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Standard Electrode Potential is the value of standard emf of a cell. In the cell, the molecular hydrogen under standard pressure is oxidized to solvated protons. Electrode Potential is an electromotive force between two electrodes in a cell. Measuring the exact value of a single electrode is impossible. Therefore, through experimentation, one can measure the difference between the electrodes.
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Key Takeaways: Redox reactions, Standard electrode potential, Standard hydrogen electrode, Electrode, Electrolysis, Electrode potential, Unity, Atmospheric pressure
Definition of Standard Electrode Potential
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The potential associated with each electrode is the electrode potential. Let us assume the two chemical species at the two electrodes have equal concentrations of unity(for gaseous reactants → 1 atmospheric pressure) and the reaction between these species is carried out at 298 K, then the potential of each electrode is known as Standard Electrode Potential.
Standard Electrode Potential
The standard electrode potential is denoted as E°. The E° for hydrogen electrodes is 0.00 volts. The electrode potential value for different chemical species is a measure of the relative tendency of the element to remain in oxidized or reduced form. There are two possibilities:
- Negative E°- redox couple is a stronger reducing agent than the H+/H2 couple
- Positive E°- redox couple is a weaker reducing agent than the H+/H2 couple
Hydrogen Electrodes
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Standard Electrode Potential Example
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The figure below shows the calculation of the standard electrode potential of a zinc electrode with respect to the Standard Hydrogen Electrode (SHE). Potential is measured under standard conditions where the temperature is 298K, the pressure is 1 atm, and the concentration of the electrolytes is 1M.
Standard Electrode Potential Example
The potential is measured under the following standard conditions:
- Temperature - 298 K
- Pressure - 1 atm
- The concentration of the electrolytes - 1M
Uses of Standard Electrode Potential
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There are various uses of Standard Electrode Potential.
- We can measure the relative strength of different oxidants and reactants.
- Standard Cell Potential Value is calculated for various chemical species.
- We can predict whether certain chemical species will react with each other and to what extent that reaction will be.
- Standard Electrode Potential is used to predict the point of equilibrium in a chemical reaction.
Significance of Standard Electrode Potential
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- An electrochemical cell is made up of two half-reactions where redox reactions take place.
- At the anode end, oxidation takes place and at the cathode end, reduction takes place. Oxidations lead to the loss of electrons at the anode end and the gain of electrons at the cathode end. Thus, electrons from anode to cathode lead to the conduction of electricity.
- Each electrode dipped in its electrolyte has a difference in potentials between the cathode and anode thereby causing electric potential. This cell potential is measured using a voltmeter.
- Obtaining the potentials of individual half-cells is not possible. This is where the real significance of Standard Electric Potential arises because the individual potential may change with a change in pressure, temperature, or concentration of the electrolytes. The individual reduction potential of a half-cell is therefore determined by using a Standard Hydrogen Electrode (SHE). The electrode potential of SHE is 0 volts.
- By connecting an electrode to the SHE and measuring the cell potential of the resulting galvanic cell, the standard electrode potential of an electrode can be calculated. An electrode's oxidation potential is the inverse of its reduction potential. As a result, an electrode's standard electrode potential is characterized by its standard reduction potential.
- The standard reduction potentials of good oxidizing agents are high, while the standard reduction potentials of good reducing agents are low.
The spontaneity of Redox Reactions
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Gibbs free energy or ΔG° is used to know the spontaneity of redox reactions. It must have a negative value for the reaction to be feasible. The formula for finding if a reaction is feasible or not is:
ΔG°=-n x F x E° cell
- n can be found out by balancing the reactions and finding out changes in the oxidation state of the elements i.e electrons exchanged.
- F is Faraday’s constant (96485 C.mol-1 approximately). You may be given a different value which can be close to this value in the question. Use the value given in the question
- The E° cell is E° cathode - E° anode.
The E° cell is generated by subtracting the anode's standard electrode potential from the cathode. The E° cell must have a positive value for a redox reaction to be spontaneous (since both n and F have positive positive values, and the ΔG° value must be negative).
