2.7 The Relative Reactivity of Species as Oxidizing Agents or Reducing Agents

The electrochemical series is not just a list of numbers; it is a powerful tool that allows us to predict how different chemical species will behave. By comparing standard reduction potentials, we can determine the relative strength of elements, compounds, and ions as oxidizing or reducing agents. This helps us predict which redox reactions will occur spontaneously.

Main Idea

• Higher (more positive) reduction potential: A greater tendency to gain electrons, making the species a stronger oxidizing agent.
• Lower (more negative) reduction potential: A greater tendency to lose electrons, making the species a stronger reducing agent.

By looking at the E^° values, you can rank species by their reactivity.

Core Concept

The core concept is that redox reactions proceed in the direction that leads to the formation of the weaker oxidizing and reducing agents from the stronger ones. In other words, a strong oxidizing agent will react with a strong reducing agent to produce a weaker oxidizing agent and a weaker reducing agent. The electrochemical series allows us to identify which species are strong and which are weak.

Detailed Explanation

The textbook provides two examples to illustrate how electrode potentials determine reactivity.

Example 1: Comparing Cu^{2+} and Zn^{2+} as Oxidizing Agents

Consider the reduction potentials for the following half-reactions: Cu^{2+} + 2e^- \longrightarrow Cu E^° = +0.34 V Zn^{2+} + 2e^- \longrightarrow Zn E^° = -0.76 V

• Analysis: Cu^{2+} ions have a higher (more positive) reduction potential (+0.34 V) compared to Zn^{2+} ions (-0.76 V).
• Conclusion: Cu^{2+} ions have a greater tendency to gain electrons and thus act as a stronger oxidizing agent than Zn^{2+} ions. Conversely, zinc metal (Zn) is a stronger reducing agent than copper metal (Cu) because it is more easily oxidized.

Example 2: Reaction Between Potassium Permanganate and Iron(II) Sulphate

Consider the reaction between potassium permanganate (KMnO_4) and iron(II) sulphate (FeSO_4) in an acidic solution: MnO_4^- + 8H^+ + 5e^- \longrightarrow Mn^{2+} + 4H_2O E^° = +1.52 V Fe^{2+} \longrightarrow Fe^{3+} + e^- E^° = +0.77 V

• Analysis: MnO_4^- ions have a higher reduction potential (+1.52 V) than Fe^{2+} ions (+0.77 V).
• Conclusion: MnO_4^- ions act as a strong oxidizing agent and will oxidize Fe^{2+} ions, which act as a strong reducing agent in this reaction.
• Why is this so? The higher the reduction potential, the more readily the species accepts electrons. Thus, MnO_4^- will take electrons from Fe^{2+}, causing Fe^{2+} to be oxidized to Fe^{3+} while MnO_4^- is reduced to Mn^{2+}.

General Rules for Reactivity

• Stronger Oxidizing Agents: The species with the more positive (higher) standard reduction potential. This means they are easily reduced (gain electrons). Examples include F_2, MnO_4^-, and Cu^{2+}.
• Stronger Reducing Agents: The species with the more negative (lower) standard reduction potential. This means they are easily oxidized (lose electrons). Examples include Li, K, Na, and Zn.
• Predicting Spontaneous Reactions: A spontaneous reaction occurs when a stronger oxidizing agent reacts with a stronger reducing agent. The products will be the corresponding weaker oxidizing and reducing agents.

Intermediate Level

The standard reduction potential (E^°) is a direct measure of a species' ability to be reduced. A more positive E^° means the species is more easily reduced (i.e., it's a stronger oxidizing agent). A more negative E^° means the species is more easily oxidized (i.e., it's a stronger reducing agent). In a redox reaction, the species with the more positive E^° will be the cathode (where reduction occurs), and the species with the more negative E^° will be the anode (where oxidation occurs). The reaction is spontaneous if the overall cell potential is positive. The species with the higher reduction potential will react with the species with the lower reduction potential to form weaker agents.

