Standard reduction potentials | Redox reactions and electrochemistry | Chemistry | Khan Academy

Khan Academy Organic Chemistry
28 Dec 201409:09
EducationalLearning
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TLDRThe video script discusses the concept of Standard Reduction Potentials, explaining how they predict the likelihood of a substance being reduced or oxidized in a redox reaction. It uses the example of a copper-zinc voltaic cell to illustrate how to calculate the standard cell potential by combining the reduction and oxidation half-reactions. The script emphasizes the relationship between the reduction potential of an ion and its strength as an oxidizing or reducing agent, highlighting that a higher (more positive) reduction potential indicates a stronger oxidizing agent, while a lower (more negative) potential indicates a stronger reducing agent.

Takeaways
  • 📊 The script discusses Standard Reduction Potentials and provides a table with various half-reactions.
  • 🔄 All half-reactions in the table are written as reduction reactions, where reduction is the gain of electrons.
  • 💰 The standard reduction potential of silver ions is +0.80 volts, while the standard hydrogen electrode serves as the reference with 0 volts.
  • ⚖️ The more positive the standard reduction potential, the more likely a substance is to undergo reduction.
  • 🔩 Comparing copper 2+ and zinc 2+ ions, copper has a higher standard reduction potential (+0.34 volts) than zinc (-0.76 volts).
  • 🌐 In a redox reaction involving copper and zinc, copper 2+ ions are reduced to solid copper, and zinc is oxidized to zinc 2+ ions.
  • 🔄 The oxidation half-reaction is derived by reversing the reduction half-reaction, changing the sign of the potential.
  • 🔋 The overall redox reaction is obtained by adding the reduction and oxidation half-reactions, with electrons cancelling out.
  • 🔌 The standard cell potential is calculated by adding the reduction potential and the oxidation potential of the half-reactions.
  • 🏷️ Zinc is the reducing agent and copper 2+ is the oxidizing agent in the redox reaction between them.
  • 📈 Higher standard reduction potentials indicate stronger oxidizing agents, while lower potentials indicate stronger reducing agents.
Q & A

  • What is the standard reduction potential and how is it used in comparing half-reactions?

    -The standard reduction potential is a measure of the tendency of a substance to be reduced, or gain electrons, in a redox reaction. It is used to compare half-reactions by looking at the potential values; the more positive the value, the more likely the substance is to be reduced.

  • What is the standard reduction potential for the silver ion gaining an electron to form solid silver?

    -The standard reduction potential for the silver ion gaining an electron to form solid silver is +0.80 volts.

  • How does the standard hydrogen electrode serve as a reference in the table of standard reduction potentials?

    -The standard hydrogen electrode serves as a reference value in the table of standard reduction potentials because it has a potential of zero volts. All other half-reactions are compared to this reference electrode.

  • Which substance is more likely to be reduced: copper 2+ ions or zinc 2+ ions?

    -Copper 2+ ions are more likely to be reduced because their standard reduction potential is +0.34 volts, which is more positive than the -0.76 volts for zinc 2+ ions.

  • What is the reduction half-reaction for copper 2+ ions gaining electrons to form solid copper?

    -The reduction half-reaction for copper 2+ ions gaining electrons to form solid copper is Cu^2+ + 2e^- → Cu, with a standard reduction potential of +0.34 volts.

  • How is the oxidation half-reaction determined for zinc in the context of the copper-zinc redox reaction?

    -The oxidation half-reaction for zinc is determined by reversing the reduction half-reaction of zinc 2+ ions. It involves solid zinc being oxidized into zinc 2+ ions, losing two electrons, and has an oxidation potential of +0.76 volts.

  • What is the overall redox reaction for a copper-zinc voltaic cell?

    -The overall redox reaction for a copper-zinc voltaaic cell is Cu^2+ + Zn → Cu + Zn^2+, where copper 2+ ions are reduced to solid copper and zinc is oxidized to zinc 2+ ions.

  • How is the standard cell potential calculated for a redox reaction?

    -The standard cell potential is calculated by adding the reduction potential of the reduction half-reaction to the oxidation potential of the oxidation half-reaction. For the copper-zinc cell, it is +0.34 volts (reduction) + 0.76 volts (oxidation) = +1.10 volts.

  • Why is zinc considered the reducing agent in the copper-zinc redox reaction?

    -Zinc is considered the reducing agent because it is being oxidized, losing electrons, which are then used to reduce copper 2+ ions to solid copper. Despite being oxidized itself, zinc facilitates the reduction of another substance, making it the reducing agent.

  • How does the position of an element on the standard reduction potential table relate to its strength as an oxidizing or reducing agent?

    -The position on the standard reduction potential table indicates the tendency of an element to be reduced or oxidized. Elements higher on the table are more easily reduced and thus are stronger oxidizing agents. Conversely, elements lower on the table are more likely to be oxidized and are stronger reducing agents.

  • What is the role of copper 2+ in the copper-zinc redox reaction?

    -In the copper-zinc redox reaction, copper 2+ acts as the oxidizing agent. It gains the electrons lost by zinc, undergoing reduction to form solid copper.

