-
E of saturated calomel electrode (SCE)
+0.241
E of normal calomel electrode (NCE)
+0.280
E of desinormal calomel electrode (DCE)
+0.334
What are metallic indicators of the first kind
Electrodes that respond directly to changing the activity of the electrode cation
Two examples of metallic indicators of the first kind
- Copper Indicator: Cu l CuSO4 (xM)
2. Silver indicator: Ag l AgNO3 (xM)
What is need to measure cation concentration with metallic indictor electrodes of the first kind
reference electrode
Why are double junction electrodes used?
Assure there is no interference
What are metallic indicators of the second kind?
Electrodes that respond to changes in anion activity though formation of a complex or precipitation
Example of a soluble complex measured by an indicator electrode of the second kind?
Cu l CuY2- (aM), Y4- (x M)
Example of a precipitate measured by an indicator electrode of the second kind?
Ag l AgCl(s) l Cl- (x M)
What are redox electrodes
electrodes made of inert metals such as Pt, Au ad Pd, which are immersed in a solution of redox pairs
What is liquid junction potential
liquid junction potential develops across the boundary between two electrolyte solutions that have different compositions
What leads to ion diffusion is liquid junction potential
the separation of charge
What is ion mobility
the terminal velocity that a particle achieves in an electric field
What ions have the highest mobility
H+ and OH-
How can liquid junction potential be minimized?
placing a salt bridge between the two solutions
When is the salt bridge most effective in minimizing mobility?
- The mobility of the negative and positive ions in the bridge are nearly equal
- The concentration of the ions is large
What are membrane indicator electrodes?
Ion-selective electrodes
- made of thin membrane capable of binding only the intended ion
- do not involve redox process
- called ion selective electrodes (ISE) because they respond selectively to one ion
Crystalline membrane electrodes
- single crystal
2. polycrystalline
Non crystalline membrane electrodes
- glass
- liquid
- immobilized liquid in a rigid polymer
Example of single crystalline membrane electrode
LaF3 for F-
Example of poly crystalline membrane electrode
Ags for S2- and Ag+
Example of glass non crystalline membrane electrode
silicate glasses for Na+ and H+
Example of liquid, noncrystalline membrane electrode
liquid ion exchangers for Ca2+
neutral carries for K+
Example of immobilized liquid in a ridged polymer, non crystalline electrode
PVC matrix for Ca2+ and NO3-
What’s a possible issue with crystalline electrodes
They can dissolve and need to be replaced
What errors are associated with pH measurements
Alkaline error/ sodium error
Acid error
What is alkaline/ sodium error in pH measurments
WHen [H+] is very low and [Na+] is high, the electrode responds to Na+ and the apparent pH is lower than the true pH.
WHat is acid error in pH measuremnts
In strong acid, the measured pH is higher than the actual pH, perhaps because the glass is saturated with H+ and cannot be further protonated.
In ion selective electrodes what is selectivity coefficient (K)
Gives relative response to different species with the same charge. (an electrode is intended to measure A but also responds to X where X is the interfering ion)
The smaller the selectivity coefficient, the ____ interference there is by X
less
In the general equation for potential of ion selective electrodes, when is ion charge zA, posititve
when the analyte is cation
In the general equation for potential of ion selective electrodes, when is ion charge zA, negative
when the analyte is an anion
What is an important consideration when designing an instrument that measures cell potential
The resistance may be large with respect to the cell being measured
What will produce a significant error in an instrument measuring cell potential
IR drop
What can ion selective electrodes be used in
Direct Potentiometry
Potentiometric Titration
What is direct potentiometry
Using the Nerst equation for determining the concentration (or activity) of an analyte by measuring the potential of an indicator electrode versus a reference electrode.
What is potentiometric titration?
