Hello everyone! Welcome to this in-depth exploration of
Standard Electrode Potential and the Electrochemical Series. This topic is a cornerstone of electrochemistry, crucial not just for your CBSE board exams but absolutely vital for cracking JEE Main and Advanced. We'll start from the very basics and build our understanding piece by piece, so buckle up!
### 1. The Genesis of Potential: Understanding Electrode Potential
Imagine you dip a strip of zinc metal (Zn) into a solution containing zinc ions (Zn²⁺). What happens at the interface between the metal and the solution? It's not just a passive dipping; there's an active exchange!
At this interface, two opposing processes occur:
1.
Oxidation: Zinc atoms from the metal strip can lose electrons and enter the solution as Zn²⁺ ions.
Zn(s) → Zn²⁺(aq) + 2e⁻
These electrons accumulate on the metal strip, making it negatively charged.
2.
Reduction: Zinc ions from the solution can gain electrons from the metal strip and deposit onto it as Zn atoms.
Zn²⁺(aq) + 2e⁻ → Zn(s)
This process removes electrons from the metal, making it positively charged.
Initially, these rates might be different. However, after some time, an equilibrium is established. This equilibrium results in a
charge separation at the interface. The metal electrode either becomes slightly negatively charged (due to excess electrons from oxidation) or slightly positively charged (due to electron consumption from reduction) with respect to the solution.
This charge separation creates an electrical potential difference across the interface, much like a tiny battery. This potential difference is called the
electrode potential. Every metal (or non-metal involved in a redox process) dipped in a solution of its own ions (or involved in its specific redox environment) develops such a potential.
* If the metal tends to get oxidized (lose electrons) more readily, it will become more negative with respect to the solution. This is its
oxidation potential.
* If the metal ion tends to get reduced (gain electrons) more readily, the metal will become more positive with respect to the solution. This is its
reduction potential.
Key Insight: Oxidation potential and reduction potential for the same half-reaction are equal in magnitude but opposite in sign. For example, if the oxidation potential of Zn is +X V, its reduction potential (for Zn²⁺ + 2e⁻ → Zn) is -X V.
### 2. The Measurement Conundrum and the Need for a Reference
Here's the catch: We cannot measure the absolute electrode potential of a single half-cell. Why? Because potential difference is always measured *between two points*. A voltmeter needs two terminals to measure a potential difference. A single half-cell represents only one of those "points."
To measure the potential of a half-cell, we must connect it to another half-cell, completing a circuit. The potential measured by the voltmeter will be the difference between the potentials of the two half-cells. This measured potential is the
cell potential (E_cell).
Analogy: Imagine trying to measure the "absolute height" of a single step. You can't. You can only measure its height *relative* to the ground, or *relative* to another step. Similarly, electrode potential is always relative.
To overcome this, scientists established a universally accepted reference electrode, whose potential is arbitrarily assigned a value of exactly zero. This allows us to measure all other electrode potentials relative to this reference.
### 3. The Standard Hydrogen Electrode (SHE): Our Zero-Point Reference
The chosen reference is the
Standard Hydrogen Electrode (SHE), also sometimes called the Normal Hydrogen Electrode (NHE).
Construction of SHE:
* A platinum electrode (inert, provides a surface for the reaction and conducts electrons) is immersed in an acidic solution.
* The solution contains H⁺ ions at
1 M concentration.
* Pure hydrogen gas is bubbled through the solution at
1 atmospheric pressure (1 bar or 1 atm).
* The temperature is maintained at
298 K (25 °C).
The Half-Reaction at SHE:
The equilibrium established at the SHE is:
2H⁺(aq, 1 M) + 2e⁻ ⇌ H₂(g, 1 atm)
The Definition: By international convention (IUPAC), the
Standard Electrode Potential (E°) of the SHE is arbitrarily assigned a value of
0.00 Volts at all temperatures.
E°(SHE) = 0.00 V
### 4. Standard Electrode Potential (E°): The Benchmark
Now that we have a reference, we can define the standard electrode potential of any other half-cell.
The
Standard Electrode Potential (E°) of an electrode is the potential difference developed between the electrode and the electrolyte when it is connected to a Standard Hydrogen Electrode (SHE) under standard conditions.
Standard Conditions:
* Concentration of all ions in the half-cell:
1 M.
* Pressure of all gases:
1 atm (or 1 bar).
