2.6 Electrochemical Series

In the previous sections, we learned how to measure the standard electrode potential of a half-cell relative to the Standard Hydrogen Electrode (SHE). When we arrange various elements and ions in order of their standard reduction potentials, we get a powerful tool called the electrochemical series. This series allows chemists to predict the direction of redox reactions and the relative strength of oxidizing and reducing agents.

Definition

The electrochemical series is a list of elements and their standard electrode potentials (reduction potentials) arranged in order of their values relative to the Standard Hydrogen Electrode (SHE).

Core Concept

The core concept is that the electrochemical series provides a thermodynamic ranking of how easily different species gain or lose electrons.

• Species with more negative reduction potentials have a greater tendency to lose electrons (they are easily oxidized).
• Species with more positive reduction potentials have a greater tendency to gain electrons (they are easily reduced).

By comparing the positions of two half-cells in the series, you can immediately tell which one will act as the anode (oxidation) and which will act as the cathode (reduction) when they are connected.

Detailed Explanation

1. IUPAC Conventions

Under the recommendation of the International Union of Pure and Applied Chemistry (IUPAC), the half-cell reactions in the electrochemical series are written as reduction reactions. Therefore, the E^° values listed are known as standard reduction potentials.

• The value of the oxidation potential for an electrode can be obtained by reversing the sign of its reduction potential. For example, if the reduction potential of zinc is -0.76 V, its oxidation potential is +0.76 V.
• Note that the given reduction potential values relate to standard conditions only (1 M solution of ions, 25°C or 298 K, and 1 atm pressure). Changes in these conditions will alter the values, as described by the Nernst equation.

2. The Activity Series of Metals

A displacement or replacement reaction occurs when an element displaces another element that is a part of a compound. The general equation is: A + XY \longrightarrow X + AY In this reaction, atom A replaces atom X in the compound XY.

Example 1: Zinc and Copper Sulphate When Zn metal is placed in a blue solution of copper(II) sulphate, the blue colour slowly fades away and grey metal is replaced by red-orange Cu metal. Zn_{(s)} + Cu^{2+}_{(aq)} \longrightarrow Zn^{2+}_{(aq)} + Cu_{(s)} In this reaction, Cu ions in the solution are reduced to Cu metal, and Zn atoms are oxidized to Zn ions. The electrochemical series shows that the standard reduction potential of copper (+0.34 V) is greater than that of zinc (-0.76 V). This means it is easy to oxidize Zn to its ions and reduce Cu^{2+} ions to its atoms. Thus, Zn can replace Cu^{2+} ions from its solution.

Example 2: Copper and Zinc Sulphate However, when copper metal is placed in a zinc sulphate solution, no replacement reaction occurs. Copper cannot displace zinc because copper is lower in the electrochemical series (has a more positive reduction potential) and is less easily oxidized than zinc.

The Activity Series For the same reason, Mg and Al can also displace Cu^{2+} ions, but Ag cannot displace Cu^{2+} ions. Similarly, metals like Na and K can displace H_2 from water, but metals like Cu and Ag cannot displace H_2 from water. Metals are, therefore, ranked according to their ability to replace other metals and hydrogen from their compounds. In this ranking, metals and hydrogen are arranged in order of decreasing ease of oxidation to their respective ions in aqueous solution. This arrangement is called the activity series.

3. Rules for Predicting Reactions

• Metals higher on the list transfer electrons to metal cations lower on the list. The greater the separation between the species, the more vigorous the reaction will be.
• Example: When powdered barium is heated with lead(II) oxide, a replacement reaction occurs because Ba is more active than Pb.
• Example: When iron pellets are added to a solution of MgCl_2, no reaction will occur because Fe is below Mg in the activity series. Iron cannot displace magnesium.

Intermediate Level

The electrochemical series is a list of standard reduction potentials (E^°) arranged from most negative to most positive. The position of a species in the series tells you its relative strength as an oxidizing or reducing agent. A metal with a more negative reduction potential is a stronger reducing agent (it is more easily oxidized) than a metal with a more positive reduction potential. Therefore, a metal can displace another metal from its salt solution if it is higher in the series (has a more negative reduction potential). This is the basis of the activity series. For example, Zn (E^° = -0.76 V) can displace Cu (E^° = +0.34 V) from a solution because Zn is more easily oxidized.