Using potentiometry for determining the end point of titrations, inclusing acid-base, complexiometric, and redox titrations
what methods can be used for direct potentiometry?
single standard calibration
multiple standard calibration
single standard addition
multiple standard addition
How do electroanalytical techniques differ from potentiometric techniques
electroanalytical techniques involve current while potentiometric techniques do not
What are the four electroanalytical techniques
electrogravimetry
coulometry
amperometry
voltammetry
What are electrogravimetry, coulometry, amperometry, and voltammetry examples of
electrolysis
what is electrolysis
the process in which a chemical reaction is forced to occur at an electrode by applying external voltage (E)
If current is not negligible, what changes the voltage required to run the reaction
ohmic potential
overpotential (over voltage)
Concentration polarization
What is Ohmic potential/ IR drop
The force (voltage) required to overcome the resistance of the ions to movement toward the anode an the cathode
What law does ohmic potential follow?
ohm’s law
What does the net effect of IR drop increase
the potential required to operate an electrolyitc cell
What does the net effect of IR drop decrease
the measured potential of a galvanic cell
What is over potential/ over voltage
The voltage required to overcome the activation energy for the reaction at an electrode
What does applying potential to the metal increase
increases the energy of the electron in the metal
What does applying potential tot he metal decrease
the activation energy for electron transfer
What is over voltage a measure of
the kinetics of an electron transfer process
For a fast system, by increasing voltage, what reaction immediately starts
oxidation
For a fast system, by increasing or decreasing voltage, what begins to flow
current
For a fast system, by reducing voltage what reaction immediately starts
reduction
What current flows with an increase in potential
anodic current
What current flows with a decrease in potential
cathodic current
What is current exchange (Io)
the current exchanged between checmical species in solution and electrode to produce an equilibrium potential
What does the value of current exchange, Io, depend on
The kinetics (rate) of the electrochemical reaction
Is a fast system reversible or irreversible?
reversible
For a slow system, is anoidic over voltage positive or negative
positive
For a slow system, is cathoidic over voltage positive or negative
negative
Are current exchanges for slow reactions reversible or irreversible
irreversible
Why are current exchanges for slow reactions irreversible
a lot of energy is needed
The less current exchange, the more or less irreversible a system is
more irreversible
A totally reversible system has a very large or very small Io
very large Io
A totally irreversible system has a very large pot very small Io
very small… almost 0
Is the current exchange in the electrolysis of water totally reversible or totally irreversible
totally irreversible
What does the magnitude of over voltage depend on
the desired rate of each electron transfer
the nature of of the electrode surface
what assumption is relatively true with respect the the hardness/softness of an electrode?
The softer the electrode the larger n is
What is concentration polarization
when the concentration of reactants or products are not the same at the surface of the electrode as they are in the bulk solution
How mass transfer in an electrochemical cell carried out
- convection
- diffusion
- migration
What is convection
From the bulk of solution to the diffusion boundary… basically the agitation of solution
What is diffusion
from the diffusion boundary to the electrode due to concentration gradient
What is migration
If the analyte is an ion, it can be transferred by migration under the influence of an electric field
What is diffusion current
the current produced by the contribution of analytes diffused from the outer edge of the diffusion layer (nerst layer) to the surface of the electrode in the electrochemical reaction
WHat process is diffusion
mass transfer process due to concentration gradient
What can undergo diffusion
ions
molecules
what is diffusion limited to
a thin layer of solution attached to the electrode
what is the diffusion later/ nerst layer
the thin layer of solution attached to the electrode
What can only be involved in the diffusion process
materials that are brought to the diffusion layer
What does the diffusion current relate to
the rate of the electrochemical reaction
What is steady-state diffusion
When the mass transfer by diffusion happens at a constant rate
Under steady state diffusion what kind of current is measures
constant current
What is needed to reach steady-state diffusion
constant convention
constant temperature
constant stirring
If there is less convection there is less ____, and less ___
less molecules at the diffusion boundary (Cs)
less diffusion
What is migration current
The current produced when ions that undergo electrochemical reaction from migration contribution
The process of migration only occurs for
ions
cations move towards the
cathode
anions move toward the
anode
What is electrolysis current (i)
The current at any point in the reaction determined by the rate of analyte transport from the outer edge of the diffusion layer to the electrode surface by both diffusion and migration
If your electrode is + and your ion is +
diffusion occurs towards the electrode
migration occurs towards the electrode
if your electrode is + and your ion is -
diffusion occurs toward the electrode
migration occurs away from the electrode
if your electrode is - and your ion is -
diffusion occurs towards the electrode
migration occurs towards the electrode
if your electrode is - and your ion is +
diffusion occurs towards the electrode
migration occurs towards the electrode
in order to only have diffusion current controlling the electrolysis current the analyte partition (t) must be
less than 1
what can we say if t is really small?