Thus, the cathode and anode standard electrode potentials aid in forecasting the spontaneity of the cell reaction. It must be noted that in galvanic cells, the cell's ΔG° is negative, whereas, in electrolytic cells, it is positive.
Things to Remember
- There is no need to memorize the Standard electrode potential values for various chemical species as they are provided by the examiner in the paper.
- Negative E° means the redox couple is a weaker reducing agent than the H+/H2 couple and Positive E° means the redox couple is a stronger reducing agent than the H+/H2 couple.
- Standard electrode potential values are possible in aqueous equilibrium reactions only.
- The possibility of whether two given chemical species will react or not can be predicted using standard electrode potential values but the rate of the reaction between the reactants cannot be predicted.
- The arrangement of elements according to their standard electrode potential values is called electrochemical series or activity series. Elements having a higher standard electrode potential are placed above those having lower standard electrode potentials. The elements placed at the top of the series are tending to get reduced easily. While the elements placed at the bottom have the least tendency to get reduced.
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Sample Questions
Ques. A cell representation is given as Ni | Ni2+ (1M) || Ag+ (1M) | Ag where reduction potentials of electrodes are given as:
E° Ni2+| Ni = - 0.25 V
E° Ag+| Ag = 0.80 V
Find E° cell. (2 marks)
Ans. The Ni | Ni2+ represents the Anode as oxidation is taking place here while the Ag+ | Ag represents the Cathode as reduction is taking place here.
E° cell = E° cathode - E° anode
E° cell = 0.80 V - (-0.25V)
Therefore, E° cell is 1.05 V.
Ques. A Daniel Cell representation is given as Zn | Zn2+ (1M) || Cu2+ (1M) | Cu where reduction potentials of electrodes are given as:
E° Zn2+| Zn = - 0.76 V
E° Cu2+| Cu = 0.34 V
Find E° cell. (2 marks)
Ans. The Zn | Zn2+ represents the Anode as oxidation is taking place here while the Cu2+ | Cu represents the Cathode as reduction is taking place here.
E° cell = E° cathode - E° anode
E° cell = 0.34 V - (-0.76V)
Therefore, E° cell is 1.10 V.
Ques. A Cell representation is given as Zn | Zn2+ (1M) || Pb2+ (1M) | Pb where reduction potentials of electrodes are given as:
E° Zn2+| Zn = 0.76 V
E° Pb| Pb2+ = 0.13 V
Find the Standard EMF of the cell. (2 marks)
Ans. The Zn | Zn2+ represents the Anode as oxidation is taking place here while the Pb | Pb2+ represents the Cathode as reduction is taking place here.
But, the thing to note here is E° Pb| Pb2+ = 0.13 V and E° Zn2+| Zn = 0.76 V are the oxidation potentials and the formula requires reduction potentials.
Therefore, E° cathode is -0.13 V and E° anode is -0.76V.
Standard EMF of the cell i.e E° cell.
E° cell = E° cathode - E° anode
E° cell = -0.13 V - (-0.76)V
Therefore, E° cell is 0.63 V.
Ques. Distinguish between the standard electrode and standard cell potentials. (2 marks)
Ans. The standard reduction potential is found in a class known as normal cell potential or standard electrode potential. The potential difference between the cathode and anode is the inherent potential of cells. At 298 K, 1 atm, and 1 M solutions, all standard potentials are measured.
Ques. What factors influence the standard potential of an electrochemical cell? (2 marks)
Ans. The factors influencing the standard potential of an electrochemical cell are:
- The concentration of electrolyte used
- The temperature conditions used
- The surface area of rods used.
Ques. What is the use of the salt bridge in the setup to find the Standard Potential of an electrochemical cell? (2 marks)
Ans. The salt bridge completes the circuit by allowing ions to travel from one solution to another without causing the two solutions to mix. In the salt bridge, as ions are withdrawn from the solution at the electrodes, they are replaced by ions in the salt.