Important Points

• - Oxidizing Agents: Higher (more positive) reduction potential → stronger oxidizing agent.
• - Reducing Agents: Lower (more negative) reduction potential → stronger reducing agent.
• - Spontaneous Reaction: A stronger oxidizing agent reacts with a stronger reducing agent to form weaker ones.
• - Examples:
• - Cu^{2+} is a stronger oxidizing agent than Zn^{2+} because E^°(Cu^{2+}/Cu) = +0.34 V > E^°(Zn^{2+}/Zn) = -0.76 V.
• - MnO_4^- is a strong oxidizing agent (E^° = +1.52 V), and Fe^{2+} is a stronger reducing agent compared to many species.

Remember

• - Higher E^° → Stronger Oxidizing Agent.
• - Lower E^° → Stronger Reducing Agent.
• - Spontaneous reaction: Strong oxidizing agent + Strong reducing agent → Weak oxidizing agent + Weak reducing agent.
• - Example: Cu^{2+} is a stronger oxidizing agent than Zn^{2+}.
• - Example: Zn is a stronger reducing agent than Cu.

Common Mistakes

• - Confusing the terms. A strong oxidizing agent is a species that causes oxidation in another substance but is itself reduced. It has a high reduction potential. A strong reducing agent is a species that causes reduction in another substance but is itself oxidized. It has a low (more negative) reduction potential.
• - Reversing the relationship. Students often think a high reduction potential means a strong reducing agent. It is the opposite: high reduction potential means the species is easily reduced, making it a strong oxidizing agent.
• - Forgetting that the series is for standard conditions. The values change if concentrations, pressures, or temperature change (Nernst equation).
• - Assuming all oxidizing agents are equally strong. Strength depends on the E^° value. For example, F_2 is much stronger than Cu^{2+}.

Quick Revision

The relative reactivity of species as oxidizing or reducing agents is determined by their standard reduction potentials. A species with a more positive standard reduction potential is a stronger oxidizing agent because it has a greater tendency to gain electrons. Conversely, a species with a more negative standard reduction potential is a stronger reducing agent because it has a greater tendency to lose electrons. In a spontaneous redox reaction, a strong oxidizing agent reacts with a strong reducing agent to produce weaker oxidizing and reducing agents.

60-Second Revision

The strength of an oxidizing agent increases with a more positive standard reduction potential, while the strength of a reducing agent increases with a more negative standard reduction potential. For example, Cu^{2+} (+0.34 V) is a stronger oxidizing agent than Zn^{2+} (-0.76 V). In a spontaneous reaction, a strong oxidizing agent reacts with a strong reducing agent to form weaker ones.

Frequently Asked Questions (FAQs)

Q1. How does the electrochemical series help determine the relative strength of oxidizing and reducing agents? Answer: The electrochemical series lists standard reduction potentials. The species with the more positive (higher) reduction potential has a greater tendency to gain electrons and is therefore a stronger oxidizing agent. The species with the more negative (lower) reduction potential has a greater tendency to lose electrons and is therefore a stronger reducing agent.

Q2. In the reaction between Cu^{2+} and Zn, which is the stronger oxidizing agent? Answer: Cu^{2+} is the stronger oxidizing agent. Its standard reduction potential (+0.34 V) is more positive than that of Zn^{2+} (-0.76 V), so it has a greater tendency to gain electrons and be reduced. Zn is the stronger reducing agent.

Q3. Why is MnO_4^- considered a strong oxidizing agent? Answer: MnO_4^- has a very high standard reduction potential (+1.52 V), which means it has a strong tendency to gain electrons and be reduced. Therefore, it is a strong oxidizing agent, capable of oxidizing many other species, such as Fe^{2+}.

Q4. What happens when a strong oxidizing agent reacts with a strong reducing agent? Answer: A spontaneous redox reaction occurs. The strong oxidizing agent is reduced, and the strong reducing agent is oxidized, forming a weaker oxidizing agent and a weaker reducing agent. The reaction proceeds in the direction that forms the weaker agents.

Q5. Can a species with a more negative reduction potential be a strong oxidizing agent? Answer: No. A more negative reduction potential means the species has a greater tendency to lose electrons (be oxidized) and a lower tendency to gain electrons. Therefore, it is a stronger reducing agent, not an oxidizing agent.

Q6. How do you identify the strongest oxidizing agent in a list of half-reactions? Answer: Look for the half-reaction with the most positive standard reduction potential. The species on the left side (the oxidized form) is the strongest oxidizing agent.

Q7. How do you identify the strongest reducing agent in a list of half-reactions? Answer: Look for the half-reaction with the most negative standard reduction potential. The species on the right side (the reduced form) is the strongest reducing agent.

