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Physical Chemistry-Electrochemistry
This is an article about the difference between irreversible electrochemical cells, reversible electrochemical cells and irreversible electrochemical cells (the main content includes: solving problems of electrode reactions in the electrolysis process, polarization phenomena, and the relationship between electrolytic cell voltage and current.
Edited at 2024-03-31 10:00:42
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Physical Chemistry-Electrochemistry
- Avatar Character Relationship Chart
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- Outline
electrochemistry
Preface
Conductive
Conductive mechanism
Electrode reaction---Faraday's law
Ion directional migration---ion migration number
(Molar) Conductivity: Describes the ability to conduct electricity
average activity coefficient of ions
electrolyte solution activity
Electrolyte solution: the working medium of batteries and electrolytic cells
Conductivity and other properties
basic concept
Electrode naming
For electrolyte solutions
Anode: [O] loses electrons; cathode: [H]
Cations always move towards the cathode
e.g.
for the entire battery
Positive electrode: electrons flow in, the beginning of the current I; negative electrode: electrons flow out, the end of the current I
According to the level of potential: high potential (potential) - positive pole, low potential - negative pole According to the nature of the reaction: oxidation - anode
Electrode reaction
e.g.
Battery reaction: sum of 2 electrode reactions
Research objects of electrochemistry
Cell
Positive electrode <---> anode; negative electrode <---> cathode
Electrode reaction & battery reaction
Primary battery "Negative" "Yang" Zi Huan
Anode <--> negative pole; cathode <--> positive pole
Electrode reaction & battery reaction cannot be written in minus electron form
Conductive mechanism of electrolyte solutions and Faraday's law
a
Conductive mechanism of electrolyte solutions
P.S.
conductor
Class 1 conductor
Electron conductor (metal): conducts electricity due to the directional movement of electrons
The higher the temperature, the weaker the conductivity
Class II conductor
Ionic conductor (electrolyte solution): conducts electricity due to the directional movement of ions
The larger T, the stronger the conductivity
Faraday's law Unit: Q--C, z--mol, F--C/mol
F=96500C/mol=96485 C/mol
Suitable for battery discharge process and electrolysis process
Meaning: The amount of electricity passing through the electrode is proportional to the amount of electrode reactants generated (consumed)//the amount of chemical reaction substances that occur
electromigration of ions
Ion migration number The ratio of the amount of current carried by an ion to the total amount of current passing through the solution
Ion migration numbers t, t- of anions and cations *t( ) t(-)=1 t( )/t(-)=u( )/u(-)
Meaning: Describe the contribution of anions and cations to conductivity within a certain period of time
Influencing factors (take t as an example)
I: The amount of charge of cations passing through the cross-sectional area of the conductor per unit time
Ion movement rate v =The amount of material that cations migrate out of the anode area/the amount of material that reacts at the electrode
Ionic properties: size, shape, valence state, etc.
The main reason why migration number is affected by concentration: interaction between ions When the concentration is low, this effect is not obvious, but when the concentration is high, the interaction between ions increases as the distance decreases. At this time, the movement speed of both anions and cations will slow down.
If the valences of anions and cations are the same, the changes in t( ) and t(-) are not very big, especially the migration numbers of anions and cations in KCl solution are basically not affected by concentration, but the migration numbers of other ions are generally affected by varying degrees of impact.
When the valences of anions and cations are different, the migration speed of high-valent ions decreases more significantly with increasing concentration than that of low-valent ions.
C: The influence of electrostatic attraction of anions on cations etc.
Simplify the influence of E---->Extract u ion mobility
Ion mobility //Electrical mobility Unit: m^2/V/s
Ion migration rate under unit electric field strength
Influencing factors: c, C, A
maximum (u is a constant)
When c-->0 (the solution is infinitely diluted), the influence of anions is negligible = cations move independently, then the mobility reaches a maximum value
Limiting electrical mobility of ions: ion mobility of infinitely dilute solutions
Determination of ion migration number·
Utilize definition t =Q /Q
Q (the amount of electricity passing through the cross-sectional area of the conductor in a certain period of time)---measured directly by a coulomb meter//Q=Q( ) Q(-) Q --?
Root: Measure Q
Because: within a certain period of time, the number of cations (anions) passing through any interface is equal. So: you only need to find the number of cations (anions) passing through a certain interface.