* Temperature:
298 K (25 °C).
IUPAC Convention for Reporting E°:
Crucially, IUPAC recommends that all standard electrode potentials be reported as
Standard Reduction Potentials (SRPs). This means we write the half-reaction as a reduction process (gaining electrons).
Example 1: Measuring E° for Zn/Zn²⁺ couple
Let's connect a zinc half-cell (Zn electrode in 1 M ZnSO₄ solution) to a SHE.
*
Zinc half-cell: Zn(s) | Zn²⁺(aq, 1 M)
*
SHE: Pt(s) | H₂(g, 1 atm) | H⁺(aq, 1 M)
When connected, a voltmeter reads 0.76 V, and electrons flow from the zinc electrode to the SHE. This indicates that zinc is undergoing oxidation and SHE is undergoing reduction.
*
Anode (Oxidation): Zn(s) → Zn²⁺(aq) + 2e⁻
*
Cathode (Reduction): 2H⁺(aq) + 2e⁻ → H₂(g)
The cell potential (E°_cell) = E°_cathode - E°_anode.
0.76 V = E°(H⁺/H₂) - E°(Zn²⁺/Zn)
0.76 V = 0.00 V - E°(Zn²⁺/Zn)
Therefore,
E°(Zn²⁺/Zn) = -0.76 V. (This is the standard *reduction* potential for zinc).
Example 2: Measuring E° for Cu/Cu²⁺ couple
If we connect a copper half-cell (Cu electrode in 1 M CuSO₄ solution) to a SHE.
*
Copper half-cell: Cu(s) | Cu²⁺(aq, 1 M)
*
SHE: Pt(s) | H₂(g, 1 atm) | H⁺(aq, 1 M)
The voltmeter reads 0.34 V, and electrons flow from the SHE to the copper electrode. This means SHE is undergoing oxidation, and copper is undergoing reduction.
*
Anode (Oxidation): H₂(g) → 2H⁺(aq) + 2e⁻
*
Cathode (Reduction): Cu²⁺(aq) + 2e⁻ → Cu(s)
The cell potential (E°_cell) = E°_cathode - E°_anode.
0.34 V = E°(Cu²⁺/Cu) - E°(H⁺/H₂)
0.34 V = E°(Cu²⁺/Cu) - 0.00 V
Therefore,
E°(Cu²⁺/Cu) = +0.34 V. (This is the standard *reduction* potential for copper).
### 5. The Electrochemical Series: A Power Ranking of Redox Couples
The
Electrochemical Series is a list of various standard reduction potentials (SRPs) arranged in either increasing or decreasing order. By convention, it's usually presented with the most negative SRP at the top and the most positive SRP at the bottom, or vice-versa. Let's consider a common arrangement where SRPs become more positive as we go down the series.
Reduction Half-Reaction |
Standard Reduction Potential (E° in Volts) |
|---|
| Li⁺(aq) + e⁻ → Li(s) | -3.05 |
| K⁺(aq) + e⁻ → K(s) | -2.92 |
| Ca²⁺(aq) + 2e⁻ → Ca(s) | -2.87 |
| Na⁺(aq) + e⁻ → Na(s) | -2.71 |
| Mg²⁺(aq) + 2e⁻ → Mg(s) | -2.37 |
| Al³⁺(aq) + 3e⁻ → Al(s) | -1.66 |
| Zn²⁺(aq) + 2e⁻ → Zn(s) | -0.76 |
| Cr³⁺(aq) + 3e⁻ → Cr(s) | -0.74 |
| Fe²⁺(aq) + 2e⁻ → Fe(s) | -0.44 |
| Ni²⁺(aq) + 2e⁻ → Ni(s) | -0.25 |
| Sn²⁺(aq) + 2e⁻ → Sn(s) | -0.14 |
| Pb²⁺(aq) + 2e⁻ → Pb(s) | -0.13 |
| 2H⁺(aq) + 2e⁻ → H₂(g) | 0.00 (SHE) |
| Cu²⁺(aq) + 2e⁻ → Cu(s) | +0.34 |
| I₂(s) + 2e⁻ → 2I⁻(aq) | +0.54 |
| Fe³⁺(aq) + e⁻ → Fe²⁺(aq) | +0.77 |
| Ag⁺(aq) + e⁻ → Ag(s) | +0.80 |
| Br₂(l) + 2e⁻ → 2Br⁻(aq) | +1.09 |
| O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l) | +1.23 |
| Cr₂O₇²⁻(aq) + 14H⁺(aq) + 6e⁻ → 2Cr³⁺(aq) + 7H₂O(l) | +1.33 |
| Cl₂(g) + 2e⁻ → 2Cl⁻(aq) | +1.36 |
| MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ → Mn²⁺(aq) + 4H₂O(l) | +1.51 |
| F₂(g) + 2e⁻ → 2F⁻(aq) | +2.87 |
Interpretation of the Electrochemical Series:
1.