Q8. What is the relationship between the reduction potential and the tendency to be reduced? Answer: The more positive the reduction potential, the greater the tendency of the species to be reduced (gain electrons). A more negative reduction potential indicates a lower tendency to be reduced and a greater tendency to be oxidized.

Q9. In the reaction between MnO_4^- and Fe^{2+}, which species is the oxidizing agent? Answer: MnO_4^- is the oxidizing agent. It has a higher reduction potential (+1.52 V) and is reduced to Mn^{2+}, while causing Fe^{2+} to be oxidized to Fe^{3+}.

Q10. Can a weak oxidizing agent react with a weak reducing agent? Answer: Generally, no. For a spontaneous reaction to occur, the oxidizing agent must be stronger than the one being formed, and the reducing agent must be stronger than the one being formed. A weak oxidizing agent and a weak reducing agent would not react spontaneously to any significant extent.

Multiple Choice Questions (MCQs)

Q1. Which species is the strongest oxidizing agent based on the following standard reduction potentials? Cu^{2+} + 2e^- \rightarrow Cu E^° = +0.34 V Zn^{2+} + 2e^- \rightarrow Zn E^° = -0.76 V Ag^+ + e^- \rightarrow Ag E^° = +0.80 V A. Cu^{2+} B. Zn^{2+} C. Ag^+ D. Cu Correct Answer: C Explanation: Ag^+ has the most positive reduction potential (+0.80 V), so it has the greatest tendency to be reduced and is the strongest oxidizing agent.

• Q2. Which species is the strongest reducing agent based on the following standard reduction potentials?
• Zn^{2+} + 2e^- \rightarrow Zn\) \(E^° = -0.76 V
• Mg^{2+} + 2e^- \rightarrow Mg\) \(E^° = -2.37 V
• Cu^{2+} + 2e^- \rightarrow Cu\) \(E^° = +0.34 V
• A. Zn
• B. Mg
• C. Cu
• D. Zn^{2+}
• Correct Answer: B
• Explanation: Mg has the most negative reduction potential (-2.37 V), so it is the most easily oxidized and is the strongest reducing agent.

• Q3. In the reaction between MnO_4^- and Fe^{2+}, which species is reduced?
• A. MnO_4^-
• B. Fe^{2+}
• C. Mn^{2+}
• D. Fe^{3+}
• Correct Answer: A
• Explanation: MnO_4^- has a higher reduction potential (+1.52 V) and is reduced to Mn^{2+}, while Fe^{2+} is oxidized.

• Q4. Which of the following statements is true?
• A. A species with a more positive reduction potential is a stronger reducing agent.
• B. A species with a more negative reduction potential is a stronger oxidizing agent.
• C. A species with a more positive reduction potential is a stronger oxidizing agent.
• D. A species with a more negative reduction potential is a weaker reducing agent.
• Correct Answer: C
• Explanation: A more positive reduction potential means a greater tendency to gain electrons, which corresponds to a stronger oxidizing agent.

• Q5. What is the relationship between the strength of an oxidizing agent and its reduction potential?
• A. Directly proportional
• B. Inversely proportional
• C. No relationship
• D. Equal
• Correct Answer: A
• Explanation: The strength of an oxidizing agent increases as its standard reduction potential becomes more positive.

• Q6. Which of the following is a strong reducing agent?
• A. F_2
• B. Cu^{2+}
• C. Zn
• D. Ag^+
• Correct Answer: C
• Explanation: Zinc has a more negative reduction potential (-0.76 V), meaning it is easily oxidized and is a strong reducing agent.

• Q7. If E^° for Fe^{3+}/Fe^{2+} is +0.77 V and for Sn^{4+}/Sn^{2+} is +0.15 V, which is the stronger oxidizing agent?
• A. Fe^{3+}
• B. Sn^{4+}
• C. Fe^{2+}
• D. Sn^{2+}
• Correct Answer: A
• Explanation: Fe^{3+} has a more positive reduction potential (+0.77 V) than Sn^{4+} (+0.15 V), so it is the stronger oxidizing agent.