Find Q == Find n (migration)
Method 1: Material balance in anode area
1. Take " " = the reaction generates cations 2.n (back), n (front), n (reaction) can be obtained Note: Anode area: [O], cations migrate out, so "-n migration"
Method 2: Material balance in cathode area
subtopic
e.g. 43.50---n (front) [The amount of water before and after the reaction remains unchanged, and can be regarded as the water in the anode area before the reaction = the water in the anode area after the reaction]; 0.723---Q//n (reaction); anode area. ...---n(n)
Method 1 Anode area ion material balance
Method 2 Anode area-ion material balance
test methods
Hitov method--respectively measure the amount of material that ions migrate out of the corresponding polar area and the amount of material that undergoes electrode reaction.
interface movement method
Activity, activity factor and Debye-Huckel limit formula of electrolyte solution
Thermodynamic description of electrolyte solution----chemical potential Idea: Find the chemical potential of the cathode and anode to determine the reaction direction ---> Find a(-)---> Find γ and concentration
Review (points can be ignored)
Note: 1. Selection of standard state 2. Composition represents selection
Activity a
Expression of chemical potential of electrolyte solution
u(B)=cationic chemical potential anionic chemical potential a( ), a(-) cannot be measured directly, a(-) needs to be introduced
a & γ & b v=v( ) v(-)
Derivation (according to the definition of battery chemical potential u) [fill in the blank/select] b is a known quantity
e.g.
average activity factor γ
Can reflect the difference between the current solution and the ideal dilute solution
The closer it is to the ideal, the closer it is to 1
The higher the concentration, the more γ deviates from 1
In the dilute solution range (with the same concentration)
The valence type is the same and the γ value is similar. e.g. HCl, LiBr
For multiple types of solutions with the same concentration, the higher the valency, the greater the deviation (the smaller the γ value)
PS: When dealing with strong electrolyte solutions, the value of γ must be considered, unless it is given in the question
γ is related to the electrostatic interaction between ions --->Concentration and valence affect γ (-) --->Introduce ionic strength I
Ionic strength I =1/2*【b( )z( )^2 b(-)z(-)^2】 Unit: mol/kg z: the number of charges carried by the ion
The relationship between γ & I - Debye-Huckel limit equation D-H formula
Equation assumptions
1. Strong electrolytes are all dissociated in the solution
2. The main interaction between ions is Coulomb force
3. Each ion is surrounded by an ion atmosphere formed by charges of opposite signs.
Characteristics of ion atmosphere
Around positive ions, negative ions have more chances to appear, and vice versa, but the overall solution is electrically neutral. {There are more chances for charges with different signs to appear around a certain ion}
Each ion is both a central ion and a member of the ion atmosphere
The ionic atmosphere is spherically symmetrical
The ion atmosphere is not fixed
The proposal of ionic atmosphere simplifies the complex electrostatic interactions in the solution to the central ion-ion atmosphere interaction.
Only suitable for dilute solutions of strong electrolytes A: 0.509 (mol/kg)^ (-1/2) z: number of charges
z( )|z(-)| basically determines the size of lgr(-) inversely proportional to
For strong electrolytes of the 1-1 valence type
Seeking the law
1. If γ (-) is given in the question, then γ(-)--(b-)-->a(-)---->a
2. If γ (-) is not given in the question, then E--(Nernst equation)-->a( -)---(b -)-->γ( -)
Description of conductivity of electrolyte solutions----conductance, conductivity and molar conductance
Conductance G=1/R Unit: Ω^-1=S (Siemens), mS=10^-3S,uS=10^-6S
Does conductance well describe a conductor's ability to conduct electricity?
The conductance of conductors of the same length and cross-sectional area should be compared
Conductivity κ (kapa) Unit: S*m^-1,mS*cm^-1,uS*cm^-1
Meaning: equal to the conductive capacity of a conductor per unit length (1m) and unit cross-sectional area (1m^2)
eg Silver conducts electricity more easily than copper = silver has a greater electrical conductivity
Conductivity of electrolyte
The conductivity of an electrolyte is equivalent to the conductance of a certain electrolyte in an electrolytic cell per unit plate area and unit plate distance.