Strength of Oxidizing and Reducing Agents:
*
Higher (more negative) E° values: Species on the *left* of the reduction half-reaction (e.g., Li⁺, K⁺) are *weak oxidizing agents*. Their conjugate species on the *right* (e.g., Li, K) are
strong reducing agents because they have a high tendency to get oxidized (lose electrons).
*
Lower (more positive) E° values: Species on the *left* of the reduction half-reaction (e.g., F₂, Cl₂, MnO₄⁻) are
strong oxidizing agents because they have a high tendency to get reduced (gain electrons). Their conjugate species on the *right* (e.g., F⁻, Cl⁻, Mn²⁺) are *weak reducing agents*.
Trend: As you move down the series (increasing E°), the oxidizing power of the species on the left increases, and the reducing power of the species on the right decreases.
2.
Feasibility of Redox Reactions (Spontaneity):
A redox reaction is spontaneous under standard conditions if the cell potential (E°_cell) is positive.
E°_cell = E°_cathode (reduction) - E°_anode (oxidation)
For a spontaneous reaction, the oxidizing agent must have a more positive E° (greater tendency to be reduced) than the reducing agent (greater tendency to be oxidized).
Rule of Thumb: Any species on the left side of a half-reaction in the series can oxidize any species on the right side of a half-reaction that appears *above it* in the series.
Example: Will Zn react with CuSO₄ solution?
Zn²⁺/Zn: E° = -0.76 V
Cu²⁺/Cu: E° = +0.34 V
Since Cu²⁺ has a more positive E° (+0.34 V) than Zn²⁺ (-0.76 V), Cu²⁺ is a stronger oxidizing agent than Zn²⁺. Thus, Cu²⁺ will oxidize Zn (meaning Zn will reduce Cu²⁺).
Reaction: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
E°_cell = E°(Cu²⁺/Cu) - E°(Zn²⁺/Zn) = (+0.34) - (-0.76) = +1.10 V.
Since E°_cell > 0, the reaction is spontaneous.
3.
Displacement Reactions:
A metal with a more negative standard reduction potential (higher in the series) can displace a metal with a more positive standard reduction potential (lower in the series) from its salt solution. This is because the metal higher in the series is a stronger reducing agent.
Example: Can Fe displace Cu from CuSO₄?
Fe²⁺/Fe: E° = -0.44 V
Cu²⁺/Cu: E° = +0.34 V
Since E°(Fe²⁺/Fe) < E°(Cu²⁺/Cu), Fe is a stronger reducing agent than Cu. Yes, Fe can displace Cu.
Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s)
4.
Reaction with Acids (Evolution of H₂ gas):
Metals having a negative standard reduction potential (i.e., E° < 0.00 V) can displace hydrogen from acids, because they are stronger reducing agents than H₂/H⁺.
Example:** Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g)
E°_cell = E°(H⁺/H₂) - E°(Zn²⁺/Zn) = 0.00 - (-0.76) = +0.76 V. Spontaneous.
Metals having a positive standard reduction potential (i.e., E° > 0.00 V) cannot displace hydrogen from acids.
Example:** Cu(s) + 2H⁺(aq) → No reaction (Cu is a weaker reducing agent than H₂)
E°_cell = E°(H⁺/H₂) - E°(Cu²⁺/Cu) = 0.00 - (+0.34) = -0.34 V. Non-spontaneous.
5.
Corrosion Tendencies (for Advanced understanding):
Metals with more negative E° values (e.g., Fe, Zn) are more easily oxidized, meaning they are more susceptible to corrosion than metals with more positive E° values (e.g., Ag, Au). This is why zinc is used to protect iron from rusting (galvanization), as zinc will preferentially get oxidized.
### 6. CBSE vs. JEE Focus
*
CBSE Level: Focus will be on defining electrode potential, SHE, standard electrode potential, and its basic applications (identifying oxidizing/reducing agents, predicting simple displacement reactions, calculating E°_cell).
*
JEE Level: Requires a deeper conceptual understanding of *why* potentials arise, the implications of SRP values for complex reactions, relating E°_cell to spontaneity through