that the analyte doesn’t really contribute
more ions means ____ analyte partition
less
if migration current (im) is positive, diffusion and migration are going in _____ directions
the same
if migration current (im) is positive, diffusion and migration are going in _____ directions
different
how can analyte partition be minimized?
introducing a supporting electrolyte that have inactive ions
At what concentration of the supporting electrolyte doe sthe total current carried by the analyte approach zero
when the concentration of the supporting analyte is between 50-100 fold greater than that of the analyte
What are common supporting electrolytes
KCl KNO3 Na2SO4 LiClO4 LiNO3 HCl H2SO4
What is Fick’s first law of diffusion
Diffusion current is related to the diffusion flux which is related to the concentration gradient in the diffusion layer
What is diffusion flux
the rate of flow of a substance per unit area per unit time
Usually ____ molecules have a larger diffusion coefficient
smaller
What is the nerst assumption
diffusion is limited to a thin layer around the electrode and we assume there is a linear concentration gradient
When potential is applied to drive an electrolytic reaction what increases
current
Once enough potential is applied what leads to the stabilization of current in electrolysis
diffusion
what is the stabilization of current when increasign potential is applied in electrolysis called
diffusion plateau
what does a decrease in current give?
cathodic wave
what does an increase in current give
anodic wave
what is limiting current
current at which diffusion plateau determines reaction
How does limiting current increase
by increasing the concentration of the analyte in the solution
what occurs when the concentration of the analyte is very high leading to the limiting current being very high
Oxidation or reduction wall
What is an oxidation or reduction wall
when the diffusion plateau won’t appear in a regular scale
What stays constant due to agitation
Cs
What decreases when the solution is unstirred
Cs
What kind of cell is utilized by several electroanalytical techniques
Three-electrode system
What electrodes are in a three-electrode system
working
reference
auxiliary (counter)
what does the working electrode do
makes the reaction: where to reaction of interest occurs
What does the reference electrode do
has fixed potential and used to measure the potential of the working electrode
what do we want to measure with the reference electrode
the potential a polarizable working electrode with significant current with respect to a nonpolarizable reference electrode with negligible current
what is the auxiliary electrode
the electrode at which the opposite reaction occurs and is a current supporting partner of the working electrode
why do you only need two electrodes in potentiometry
there is no current
current cannot travel through the ___ electrode
reference
What is used for controlled-potential electrolysis with three electrode cell
Circut
Parts of controlled-potential electrolysis circut
electrolysis cell anode (auxiliary electrode) cathode (working electrode) Reference electrode controlled potential ammeter potentiostat
What is coulometry
measuring electrical charge ( in coulombs) ised in an electrochemical reaction
In coulometry you measure __, to find __
Q to find n
Types of coulometry
controlled potential coulometry (CPC)
controlled current coulometry (CCC)
what is CPC
the potential of working electrode is usually adjusted on the diffusion plateau of teh analyte
What do we assume with CPC
everything is absorbed at the working electrode
What is used to determine the quantity charge required to complete an electrolysis
digital integrator
in coluometry, essentially all of the ___ is consumed
the analyte
What is CCC (coulometric titrations)
a reagent is generated on an electrode by applying constant current to react with the analyte
What is proportional to the concentration of the analyte until the end of the titration in CCC
The charge consumed
How can the end of the titration be determined in CC
visual (indicator) method
intrumental method
What instrumental methods are used to determine end of CCC titration
amperommetry
potentiometry
At the generator electrode what is the titrant?