Conductivity cell coefficient K (cell) Unit: m^-1 ---Known in the question
Find κ--->Find G--->Measure resistance Rx (Wheatstone bridge method: adjust R3 so that the ammeter = 0, so that U of the two circuits is equal)
κ VS c
Factors affecting conductivity
For strong electrolytes
When the concentration doubles, N doubles. At this time, v decreases due to the increase in the electrostatic attraction of the cations by the anions. v-Similarly, the conductivity is less than doubled.
For weak electrolytes (The number of conducting ions N has always been very low, and κ has not changed much)
c has little effect on κ
κ&Λm
κ(solute)=κ(solution)-κ(H20)
Application of conductivity measurement【Examination】
Calculate the dissociation degree and equilibrium constant of weak electrolytes
step κ--->Λm--->a,K
1. List the three-part equation and find out the relationship between K and a (note c (standard))
2.Λm: Obtained by definition Λm(∞): Known look-up table independent motion rules of ions
Calculate the solubility c of poorly soluble salts
Ions produced by weak electrolytes, ions produced by insoluble salts == infinitely thin solutions
! ! Λm~Λm（∞）
step
1. Calculate the κ of the target substance κ(solute)=κ(solution)-κ(H20)
2.c=κ/Λm=κ/Λm（∞）
3.Ksp=c^(?)=（κ/Λm）^(?)
Is conductivity a good description of a conductor's ability to conduct electricity?
Ignore the influence of the concentration c of the actual solution
Molar conductivityΛm=κ/c Unit: S*m^2/mol
Meaning: Conductivity of unit concentration electrolyte solution. A solution containing 1 mol of electrolyte is placed between two parallel plate electrodes 1 m apart. At this time, the conductivity of the solution
Λm VS c
The molar conductivity of the same electrolyte is related to the concentration!
1.κ=f(N,v,v-)--(Λm=κ/c, equivalent to quantity normalization)-->κ=f(v,v-) 2. After c increases, v, v- decreases, causing Λm to decrease
Λm & root c
For strong electrolytes (The above formula is satisfied in dilute solution)
B=Λm（∞） (molar conductivity of infinitely dilute solution)
How to find it: extrapolation, find the intercept
Equivalent to the molar conductivity when the solution concentration is 0 = Λm (∞)
usually as a known quantity
c becomes larger, Λm becomes smaller (u(B) becomes smaller)
Kohlrausch's rule of thumb
Within the dilute concentration range, Λm~√c has a linear relationship
For weak electrolytes
Λm~c approximates a hyperbola
Within the dilute concentration range, Λm is sensitive to c (c becomes smaller, α becomes larger, and N becomes larger)
Limit molar conductivity Λm (∞) [Test]
Why is Λm(∞) the largest?
Λm=f(v,v-), when the solution is infinitely dilute, v and v- are maximum, anions and cations move independently of each other, conductivity is maximum, and molar conductivity is maximum
Seeking the law
For strong electrolytes: Extrapolation method/Kohlrausch's law of independent motion of ions
Kohlrausch's law of independent motion of ions
In an infinitely diluted solution, ions move independently of each other and do not affect each other. The molar conductivity of the infinitely diluted electrolyte Λm (∞) = the sum of the molar conductivities of anions and cations at infinite dilution.
e.g. CaCl2: Λm（∞）=1*Λm, (∞) 2*Λm,-(∞)
For weak electrolytes Kohlrausch's law of independent motion of ions
Method 1: Kohlrausch's law of independent motion of ions Known experimental data Λm (∞) (H ), Λm (∞) (Ac-)
egHAc: Λm（∞）=1*Λm（∞）(H ) 1*Λm（∞）(Ac-)
Method 2: Use strong electrolytes (Reason: Kohlrausch's law of independent motion of ions - independent motion of anion and cation)
The relationship between Λm(∞) and rate Λm
Λm=Λm（∞）*a
a: degree of dissociation
Measurement of Λm---Wheatstone bridge method
principle [Measure R-->κ-->Λm]
κ=G*（l/As）=1/R*（l/As）=1/R*K（cell） K (cell): can be measured from a solution with known conductivity
For any electrode area
Principles: Conservation of Matter and Electroneutrality of Solutions
n (electrolyte) = n (ion migration)
n (electrolyte) = n (ion migration)
U1 Ⅱ reversible electrochemical cell Design Battery--->Er--->Thermodynamic Analysis--->Energy Conversion, Thermodynamic Data
Reversible electrochemical cell basics
Battery symbol//battery icon representation
in principle
The anode is on the left and the cathode is on the right
Indicate the state of matter (phase, temperature, pressure, activity)
solid state - phase state Solution--activity/concentration
Clear phase interface "|" [between different phases//between different solid phases]
Content: Agar saturated KCl//saturated KNO3
Purpose: Eliminate liquid junction potential
liquid junction potential
Diffusion proceeds from high concentration to low concentration. The diffusion rates of H & Cl- are different, resulting in a relative surplus of anions and cations on both sides, leading to the emergence of ΔE
K &Cl-Move at the same rate and the ion concentration within the salt bridge is very large
e.g.