• Q8. Which of the following would be the strongest oxidizing agent?
• A. A species with a standard reduction potential of -2.87 V
• B. A species with a standard reduction potential of -0.76 V
• C. A species with a standard reduction potential of +0.34 V
• D. A species with a standard reduction potential of +1.52 V
• Correct Answer: D
• Explanation: The species with the most positive reduction potential (+1.52 V) has the greatest tendency to gain electrons and is the strongest oxidizing agent.

• Q9. In a spontaneous redox reaction, the stronger oxidizing agent reacts with:
• A. A weaker reducing agent.
• B. A stronger reducing agent.
• C. Another strong oxidizing agent.
• D. A non-metal.
• Correct Answer: B
• Explanation: A spontaneous redox reaction involves a strong oxidizing agent reacting with a strong reducing agent to form weaker agents.

• Q10. Which of the following is true about the relative strength of reducing agents?
• A. The strength of a reducing agent increases as its reduction potential becomes more positive.
• B. The strength of a reducing agent increases as its reduction potential becomes more negative.
• C. The strength of a reducing agent is independent of its reduction potential.
• D. All reducing agents have equal strength.
• Correct Answer: B
• Explanation: A more negative reduction potential means the species has a greater tendency to lose electrons, making it a stronger reducing agent.

Multiple Choice Questions (MCQs)

Q1. What determines the relative strength of an oxidizing agent? A. Its atomic mass B. Its standard reduction potential C. Its concentration D. Its melting point Correct Answer: B Explanation: The standard reduction potential is a measure of a species' tendency to gain electrons. A higher (more positive) reduction potential indicates a stronger oxidizing agent.

• Q2. Which species is a stronger oxidizing agent?
• A. Species with E^° = -0.76 V
• B. Species with E^° = +0.34 V
• C. Species with E^° = -2.87 V
• D. Species with E^° = 0.00 V
• Correct Answer: B
• Explanation: The species with the most positive reduction potential (+0.34 V) has the greatest tendency to be reduced and is the strongest oxidizing agent.

• Q3. In the reaction Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu, which species is the reducing agent?
• A. Zn
• B. Cu^{2+}
• C. Zn^{2+}
• D. Cu
• Correct Answer: A
• Explanation: Zn is oxidized to Zn^{2+} and is therefore the reducing agent.

• Q4. Which of the following is a strong reducing agent?
• A. Ag^+
• B. Cu^{2+}
• C. Na
• D. F_2
• Correct Answer: C
• Explanation: Sodium has a very negative reduction potential (-2.71 V), making it a strong reducing agent.

Q5. What is the general rule for a spontaneous redox reaction? A. A weak oxidizing agent reacts with a weak reducing agent. B. A strong oxidizing agent reacts with a strong reducing agent. C. A strong oxidizing agent reacts with a weak reducing agent. D. Any two species can react spontaneously. Correct Answer: B Explanation: A spontaneous reaction occurs when a strong oxidizing agent (high reduction potential) reacts with a strong reducing agent (low reduction potential).

• Q6. Which of the following will be the strongest oxidizing agent?
• A. F_2 (E^° = +2.87 V)
• B. Cl_2 (E^° = +1.36 V)
• C. Br_2 (E^° = +1.07 V)
• D. I_2 (E^° = +0.54 V)
• Correct Answer: A
• Explanation: F_2 has the most positive reduction potential, so it is the strongest oxidizing agent among the halogens.

• Q7. Which of the following will be the strongest reducing agent?
• A. Li (E^° = -3.04 V)
• B. Na (E^° = -2.71 V)
• C. K (E^° = -2.93 V)
• D. Mg (E^° = -2.37 V)
• Correct Answer: A
• Explanation: Lithium has the most negative reduction potential (-3.04 V), so it is the strongest reducing agent among the choices.

• Q8. In a reaction, a species with E^° = +1.52 V reacts with a species with E^° = -0.76 V. Which is the oxidizing agent?
• A. The species with +1.52 V
• B. The species with -0.76 V
• C. Both
• D. Neither
• Correct Answer: A
• Explanation: The species with the more positive reduction potential (+1.52 V) has a greater tendency to be reduced, so it is the oxidizing agent.

• Q9. What is the relationship between the strength of a reducing agent and its reduction potential?
• A. Directly proportional
• B. Inversely proportional
• C. No relationship
• D. Equal
• Correct Answer: B
• Explanation: The strength of a reducing agent increases as its reduction potential becomes more negative. It is an inverse relationship.

