What is Electrode Potential?
According to IUPAC electrode potential has been defined as follows-
In electrochemistry the electrode potential is the electromotive force of a cell built of two electrodes.
It is denoted by the sign E. It is not possible to measure accurately the absolute value of single electrode potential directly. Only the difference in potential between two electrodes can be measured experimentally. So, in an experiment to measure electrode potential, in a cell one electrode is used as a reference electrode whose potential is already known and another electrode is used of unknown potential. Cell potential is measured experimentally which is equal to the sum of potentials on the two electrodes.
ECell = ECathode + EAnode
Ecell is measured by voltameter experimentally and electrode potential of one electrode is already known so electrode potential of another (electrode with unknown electrode potential) can be calculated.
What is Standard Electrode Potential?
The potential of the half-reaction (half-cell) measured against the standard hydrogen electrode under standard conditions is called the standard electrode potential for that half-cell or half reaction. Standard conditions mean temperature at 298K, pressure should be 1 atm, the concentration of the electrolyte used should be 1M. It is measured with respect to standard hydrogen electrodes.
Standard hydrogen electrode is a gas – ion electrode. It is used as a reference electrode for determination of standard electrode potential of elements and other half cells. It can act as anode half - cell as well as cathode half-cell. Value of its standard reduction potential and standard oxidation potential is always zero at 25℃ or 298K. It is the base of the thermodynamic scale of oxidation-reduction potentials.
Standard electrode potential is denoted by E0. Either Standard reduction potential or standard oxidation potential can be calculated for an electrode using a standard hydrogen electrode. The standard cell potential is the difference between standard reduction potentials of two half–cells or half-reactions. It can be represented as –
E0cell = Ecathode – Eanode
Calculating Standard Reduction Potential for Zinc Electrode
We are describing here a method of calculating standard reduction potential by taking a zinc electrode in a half cell with zinc sulfate (electrolyte) against a standard hydrogen electrode. Standard reduction potential and standard oxidation potential for standard hydrogen potential are always taken 0.00. The experiment is described below-
Construction of standard hydrogen electrode - To construct a standard hydrogen electrode, we take a hydrogen chloride solution of 1M in a glass beaker. Now a platinum inert electrode with platinum black foil at one end is immersed in the beaker and a glass jacket is kept on it to prevent the entry of oxygen. It has an inlet for pure hydrogen gas (1atm) to enter the solution. Temperature is maintained at 25℃. A figure of Standard Hydrogen Electrode is shown below-
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This half cell of standard hydrogen electrode is connected with a half cell of zinc electrode. For this, zinc sulfate is taken in a beaker and a zinc rod is dipped in it. 1M concentration of electrolyte zinc sulfate is taken. Temperature is maintained at 25℃. Now, this zinc electrode is connected to a standard hydrogen electrode by using a voltmeter which will measure the electrode potential of the cell. A salt bridge is also used which prevents intermixing of the solutions and maintains the electrical neutrality of the solutions. Zinc half cell is taken as cathode and hydrogen half cell is taken as the anode.
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As we know, the standard reduction potential of standard hydrogen electrode is always taken as 0 in standard conditions and we are using standard conditions in the experiment. So, E0H+/ H2 = 0
The value of the standard reduction potential of the cell is measured by reading the voltmeter used. Thus, by the experiment, we know the value of E0cell, and the value of E0H+/H2 is already known. So, by equation (1), we can calculate the value of E0Zn2+/Zn.
The value of E0cell comes out to -0.76V by the experiment. So, the value of E0Zn2+/Zn is -0.76V as the standard reduction potential for SHE is 0.
By the same method, we can calculate the standard reduction potential of the copper electrode by using a half cell with copper electrode and copper sulfate electrolyte in place of zinc electrode and zinc sulfate electrolyte. By the experiment, the value of E0Cu2+/Cu comes out +0.34V.
If Daniel cell representation is given as Zn(s)/Zn2+(aq)||Cu2+(s)/Cu(aq) and standard conditions are used such concentrations of electrolyte is 1M, temperature is 298K and pressure is 1 atm. Then we can calculate the standard electrode potential for the cell as follows –
E0cell = E0cathode – E0anode
E0cell = E0Cu2+/Cu - E0Zn2+/Zn
(if you use + sign in place of – in the equation then you have to write zinc electrode as oxidation electrode it means it will be written as E0cell = E0Cu2+/Cu + E0Zn2+/Zn )
E0cell = +0.34 – (-0.76)
E0cell = 1.1V
Thus, the EMF or standard cell potential of Daniel’s cell is 1.1V.
The positive value of E0cell shows that the reaction occurs spontaneously while the negative value of E0cell shows that the reaction proceeds spontaneously in the opposite direction.
Standard hydrogen electrode which is used as a reference electrode should not be affected by the properties of the solution to be analyzed and it must be physically isolated. Apart from standard hydrogen electrodes, many other electrodes are used as reference electrodes such as calomel electrodes, quinhydrone electrodes, etc.