Reversible primary battery Any reaction and process in the battery is reversible
Conditions for reversible expansion
1. The piston has no mass
2. Frictionless
3. System pressure is greater than ambient pressure
expansion under mechanical equilibrium
Conditions for reversible batteries
1. Chemical reactions are reversible Electrolysis proceeds in reverse
Thermodynamic reversibility
2. No liquid junction No Diffusion – Salt Bridge
3. The working current is 0 Infinitely slow discharge (reversible discharge)
The electrode reaction is infinitely close to electrochemical equilibrium
Batteries operating in electrical balance
There is a diaphragm --- irreversible
egD-irreversible battery, W-reversible battery
Determination of reversible primary cell electromotive force Er
The measurement of battery electromotive force must be carried out when the current is infinitely close to 0
Poggendorff cancellation method? ? ?
Nernst equation
E (standard) = E (standard - cathode) - E (standard - anode) Then substitute it into Nernst equation
Be sure to pay attention to the applicable equation of the nernst equation---reduction reaction
P.S.
Oxidation state: the product of a substance that has lost electrons
A B ---> C D Whoever is reduced (gets electrons) between A and B is in the oxidation state
By ΔrGm
E=E cathode-E anode
Electrode potential and liquid junction potential
Electrode potential
1) Electrode potential regulations
Standard acidic hydrogen electrode potential = 0, and the electrode to be tested is used as a cathode for comparison.
E (standard) of concentration difference battery = 0
2) When writing the electrode potential, it must be written according to the reduction reaction, which is the reduction potential.
Therefore, when calculating the potential of the cathode and anode, it must be converted into a reduction reaction (electron gain) formula
3) Determination of cathode and anode of primary battery
The reduction potential E (electrode) is high --> / prone to reduction reaction --> cathode
The level of the reduction electrode potential is a measure of the tendency of the reaction in which the oxidized substance of the electrode obtains electrons and is reduced to the reduced state.
4) Battery electromotive force
a. Nernst equation of battery
b. Battery potential = positive electrode potential - negative electrode potential = E cathode - E anode
Liquid junction potential and its elimination
Liquid junction potential//diffusion potential: the potential difference existing at the interface of two different solutions [need to be remembered, not taken]
This formula is applicable when the electrolytes in the two junction solutions are of the same type and are type 1-1 electrolytes.
e.g.
Essence: Caused by the different diffusion rates of ions in the solution
Elimination method: Add salt bridge (the migration numbers of anion and cation of the electrolyte should be close)
1. The electrolyte used in the salt bridge must have extremely close migration numbers of anions and cations. 2. KCI saturated solution is most suitable for salt bridge conditions, but the salt bridge solution cannot react with the original solution. For example, solutions containing Ag cannot be used.
Thermodynamics【Test filling, selection, calculation】
Find ΔrGm
ΔG physical meaning
Under constant temperature and constant voltage reversible conditions, ΔG=Wr'=reversible electrical power --->ΔrGm=Wr,m’=ΔrHm-TΔrSm (= reversible electrical work done when the reaction proceeds to 1 mol)
The change in the Gibbs function represents the system's ability to do non-volume work.
ΔrGm=-zFEr
E has nothing to do with the coefficients of the equation == non-summable, while ΔrGm is related to the coefficients
-: The system does work on the environment
Find ΔrGm (standard)
E (standard): E when the electrode reactant is in the standard state
For gases: pure ideal gas with pressure p (standard)
For liquids: pure liquid with pressure p (standard)
Pure liquid activity=1
For ions: pressure p (standard) ion with activity 1
Find ΔrSm
Find ΔrHm
ΔrHm=ΔrGm TΔrSm
Asking for Q
Constant temperature, reversible: Q=ΔrSm*T=ΔrHm-ΔrGm=ΔrHm-W'
Determine whether there is thermal effect
Note that the calculation is J/mol according to the international standard unit, and the result needs to be converted into kJ/mol.
