4.6 Electrochemical Series

Introduction

In the previous section, we learned that in a galvanic cell, zinc acts as the anode and copper acts as the cathode. But how do we know which metal will act as the anode and which will act as the cathode? The answer lies in the electrochemical series.

The electrochemical series is a list of metals arranged according to their tendency to lose or gain electrons. It helps chemists predict:

• Which metal will act as the anode and which as the cathode in a galvanic cell
• Which substances will form at the electrodes during electrolysis
• Whether a metal will react with an acid or another metal's salt solution

Definition

Electrode Potential (E^°): The tendency of a metal to become oxidized or reduced when placed in its salt solution is called electrode potential (E^°).

Electrochemical Series: A table showing the arrangement of metals based on increasing reduction potentials is called an electrochemical series.

In simple words: The electrochemical series is a list of metals arranged by their tendency to gain or lose electrons.

Main Idea

• The electrode potential of an electrode is measured in volts.
• The International Union of Pure and Applied Chemistry (IUPAC) has recommended the representation of electrode potential as reduction potentials.
• A metal higher in the series has a lower value of reduction potential than the metal below it in the series.
• So, it has a higher tendency to lose electrons and is more active than the metals below it.
• In a voltaic cell:
• The electrode with the lower value of reduction potential acts as the anode.
• The electrode with the higher reduction potential acts as the cathode.

Core Concept

Key point:

• More active metal (higher in series) → anode → oxidation.
• Less active metal (lower in series) → cathode → reduction.

Detailed Explanation

What is Electrode Potential?

Electrode potential (E^°): The tendency of a metal to become oxidized or reduced when placed in its salt solution.

• It is a measure of the potential of a reaction that can occur at the electrode.
• It is measured in volts.
• The electrode potential of an electrode is determined experimentally by using a standard hydrogen electrode.
• IUPAC has recommended the representation of electrode potential as reduction potentials.

What is the Electrochemical Series?

Electrochemical series: A table showing the arrangement of metals based on increasing reduction potentials.

Key features:

• A metal higher in the series has a lower value of reduction potential than the metal below it.
• So, it has a higher tendency to lose electrons and is more active than the metals below it.
• In a voltaic cell:
• The electrode with the lower value of reduction potential acts as the anode.
• The electrode with the higher reduction potential acts as the cathode.

How the Electrochemical Series Helps Predict Products

The electrochemical series helps us predict which substances will form at the electrodes during electrolysis.

At the Cathode (Negative Electrode):

• At the cathode, positive ions (cations) gain electrons (reduction happens).
• The ion lower in the electrochemical series (less reactive) is more likely to get reduced.

Example: If the solution contains sodium ions (Na⁺) and hydrogen ions (H⁺), hydrogen gas (H₂) forms because hydrogen is lower in the series than sodium.

At the Anode (Positive Electrode):

• At the anode, negative ions (anions) lose electrons (oxidation happens).
• Halide ions (Cl^-, Br^-, I^-) are discharged before hydroxide (OH^-) unless halides are absent.

Example: In a concentrated solution of sodium chloride, chloride ions (Cl^-) are discharged, producing chlorine gas (Cl₂) because chlorine is lower in the series than water.

Concentration Effect

However, in the electrolysis of dilute aqueous NaCl, OH^- ions are discharged instead of Cl^- ions, even though Cl^- ions are lower in the electrochemical series.

This is due to the following reasons:

Concentration Effect:

• The discharge of ions during electrolysis is influenced not only by their position in the electrochemical series but also by their concentration.
• In dilute aqueous NaCl, the concentration of Cl^- ions is relatively low compared to OH^- ions, which are present in larger amounts due to the ionization of water.

Overpotential of Chlorine Gas:

• The overpotential (extra voltage required) for the oxidation of Cl^- ions to chlorine gas is higher than that for the oxidation of OH^- ions to oxygen gas.
• This makes it energetically less favourable to discharge Cl^- ions.

Table 1: Electrochemical Series

Intermediate Level

How the electrochemical series helps predict products:

At the Cathode:

• Cations gain electrons (reduction).
• The ion lower in the series is more likely to be reduced.

Example: If a solution contains Na⁺ and H⁺, H₂ forms because H⁺ is lower in the series than Na⁺.

At the Anode:

• Anions lose electrons (oxidation).
• Halide ions (Cl^-, Br^-, I^-) are discharged before OH^- unless halides are absent.

Example: In concentrated NaCl, Cl^- is discharged, producing Cl₂.

Concentration Effect:

• In dilute NaCl, OH^- is discharged instead of Cl^-, even though Cl^- is lower in the series.
• This is due to the concentration effect and the overpotential of chlorine gas.

Key point:

• Electrochemical series predicts which ion is discharged.
• Concentration and overpotential can affect the actual product.

Solved Example

Example 1

Question: Which of the following metals is more likely to undergo reduction?

Al³⁺(aq) + 3e^- ⇌ Al(s) E^° = -1.66\ V

Fe³⁺(aq) + 3e^- ⇌ Fe(s) E^° = -0.44\ V

Solution:

• The reduction potential of Fe³⁺ (−0.44 V) is higher (less negative) than that of Al³⁺ (−1.66 V).
• A higher reduction potential means a greater tendency to be reduced.
• Therefore, Fe³⁺ is more likely to undergo reduction than Al³⁺.