Important Points

• - Electrochemical Series: A list of standard reduction potentials.
• - IUPAC Convention: Values are for reduction reactions.
• - Oxidation Potential: The negative of the reduction potential.
• - Activity Series: Metals arranged by their ability to displace other metals and hydrogen.
• - Metals higher in the series (more negative reduction potential) are more easily oxidized and are stronger reducing agents.
• - Displacement Reaction: A more reactive metal (higher in the series) can displace a less reactive metal (lower in the series) from its salt solution.
• - Example: Zn can displace Cu, but Cu cannot displace Zn.

Remember

• - Electrochemical series: Ranking of reduction potentials.
• - More negative E^° → more easily oxidized → stronger reducing agent.
• - More positive E^° → more easily reduced → stronger oxidizing agent.
• - Displacement occurs if the metal is higher in the series (more negative E^°).
• - Na, K can displace H_2 from water; Cu, Ag cannot.
• - Zn displaces Cu; Cu does not displace Zn.

Common Mistakes

• - Confusing the electrochemical series with the activity series. The electrochemical series is a list of reduction potentials, while the activity series is a ranking of metals by their reactivity (ease of oxidation). They are related, but the electrochemical series includes non-metals and ions, while the activity series is typically just for metals.
• - Reversing the trend. Remember that a more negative reduction potential means the species is more easily oxidized (more reactive as a metal). A more positive reduction potential means the species is more easily reduced (less reactive as a metal).
• - Thinking that the sign of the reduction potential tells you the direction of the reaction. The sign is just a convention. To predict the direction of a reaction, you must compare the potentials of the two half-cells and determine which one will be the cathode (higher E^°) and which will be the anode (lower E^°).
• - Forgetting that standard conditions apply. The values in the table are for 1 M concentration, 1 atm pressure, and 25°C. Changing the conditions will change the potentials (Nernst equation).
• - Adding potentials instead of subtracting them. Use E^°_cell = E^°_cathode - E^°_anode.

Quick Revision

The electrochemical series is a list of standard reduction potentials of half-cells, measured relative to the SHE (0.00 V). It is arranged from the most negative to the most positive values. Metals with a more negative reduction potential are more easily oxidized and are stronger reducing agents. Metals with a more positive reduction potential are more easily reduced and are stronger oxidizing agents. This series allows us to predict whether a displacement reaction will occur: a metal higher in the series can displace a metal lower in the series from its solution. For example, zinc can displace copper, but copper cannot displace zinc.

60-Second Revision

The electrochemical series ranks elements by standard reduction potential. A more negative value means the element is more easily oxidized (a stronger reducing agent). A more positive value means it is more easily reduced (a stronger oxidizing agent). This allows us to predict displacement reactions: a metal can displace another metal from solution if it is higher in the series (more negative E^°). For example, zinc displaces copper, but copper cannot displace zinc.

Frequently Asked Questions (FAQs)

Q1. What is the electrochemical series? Answer: The electrochemical series is a list of standard electrode (reduction) potentials of various half-reactions, arranged in order of their values relative to the Standard Hydrogen Electrode (SHE). It is used to predict the direction of redox reactions and the relative strengths of oxidizing and reducing agents.

Q2. What is the significance of the activity series of metals? Answer: The activity series ranks metals by their ability to displace other metals and hydrogen from their compounds. It is based on the ease of oxidation of the metals. A metal higher in the series (more easily oxidized) can displace a metal lower in the series from its salt solution. This allows prediction of whether a single displacement reaction will occur.

Q3. Why does zinc displace copper from copper sulphate solution? Answer: The standard reduction potential of zinc (-0.76 V) is more negative than that of copper (+0.34 V). This means zinc is more easily oxidized than copper. Therefore, zinc atoms can transfer electrons to copper ions, causing zinc to be oxidized (to Zn^{2+}) and copper ions to be reduced (to Cu). This is a spontaneous displacement reaction.

Q4. Why does copper not displace zinc from zinc sulphate solution? Answer: Copper has a more positive standard reduction potential (+0.34 V) than zinc (-0.76 V). Copper is less easily oxidized than zinc, so it does not have enough reducing power to transfer electrons to zinc ions. Therefore, no reaction occurs.

Q5. What is the relationship between the electrochemical series and the ease of oxidation? Answer: The electrochemical series lists standard reduction potentials. The more negative the reduction potential, the greater the tendency of the species to be oxidized (to lose electrons). Therefore, metals with more negative reduction potentials are more easily oxidized and are stronger reducing agents.