pre-reagent
reagent
In the solution, what is the titrant
analyte + reagents
products
What does the generator electrode act like in CCC
burette in a titration
What is measured instead of reagent volume in CCC
charge consumed at the generator electrode
What is voltammetry
the realtion between current and voltage obsevred during electrochemical processes
what is obtained in voltammetry
a current-potential curve (votammogram)
What is present in voltammetry
significant current
due to significant current in voltammtry, what occuRS
polarization
How does voltammtry differ from coulometry
minimal consumption of the analyte in voltammetry
How is the electrochemical cell set up in voltammetry
three electrodes immersed in solution containing analyte and an excess of supporting electrolyte
In voltammtry, the working electrode’s potential
is varied linearly with time.
why are the dimensions of the working electrode in voltammetry kept small
to enhance the tendency to become polarized
What is the reference electrode commonly in voltammetry
SCE
Ag/AgCl
What remains constant at the reference electrode in voltammetry
the potential
What is the auxiliary electrode commonly in voltammetry
coil of platinum wire
what does the platinum wire auxiliary electrode do in voltammetry
conducts electricity from he signal source through the solution to the working electrode
Types of working electrodes in voltammetry
disk electrodes
hanging mercury drop electrode (HMDE)
microelectronic
What is a disk working electrode?
disk conductors compressed into a rod of an inert material, such as teflon or kel-f, that has embedded in it a wire contact
what can the conductors in dick electrodes be made out of?
noble metal (Pt, Au,…)
Carbon material (paste, pyrolytic graphite, glassy carbon, diamond, carbon nanotubes)
Semiconductor (tin, indium oxide)
metal coated with film of mercury (thin film mercury electrode)
What is a hanging mercury drop electrode?
electrode consisting of a veyr fine capillary tube connected to a mercury-containing reservoir
What forces the metal out of the capilary piston arrangement
micrometer screw
what does the micrometer screw promote
the formation of mercury drops
What’s the advantage of HDME
a fresh electrode electrode surface can be readily formed by simply producing a new drop
What are working microelectrodes?
small diameter metal wires or fivers within tempered glass bodies
What is the tip of the microelectrode polished with
alimunia or diamond polish
what is the electrical connection in microelectrodes
0.060 inch gold plated pin
what kinds of materials can make up microelectrodes
carbon fiber
platinum
gold
silver
What is hydrodynamic voltammetry
voltammetry in the presence of convection (agitation)
What can be used in hydrodynamic voltammatry
stationary electrode in stirred solution
rotating disk electrode
a flow through cell
dropping mercury electrode
What is voltammetry in the absence of convection
using a stationary electrode in an unstirred soltuion
What is used in voltammetry in the absence of convection
line sweep voltametry (LSV) cyclic voltammetry (CV) pulse voltammetry Square wave voltammetry (SWV)
How is hydrodynamic voltammatry perfromed
magnetic stirrer
rotation
passing analyte solution through a tube fitted with a working electrode
How is rotating performed in hydrodynamic voltammetry
the workings electrode is rotated at a constant high speed in the solution to provide stirring
(calm solution, electrode agitation)
what is residual current
the current due to reaction of impurities in the solution
in a reversible hydrodynamic voltammetric system including the reductant only what is the half wave potential equal to
Eo
in an irreversible hydrodynamic voltammetric system including the reductant only what is the half wave potential NOT equal to
Eo
in a reversible hydrodynamic voltammetric system including the oxidant only what is the half wave potential equal to
Eo
in an irreversible hydrodynamic voltammetric system including the oxidant only what is the half wave potential NOT equal to
Eo
what is polarography
voltammetry conducted with a dropping mercury electrode
what electrochemical cell is used polarography
three-electrode cell
What is polarograhy useful for?
studying reduction because H+ at the Hg surface has a large cathodic overpotential