Find ΔrHm (defined according to ΔrGm)
ΔrHm=ΔrGm T*ΔrSm =Wr,m' TQr,m =Qp,m
Find K (standard) (according to ΔrGm=0)
review
Find E
nernst equation ---Reversible electromotive force & concentration c ---E&E (standard) relationship
Pay attention to the difference between lg and ln formulas [25℃---lg] z: The amount of transferred electrons in the reaction! =Sum of electrons added a: liquid--a; gas--p
Notice:
ΔrGm (standard) = -zE (standard) F When the same, E (standard) is not necessarily the same
E is the intensity quantity
E has nothing to do with the way the reaction is written, but is related to ΔrGm (related to n)
Find γ(-)
e.g.
eg ΔrGm==test Er (Boggendorff cancellation method) ΔrSm===Make E-T curve (measure E at variable temperature)===Get the slope of the target temperature
Applications of Electromotive Force Measurement
①Measurement of activity
E--->a---->a( -)---->a( )
②Measurement of standard electromotive force
③Determine the reaction direction--ΔrGm
Primary battery design
Electrode classification
Type 1 electrode
Metal electrode: An electrode in which metal and its ions are in different phases and in direct contact
Non-metal electrode: non-metal (gas element) additional inert electrode (eg graphite, Pt, Pd) and its ions are in different phases and are in direct contact with each other
hydrogen electrode
a. Acidic hydrogen electrode Electrode reactions and electrode symbols are written as cathodes.
b.Alkaline hydrogen electrode
How to find the standard electrode potential of alkaline hydrogen electrode?
oxygen electrode
a. Acid oxygen electrode
b.Alkaline oxygen electrode
Alkaline... = acidic... ....
Type II electrode
Metal-refractory salt electrode: an electrode composed of a metal, its refractory salt and the anions of the refractory salt in the electrolyte
a. Calomel electrode---reference electrode (The electrode potential is usually a known quantity)
Application: Measure pH a(H2)=1 under 100kPa
b.Silver-silver chloride electrode
Metal-metal oxide electrode: an electrode composed of metal and its insoluble salts and H //OH-
Antimony-antimony oxide electrode
The third type of electrode - redox electrode The substances participating in the reaction are in the same solution, and the electrode plate does not participate in the reaction
battery design
Principle: Electrodes belong to the above three types of electrodes
Notice
If KCl solution is used as electrolyte solution, then KCl salt bridge cannot be used as salt bridge.
When the electrolyte solution is an alkaline solution, H2O cannot be used as the alkaline medium.
Design of concentration difference battery
E (standard) of concentration electrode = 0
step
1. Design the cathode first (reduction reaction-getting electrons)
First, write down the substances that must gain and lose electrons in the reaction.
Then through charge conservation, use H /H2O balance
2. Anode = total reaction formula - cathode formula
3. Check
a. Conservation of charge: The number of charges on the cathode and anode must be the same
b.Electrode classification! !
e.g.
Electrochemistry question routine
1. Electrode reaction and total reaction
2. List the Nernst equation
Ⅲ.Irreversible electrochemical battery The difference between reversible electrochemical cells and irreversible electrochemical cells (Er! = Eir) --- Reason: polarization Electrode reaction in electrolysis process
Electrolytic cell voltage and current relationship
Theoretically
Think of this electrolytic cell as a large resistor; E=IR---in a straight line
actually
three-segment curve
stage a:
External voltage! =0, the two electrodes react (H2, Cl2), thus forming a primary battery with the primary ions that resists the external power supply; [Figure 3] Therefore, a primary battery is connected to the external circuit (to resist the external power supply). When E (external) = E (primary battery) IR, E (external) ~ E (primary battery), resulting in I being very small
When the external voltage increases, I remains unchanged: the external primary battery is regarded as a reversible primary battery, and Er is listed; when the external voltage increases, the back electromotive force increases, so that I does not change much.
stage b
After the external voltage reaches a certain value, E (primary battery) reaches a maximum value---E decomposes, and the current increases linearly with the external voltage.