Q6. How can the electrochemical series be used to predict the feasibility of a redox reaction? Answer: To predict feasibility, identify the two half-cells involved. The half-cell with the more positive reduction potential will undergo reduction (cathode), and the one with the more negative reduction potential will undergo oxidation (anode). The reaction is feasible (spontaneous) if the calculated cell potential is positive. If the reaction as written has a negative potential, it is not feasible, and the reverse reaction would be spontaneous.

Q7. What is the activity series of metals? Answer: The activity series is a ranking of metals in order of decreasing ease of oxidation (i.e., decreasing reactivity). Metals at the top are most easily oxidized and are the most reactive, while metals at the bottom are least easily oxidized and are the least reactive. It is derived from the electrochemical series and is used to predict displacement reactions.

Q8. Can a metal displace hydrogen from water? Answer: Metals that are above hydrogen in the activity series (i.e., have a more negative reduction potential than hydrogen, which is 0.00 V) can displace hydrogen from water or acids. For example, sodium and potassium react vigorously with water to produce hydrogen gas. Metals below hydrogen, such as copper and silver, cannot displace hydrogen from water.

Q9. What is the IUPAC convention for the electrochemical series? Answer: IUPAC recommends that the half-cell reactions in the electrochemical series be written as reduction reactions. Therefore, the listed E^° values are standard reduction potentials. The oxidation potential is obtained by reversing the sign of the reduction potential.

Q10. Why does the electrochemical series only apply to standard conditions? Answer: The standard reduction potentials in the series are measured under standard conditions (1 M concentration, 1 atm pressure, 25°C). Changes in temperature, pressure, or concentration will change the electrode potential, as described by the Nernst equation. Therefore, the tabulated values are only directly applicable under standard conditions.

Multiple Choice Questions (MCQs)

• Q1. What does the electrochemical series rank?
• A. Elements by atomic number
• B. Elements by standard reduction potential
• C. Compounds by solubility
• D. Gases by density
• Correct Answer: B
• Explanation: The electrochemical series is a list of standard electrode (reduction) potentials of half-reactions, arranged in order of their values.

• Q2. In the electrochemical series, which metal is more easily oxidized?
• A. The one with the more positive reduction potential
• B. The one with the more negative reduction potential
• C. The one with the higher atomic mass
• D. The one with the lower atomic number
• Correct Answer: B
• Explanation: A more negative reduction potential indicates a greater tendency to be oxidized (to lose electrons).

• Q3. Which of the following metals can displace copper from a copper sulphate solution?
• A. Silver
• B. Zinc
• C. Gold
• D. Platinum
• Correct Answer: B
• Explanation: Zinc is higher than copper in the electrochemical series (more negative reduction potential), so it can displace copper from its salt solution.

• Q4. Which of the following metals cannot displace hydrogen from water?
• A. Sodium
• B. Potassium
• C. Copper
• D. Calcium
• Correct Answer: C
• Explanation: Copper is below hydrogen in the activity series (more positive reduction potential), so it cannot displace hydrogen from water.

• Q5. What is the standard reduction potential of the hydrogen electrode?
• A. 0.00 V
• B. 1.00 V
• C. -1.00 V
• D. 0.76 V
• Correct Answer: A
• Explanation: The Standard Hydrogen Electrode (SHE) is defined as having a potential of exactly 0.00 V.

• Q6. If a metal has a more positive reduction potential, it is a stronger:
• A. Reducing agent
• B. Oxidizing agent
• C. Base
• D. Acid
• Correct Answer: B
• Explanation: A more positive reduction potential means the species has a greater tendency to be reduced (gain electrons), making it a stronger oxidizing agent.

• Q7. Which of the following is true about the activity series?
• A. Metals lower in the series are more reactive.
• B. Metals higher in the series are more easily oxidized.
• C. Metals higher in the series are stronger oxidizing agents.
• D. The activity series is the same as the electrochemical series.
• Correct Answer: B
• Explanation: The activity series ranks metals in order of decreasing ease of oxidation. Metals higher in the series are more easily oxidized (more reactive).

• Q8. When zinc is placed in copper sulphate solution, what is the reducing agent?
• A. Zn
• B. Cu^{2+}
• C. Zn^{2+}
• D. Cu
• Correct Answer: A
• Explanation: Zinc is oxidized to Zn^{2+}, so it acts as the reducing agent. It reduces copper ions to copper metal.