Electrochemical Series
The arrangement of elements according to their standard electrode potential values is called electrochemical series. It is also called an activity series. Elements having higher standard electrode potential are placed above those having lower standard electrode potentials. The elements placed at the top of the series are having a tendency to get reduced easily. While the elements placed at the bottom have the least tendency to get reduced.
Fluorine has a maximum tendency to get reduced as it has the highest standard electrode potential. While lithium has the least tendency to get reduced as it has the lowest value of standard electrode potential. Thus, fluorine is a powerful oxidizing agent and lithium is a powerful reducing agent.
Uses of Standard Electrode Potentials
Uses of standard electrode potentials are listed below –
It is used to measure the relative strengths of various oxidants and reductants.
It is used to calculate standard cell potential.
It is used to predict possible reactions.
Prediction of equilibrium in the reaction.
Limitation of Standard Electrode Potentials
Standard electrode potentials can be applied to aqueous equilibrium only. We can predict reaction possibilities, but we can’t predict the rate of reaction by using standard electrode potentials.
This was brief on standard electrode potential and its calculations explained with examples. Focus on the concept and understand how it is calculated.
FAQs on Standard Electrode Potential
1. What is meant by the standard electrode potential of an element as per the NCERT syllabus?
The standard electrode potential (E°) of an element is the potential difference developed when its electrode is in contact with a 1 Molar (1M) solution of its own ions at a temperature of 298 K (25°C). It is a measure of the tendency of a chemical species to be reduced, and it is measured relative to the Standard Hydrogen Electrode (SHE), which is assigned a potential of 0.00 volts.
2. What is the importance of understanding standard electrode potential for Class 12 Chemistry?
Understanding standard electrode potential is crucial for several reasons in electrochemistry. Its primary importance includes:
- Comparing Oxidising and Reducing Strength: It allows us to compare the relative tendencies of different substances to act as oxidising or reducing agents.
- Predicting Reaction Spontaneity: It helps predict whether a given redox reaction will occur spontaneously under standard conditions.
- Calculating Cell EMF: It is used to calculate the standard electromotive force (EMF) or E°cell of any electrochemical cell.
3. How is the standard EMF (E°cell) of a galvanic cell calculated using standard electrode potentials?
The standard EMF of a galvanic cell (E°cell) is calculated by finding the difference between the standard reduction potential of the cathode (where reduction occurs) and the standard reduction potential of the anode (where oxidation occurs). The formula is:
E°cell = E°(cathode) - E°(anode)
A positive E°cell value indicates a spontaneous reaction under standard conditions.
4. What are the specific standard conditions required for measuring standard electrode potential (E°)?
To ensure a consistent and comparable measurement, the standard electrode potential (E°) must be determined under the following specific conditions:
- Temperature: The measurement must be taken at a constant temperature of 298 K (25°C).
- Concentration: All dissolved species (ions) in the half-cell must have a concentration of 1 Molar (1 mol/L).
- Pressure: If any gaseous species are involved in the half-reaction, their pressure must be maintained at 1 bar.
5. How can you use standard electrode potential values to predict if a redox reaction is spontaneous?
You can predict the spontaneity of a redox reaction by calculating its standard cell potential (E°cell). The rule is simple: if the calculated E°cell is positive, the reaction is spontaneous as written under standard conditions. If the E°cell is negative, the reaction is non-spontaneous in the forward direction but is spontaneous in the reverse direction. This is directly related to Gibbs free energy, where a positive E°cell corresponds to a negative ΔG°.
6. Why is the Standard Hydrogen Electrode (SHE) used as the universal reference for potential measurements?
The absolute potential of a single half-cell cannot be measured independently; only the potential difference between two half-cells can be determined. By international convention, the Standard Hydrogen Electrode (SHE) is assigned an arbitrary potential of exactly 0.00 volts under standard conditions. This provides a fixed, universal baseline against which the electrode potentials of all other half-cells can be measured and compared, creating a standardised scale known as the electrochemical series.
7. Why does the copper electrode (Cu²⁺/Cu) have a positive standard reduction potential (+0.34 V)?
A positive standard reduction potential indicates the tendency of a species to be reduced compared to H⁺ ions. The value of +0.34 V for the Cu²⁺/Cu electrode means that Cu²⁺ ions have a stronger tendency to gain electrons (get reduced) than H⁺ ions from the Standard Hydrogen Electrode (SHE). Therefore, when a copper half-cell is connected to the SHE, copper acts as the cathode, and hydrogen is oxidised.
8. What is the key difference between electrode potential (E) and standard electrode potential (E°)?
The key difference lies in the conditions of measurement. Standard Electrode Potential (E°) is the potential measured under a strict set of standard conditions (298 K, 1M concentration, 1 bar pressure). In contrast, Electrode Potential (E) is the potential of an electrode under any non-standard conditions, where the concentration, temperature, or pressure may vary. The Nernst equation is used to calculate the electrode potential (E) at non-standard conditions.