• Q10. Which of the following statements is correct?
• A. A strong oxidizing agent is easily oxidized.
• B. A strong reducing agent is easily reduced.
• C. A strong oxidizing agent is easily reduced.
• D. A strong reducing agent is a strong oxidizing agent.
• Correct Answer: C
• Explanation: A strong oxidizing agent has a high tendency to gain electrons, so it is easily reduced.

Multiple Choice Questions (MCQs)

• Q1. Based on standard reduction potentials, which is the strongest oxidizing agent?
• A. Zn^{2+} (-0.76 V)
• B. Cu^{2+} (+0.34 V)
• C. Ag^+ (+0.80 V)
• D. Fe^{2+} (-0.44 V)
• Correct Answer: C
• Explanation: The species with the most positive reduction potential (+0.80 V) is the strongest oxidizing agent.

• Q2. Which is the strongest reducing agent?
• A. Li (-3.04 V)
• B. Na (-2.71 V)
• C. Zn (-0.76 V)
• D. Cu (+0.34 V)
• Correct Answer: A
• Explanation: The species with the most negative reduction potential (-3.04 V) is the most easily oxidized and is the strongest reducing agent.

• Q3. In the reaction MnO_4^- + 8H^+ + 5Fe^{2+} \rightarrow Mn^{2+} + 4H_2O + 5Fe^{3+}, which species is the oxidizing agent?
• A. MnO_4^-
• B. Fe^{2+}
• C. Mn^{2+}
• D. Fe^{3+}
• Correct Answer: A
• Explanation: MnO_4^- has a higher reduction potential (+1.52 V) and is reduced, so it acts as the oxidizing agent.

• Q4. What is the relationship between the reduction potential and the strength of an oxidizing agent?
• A. A more positive reduction potential means a weaker oxidizing agent.
• B. A more positive reduction potential means a stronger oxidizing agent.
• C. A more negative reduction potential means a stronger oxidizing agent.
• D. There is no relationship.
• Correct Answer: B
• Explanation: A more positive reduction potential indicates a greater tendency to gain electrons, which corresponds to a stronger oxidizing agent.

• Q5. Which of the following will spontaneously react?
• A. Cu + Zn^{2+}
• B. Zn + Cu^{2+}
• C. Ag + Cu^{2+}
• D. Au + Zn^{2+}
• Correct Answer: B
• Explanation: Zinc is higher than copper in the activity series (more negative reduction potential), so it can displace copper from its solution.

• Q6. Which species is the stronger reducing agent?
• A. A species with E^° = +0.34 V
• B. A species with E^° = -0.76 V
• C. A species with E^° = 0.00 V
• D. A species with E^° = +0.80 V
• Correct Answer: B
• Explanation: The species with the most negative reduction potential (-0.76 V) is the most easily oxidized and is the strongest reducing agent.

• Q7. In a spontaneous reaction, the stronger oxidizing agent reacts with:
• A. A weaker reducing agent.
• B. A stronger reducing agent.
• C. Another strong oxidizing agent.
• D. A non-metal.
• Correct Answer: B
• Explanation: A spontaneous reaction occurs between a strong oxidizing agent and a strong reducing agent.

• Q8. Which of the following statements about oxidizing agents is correct?
• A. They are easily oxidized.
• B. They are easily reduced.
• C. They have low reduction potentials.
• D. They are strong reducing agents.
• Correct Answer: B
• Explanation: Oxidizing agents have a high tendency to gain electrons, so they are easily reduced.

• Q9. If E^°(Fe^{3+}/Fe^{2+}) = +0.77 V and E^°(I_\frac{2}{I}^-) = +0.54 V, which is the stronger oxidizing agent?
• A. Fe^{3+}
• B. I_2
• C. Fe^{2+}
• D. I^-
• Correct Answer: A
• Explanation: Fe^{3+} has the more positive reduction potential (+0.77 V), so it is the stronger oxidizing agent.

• Q10. What is the defining characteristic of a strong reducing agent?
• A. A high (positive) reduction potential.
• B. A low (negative) reduction potential.
• C. A neutral charge.
• D. A high atomic mass.
• Correct Answer: B
• Explanation: A strong reducing agent is easily oxidized, which means it has a low (more negative) standard reduction potential.