• Q9. Which of the following statements about the electrochemical series is correct?
• A. It lists oxidation potentials.
• B. It is arranged in order of increasing reduction potential.
• C. It is a list of standard reduction potentials relative to the SHE.
• D. It can only be used for metals.
• Correct Answer: C
• Explanation: The electrochemical series is a list of standard reduction potentials of half-reactions relative to the Standard Hydrogen Electrode (SHE).

• Q10. Which of the following will not occur based on the electrochemical series?
• A. Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu
• B. Cu + Zn^{2+} \rightarrow Cu^{2+} + Zn
• C. Mg + Cu^{2+} \rightarrow Mg^{2+} + Cu
• D. Ba + PbO \rightarrow BaO + Pb
• Correct Answer: B
• Explanation: Copper has a more positive reduction potential than zinc, so it cannot reduce zinc ions. This reaction is non-spontaneous and will not occur.

Multiple Choice Questions (MCQs)

• Q1. What is the electrochemical series?
• A. A list of elements by atomic mass.
• B. A list of standard reduction potentials.
• C. A list of standard oxidation potentials.
• D. A list of solubility products.
• Correct Answer: B
• Explanation: The electrochemical series is a list of standard reduction potentials arranged relative to the SHE.

• Q2. In the electrochemical series, the most negative reduction potentials correspond to the:
• A. Strongest oxidizing agents.
• B. Strongest reducing agents.
• C. Most stable elements.
• D. Least reactive metals.
• Correct Answer: B
• Explanation: A more negative reduction potential indicates a greater tendency to lose electrons (be oxidized), making the species a stronger reducing agent.

• Q3. Why can zinc displace copper from copper sulphate solution?
• A. Zinc has a more positive reduction potential than copper.
• B. Zinc has a more negative reduction potential than copper.
• C. Zinc is more dense than copper.
• D. Zinc has a higher atomic number than copper.
• Correct Answer: B
• Explanation: Zinc has a more negative reduction potential (-0.76 V) than copper (+0.34 V), so it is more easily oxidized and can reduce copper ions.

Q4. Which of the following metals can displace hydrogen from dilute acids? A. Copper B. Silver C. Gold D. Zinc Correct Answer: D Explanation: Metals above hydrogen in the electrochemical series (more negative reduction potential) can displace hydrogen. Zinc is above hydrogen (E° = -0.76 V), while copper, silver, and gold are below.

• Q5. What is the standard reduction potential of the SHE?
• A. 0.00 V
• B. 1.00 V
• C. -1.00 V
• D. 0.34 V
• Correct Answer: A
• Explanation: The standard hydrogen electrode (SHE) is assigned a potential of exactly 0.00 V.

• Q6. A metal with a more positive reduction potential is a stronger:
• A. Reducing agent
• B. Oxidizing agent
• C. Base
• D. Acid
• Correct Answer: B
• Explanation: A more positive reduction potential means a greater tendency to gain electrons (be reduced), making the species a stronger oxidizing agent.

Q7. Which of the following reactions will occur spontaneously? A. Cu + Zn^{2+} \rightarrow Cu^{2+} + Zn B. Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu C. Ag + Cu^{2+} \rightarrow Ag^+ + Cu D. Au + Zn^{2+} \rightarrow Au^{3+} + Zn Correct Answer: B Explanation: Zinc is higher than copper in the electrochemical series (more negative reduction potential), so it can displace copper. The other reactions involve metals lower in the series trying to displace metals higher in the series, which is non-spontaneous.

• Q8. The activity series of metals is arranged in order of:
• A. Increasing atomic number.
• B. Decreasing ease of oxidation.
• C. Increasing reduction potential.
• D. Decreasing atomic mass.
• Correct Answer: B
• Explanation: The activity series ranks metals by their ability to displace other metals and hydrogen, which is based on their ease of oxidation.

• Q9. Which of the following is a correct statement about the electrochemical series?
• A. It lists elements by their atomic number.
• B. It is a list of standard reduction potentials relative to the SHE.
• C. It is only applicable to non-metals.
• D. It cannot be used to predict the feasibility of a reaction.
• Correct Answer: B
• Explanation: The electrochemical series is a list of standard reduction potentials measured relative to the SHE.

Q10. If E^° for Zn^{2+}/Zn is -0.76 V and E^° for Cu^{2+}/Cu is +0.34 V, what is the cell potential for the reaction Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu? A. +1.10 V B. -1.10 V C. +0.42 V D. -0.42 V Correct Answer: A Explanation: E^°_cell = E^°_cathode - E^°_anode = 0.34 - (-0.76) = 1.10 V.