Final Answer: Fe³⁺ is more likely to undergo reduction.

Example 2

Question: Arrange the following metals in order of increasing ability to get reduced.

Na⁺(aq) + e^- ⇌ Na(s) E^° = -2.71\ V

Zn²⁺(aq) + 2e^- ⇌ Zn(s) E^° = -0.76\ V

Cu²⁺(aq) + 2e^- ⇌ Cu(s) E^° = +0.34\ V

Solution:

• Increasing ability to get reduced means increasing reduction potential (from most negative to most positive).
• Order: Na⁺ (−2.71 V) < Zn²⁺ (−0.76 V) < Cu²⁺ (+0.34 V)

Final Answer: Na < Zn < Cu (in increasing ability to get reduced).

Example 3

Question: Identify the half-cell in which oxidation occurs and the half-cell in which reduction occurs in the given voltaic cell.

Following reactions occur in the cell:

At Anode: Zn(s) → Zn²⁺(aq) + 2e^-

At Cathode: 2H⁺(aq) + 2e^- → H₂(aq)

Solution:

• Oxidation occurs at the anode (Zn half-cell).
• Reduction occurs at the cathode (H⁺ half-cell).

Final Answer:

• Anode: Zn half-cell (oxidation)
• Cathode: H⁺ half-cell (reduction)

Important Points

• - Electrode potential (E^°): Tendency of a metal to become oxidized or reduced.
• - Electrochemical series: Table showing arrangement of metals based on increasing reduction potentials.
• - A metal higher in the series has a lower reduction potential and is more active.
• - A metal lower in the series has a higher reduction potential and is less active.
• - In a voltaic cell:
• - Anode = lower reduction potential (oxidation).
• - Cathode = higher reduction potential (reduction).
• - At the cathode: ion lower in the series is more likely to be reduced.
• - At the anode: halide ions are discharged before OH^- unless halides are absent.
• - Concentration effect: In dilute solutions, OH^- may be discharged instead of Cl^-.
• - Overpotential: Extra voltage required for a reaction.

Remember

• - Electrode potential = tendency to be oxidized or reduced.
• - Electrochemical series = arranged by increasing reduction potentials.
• - Higher in series → more active → anode.
• - Lower in series → less active → cathode.
• - Cathode: ion lower in series is reduced.
• - Anode: halide ions discharged before OH^-.
• - Concentration effect: dilute solutions may give different products.
• - Overpotential affects discharge of ions.
• - IUPAC recommends reduction potentials.

Common Mistakes

Quick Revision

Electrochemical Series:

The tendency of a metal to become oxidized or reduced, when placed in its salt solution is called electrode potential (E^°). Electrode potential is the measure of the potential of a reaction that can occur at the electrode. It is measured in volts. The electrode potential of an electrode is determined experimentally by using a standard hydrogen electrode. The International Union of Pure and Applied Chemistry (IUPAC) has recommended the representation of electrode potential as reduction potentials. A table showing the arrangement of metals based on increasing reduction potentials is called an electrochemical series.

A metal higher in the series has a lower value of reduction potential than the metal below it in the series. So, it has a higher tendency to lose electrons and is more active than the metals below it. In a voltaic cell electrode with the lower value of reduction potential acts as anode and the electrode having a higher reduction potential act as a cathode.

Electrochemical series helps us predict which substances will form at the electrodes during electrolysis:

At the Cathode (Negative Electrode): At the cathode positive ions (cations) gain electrons (reduction happens). The ion lower in the electrochemical series (less reactive) is more likely to get reduced. Example: If the solution contains sodium ions (Na⁺) and hydrogen ions (H⁺), hydrogen gas (H₂) forms because hydrogen is lower in the series than sodium.

At the Anode (Positive Electrode): At the anode negative ions (anions) lose electrons (oxidation happens). Halide ions (Cl^-, Br^-, I^-) are discharged before hydroxide (OH^-) unless halides are absent. Example: In a concentrated solution of sodium chloride, chloride ions (Cl^-) are discharged, producing chlorine gas (Cl₂) because chlorine is lower in the series than water.

However in the electrolysis of dilute aqueous NaCl, OH^- ions are discharged instead of Cl^- ions, even though Cl^- ions are lower in the electrochemical series. This is due to the following reasons.

Concentration Effect: The discharge of ions during electrolysis is influenced not only by their position in the electrochemical series but also by their concentration. In dilute aqueous NaCl, the concentration of Cl^- ions is relatively low compared OH^- ions, which are present in larger amounts due to the ionization of water.

Overpotential of Chlorine Gas: The overpotential (extra voltage required) for the oxidation of Cl^- ions to chlorine gas is higher than that for the oxidation of OH^- ions to oxygen gas. This makes it energetically less favourable to discharge Cl^- ions.

60-Second Revision

• Electrode potential (E^°) = tendency to be oxidized or reduced.
• Electrochemical series = arranged by increasing reduction potentials.
• Higher in series → more active → anode.
• Lower in series → less active → cathode.
• Cathode: ion lower in series is reduced.
• Anode: halide ions discharged before OH^-.
• Concentration effect: dilute solutions may give different products.
• Overpotential: extra voltage required.
• IUPAC: reduction potentials.
• Standard hydrogen electrode: reference electrode.
• ⚠️ Warning: in dilute solutions, OH^- may be discharged instead of Cl^- at the anode.