The K+ ions replace the cations reduced in the reduced half cell, and the NO-3 ions replace the anions oxidized in the oxidized half cell in the salt bridge. Because almost all potassium salts and nitrate salts are soluble and do not readily undergo reactions in the half cells, KNO3 is a good candidate.
Ques. Can we stir a CuSO4 aqueous solution with a Zn rod (under standard conditions)?
The reduction potentials of electrodes are given as:
E° Zn2+| Zn = - 0.76 V
E° Cu2+| Cu = 0.34 V (3 marks)
Ans. The Zn | Zn2+ represents the Anode as oxidation is taking place here while the Cu2+ | Cu Canceling represents the Cathode as reduction is taking place here.
E° cell = E° cathode - E° anode
E° cell = 0.34 V - (-0.76V)
Therefore, E° cell is 1.10 V.
1.10 > 0 . Therefore, this reaction will take place so we cannot stir a CuSO4 aqueous solution with Zn rod.
Ques. Predict feasibility of the reaction given under standard conditions.
Ni (s) + 2H+ (aq) → Ni2+ (aq) + H2 (g)
where reduction potentials of electrodes are given as:
E° Ni2+| Ni = - 0.25 V
E° H+| ½ H2 = 0 V which is the Standard Hydrogen Electrode (4 marks)
Ans. The Ni2+| Ni represents the Anode as oxidation is taking place here while the H+| ½ H2 represents the Cathode as reduction is taking place here.
E° cell = E° cathode - E° anode
E° cell = 0 V - (-0.25V)
Therefore, E° cell is 0.25 V.
The formula to calculate Standard Free Energy is:
ΔG°=-n x F x E° cell
ΔG°=-n x 96500 x 0.25
Yes, the cell is feasible as standard free energy is a negative value Because E° cell is a positive value.
Ques. Find the electrode potential at 25° C for H2SO4 aqueous solution with concentration 0.05M and 1 atm partial pressure. (5 marks)
Ans. H2SO4 is a highly concentrated dibasic acid. Therefore, 0.05M of H2SO4 will give 2 H+ ions which will have concentration → 2 x 0.05 = 0.1 M.
These are not standard conditions as concentration is not 1M. Therefore, using Nernst Equation
E = E° -0.059nlog10[P][R]
[P] is the product and [R] is the reactant.
We have to now write the balanced equation of the half-cell reaction:
H2 + 2e- → H2 where n=2 as 2 electrons are involved.
Now, E° of H+ | H2 is 0 which is the Standard Oxidation Potential of hydrogen electrodes.
So, E = 0 - 0.0592log10[Partial pressure of Hydrogen][H+ x H+]
= - 0.0592log10[Partial pressure of Hydrogen][H+ x H+]
Partial pressure of Hydrogenis given as 1 atm.
H+ concentration is 0.1 M.
= - 0.0592log1010.1 x 0.1
= - 0.059 V.
Ques. Find Standard Free Energy and predict the feasibility of the cell.
The cell representation is given as Zn | Zn2+ (1M) || Cu2+ (1M) | Cu where reduction potentials of electrodes are given as:
E° Zn2+| Zn = - 0.76 V
E° Cu2+| Cu = 0.34 V
Use 1F=96,500C (5 marks)
Ans. The Zn | Zn2+ represents the Anode as oxidation is taking place here while the Cu2+ | Cu represents the Cathode as reduction is taking place here.
E° cell = E° cathode - E° anode
E° cell = 0.34 V - (-0.76V)
Therefore, E° cell is 1.10 V.
The formula to calculate Standard Free Energy is:
ΔG°=-n x F x E° cell
ΔG°=-n x 96500 x 1.10
To find the value of n, we have to write the balanced equation of the cell reaction.
Zn → Zn2+ + 2e-
Cu2+ + 2e- → Cu
Canceling 2e- from both sides, we get the following reaction:
Zn + Cu2+ → Zn2+ + Cu
And we see there is a net change of oxidation number is 2. Therefore,
n=2.
ΔG°=-2 x 96500 x 1.10
ΔG°=-2,12,300 J = - 212.3 KJ
Yes, the cell is feasible as standard free energy is a negative value.
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