Decomposition voltage V
When performing electrolysis operation, the minimum external voltage required to enable the electrolyte to continuously decompose at the two poles
c stage
When the external pressure reaches a certain level, the carrier velocity reaches the limit and I remains unchanged.
polarization phenomenon ---Cause E(minute)=Eir>E(reason)=Er
definition
The phenomenon that the electrode potential of an irreversible electrochemical cell deviates from the electrode potential of a reversible electrochemical cell caused by current flow
reason
Irreversible electrochemical cells contain various process rates that do not match
1. Concentration polarization: slow diffusion and fast reaction (ion diffusion rate < electrode reaction rate)
Eliminate---Stir
Reason: Caused by the slowness of the diffusion process
Result (compared to reversible electrode potential): cathodic polarization --- potential reduction Anodic polarization---increased potential
eg. Primary battery ir: The ion concentration around the anode electrode is too high and there is no time to diffuse, making the electrode potential Eir>Er
The ions around the cathode electrode have no time to diffuse to its surroundings, and the concentration is too low, making Eir<Er
The same principle applies to electrolytic cells
2. Electrochemical polarization: slow reaction and fast diffusion (electron transmission rate > reaction rate)
Reason: Caused by the slowness of the electrochemical reaction itself
Result (compared to reversible electrode potential): cathodic polarization --- potential reduction Anodic polarization---increased potential
eg. Primary battery ir: The cathode needs ions to obtain electrons, and the electrons are transported from the anode reaction. The electrochemical polarization of the cathode causes the ions to accept electrons slowly, causing electrons to accumulate at the cathode, thereby reducing the potential.
Result (compared to reversible electrode potential): cathodic polarization --- potential reduction Anodic polarization---increased potential
For the entire primary battery: Er=E (yin)-E (yang)>Eir
Negative Effects
E (output), the same as doing electrical work
For the entire electrolytic cell: Er (decomposition) = Er ( ) - Er (-) = Er (yang) - Er (yin) < Eir (decomposition)
Negative Effects
E (output), energy consumption is the same
Polarization reduces the electromotive force of the original battery and increases the decomposition voltage of the electrolytic cell.
Description of polarization degree---overpotential eta (must be a positive value)
Definition: The absolute value of the difference between the electrode potential Eir and its equilibrium electrode potential Er at a certain current density.
Influencing factors: current density J
Tafel empirical formula
How to draw polarization curve---find the relationship between overpotential and current density J (Relationship between cathode electrode potential Eir (cathode) and J)
Polarization curve measurement experiment
When the power supply voltage changes, the size of the current I changes, thereby measuring the current density J; Er/ir (cathode) = E (potentiometer) - E (reference)
Need to consider the situation of overpotential
Fe, Co, and Ni electrodes among gas electrodes and metal electrodes
Polarization curve E-J curve
Primary battery
1. When J=0, E (yin) = Er (yin), E (yang) = Er (yang) The numerical value can be known 2.E (cathode)>E (anode) [the positive electrode potential is high, the cathode loses electrons ---> cathode] 3. Effect of polarization on cathode and anode
Cell
1.E (Yang)>E (Yin) 2. Effect of polarization on cathode and anode
Electrode reaction in electrolysis process Solve the problem--(There are several ions in the solution at the same time, which one reacts first)
electrolysis
Anode--[O], the one with low reduction potential is oxidized first (reaction) Cathode--[H], the one with high reduction potential is reduced first (reaction)
Cathodic reaction: [Cu2, Zn2, Ag] 1. E=Er obtained from the Nernst equation, Eir can be known from the polarization curve Eir=Er-η 2. Those with high reduction potential are prone to reduction reactions and precipitate at the cathode first. 3. The potential is so low that oxidation reactions easily occur and precipitate at the anode first.
To determine the direction sequence, use the polarized electrode potential E (X, polarization) = Eir = E (X, balance) - η
cathode
Zn2 has no hyperpolarization effect, and Er is calculated directly using the Nernst equation; H2 is a gas electrode, and hyperpolarization needs to be considered. After calculating Er, you still need to calculate Eir=E (X, polarization)
anode
During electrolysis: Anode---substances with low polarization potential are oxidized first Cathode - substances with high polarization potential are reduced preferentially
No matter the cathode or anode, the closer the E of a substance is to the Er, the easier it is to precipitate. (Special substances are considered beyond the plan)