  • How does the standard reduction potential of lithium compare to that of zinc?

    -Lithium has a more negative standard reduction potential than zinc, indicating that it is more likely to undergo oxidation. Therefore, lithium is a stronger reducing agent than zinc.

Outlines
00:00
🔋 Understanding Standard Reduction Potentials and Redox Reactions

This paragraph introduces the concept of Standard Reduction Potentials (SRP) and how they relate to reduction half-reactions. It explains that SRP is a measure of a substance's tendency to gain electrons (reduction), with more positive values indicating a greater likelihood of reduction. The standard hydrogen electrode serves as the reference point with a potential of zero volts. The paragraph uses the example of copper (Cu^2+) and zinc (Zn^2+) ions to illustrate how to compare their reduction potentials and how this relates to a redox reaction. It explains that Cu^2+ has a higher reduction potential (+0.34 volts) than Zn^2+ (-0.76 volts), making Cu^2+ more likely to be reduced. The paragraph also describes how to identify the oxidation half-reaction by reversing the reduction half-reaction and how to calculate the standard cell potential by adding the reduction and oxidation potentials. The example given is a spontaneous redox reaction forming a voltaic cell with a potential of +1.10 volts under standard conditions.

05:00
🔄 Role of Reducing and Oxidizing Agents in Redox Reactions

This paragraph delves deeper into the roles of reducing and oxidizing agents in redox reactions, using the previous example of copper and zinc. It clarifies that although zinc is being oxidized (losing electrons), it is the reducing agent because it facilitates the reduction of copper ions. The paragraph emphasizes that the position of a substance in the SRP table indicates its tendency to be oxidized (lower on the table) or reduced (higher on the table), and thus its strength as a reducing or oxidizing agent. It also explains that a stronger oxidizing agent (like Cu^2+) is more easily reduced, while a stronger reducing agent (like Zn) is more likely to be oxidized. The paragraph concludes by discussing how lithium, with an even more negative SRP than zinc, is a stronger reducing agent due to its higher tendency to be oxidized.

Mindmap
Keywords
💡Standard Reduction Potentials
Standard Reduction Potentials refer to the voltages at which half-reactions occur in their standard states. In the context of the video, these potentials are used to predict the spontaneity of redox reactions and to determine which species will be reduced or oxidized. For example, silver ions have a high standard reduction potential of +.80 volts, indicating they are likely to be reduced, while zinc ions have a lower potential of -.76 volts, making them more likely to be oxidized.
💡Half-Reactions
Half-reactions are portions of redox reactions that involve either the oxidation or reduction of a single species. They are used to analyze the individual processes occurring in a redox reaction. In the video, half-reactions are written as reduction reactions and are compared using their standard reduction potentials. The copper 2+ to solid copper half-reaction and the zinc to zinc 2+ half-reaction are examples used to illustrate this concept.
💡Reduction
Reduction is a chemical process in which a species gains electrons. In the context of the video, reduction is associated with substances that have higher standard reduction potentials, indicating a greater likelihood of undergoing reduction. The video uses the example of silver ions gaining an electron to form solid silver to illustrate a reduction half-reaction.
💡Oxidation
Oxidation is the loss of electrons from a species during a chemical reaction. In the video, oxidation is linked to substances with lower standard reduction potentials, which are more likely to be oxidized. The script explains that zinc, which has a lower standard reduction potential, is oxidized to zinc 2+ ions, losing two electrons in the process.
💡Redox Reaction
A redox reaction, short for reduction-oxidation reaction, involves the transfer of electrons between two species. One species is reduced (gains electrons), and the other is oxidized (loses electrons). The video uses the example of a reaction between copper and zinc to illustrate how to identify the reduction and oxidation half-reactions and how they combine to form the overall redox reaction.
💡Standard Hydrogen Electrode
The standard hydrogen electrode is a reference electrode used in electrochemistry to define the zero point on the standard electrode potential scale. It is used as a benchmark for comparing the reduction potentials of other half-reactions. In the video, all other half-reactions are compared to the standard hydrogen electrode, which has a potential of zero volts.
💡Cell Potential
Cell potential, also known as the electromotive force (EMF) of a cell, is the voltage or electrical potential difference that drives the flow of electrons in an electrochemical cell. It is calculated by summing the reduction potential of the reduction half-reaction and the oxidation potential of the oxidation half-reaction. The video explains how to calculate the standard cell potential for a voltaic cell involving copper and zinc.
💡Reducing Agent
A reducing agent is a substance that donates electrons to another species in a redox reaction, thereby causing the reduction of that species. In the process, the reducing agent itself is oxidized. The video identifies zinc as the reducing agent in the copper-zinc redox reaction because it loses electrons and is oxidized to zinc 2+ ions.
💡Oxidizing Agent
An oxidizing agent is a substance that accepts electrons from another species during a redox reaction, leading to the oxidation of that species. In the context of the video, copper 2+ ions act as the oxidizing agent because they gain electrons and are reduced to solid copper, while causing the oxidation of zinc.
💡Voltaic Cell
A voltaic cell, also known as a galvanic cell, is an electrochemical cell that generates electricity through spontaneous redox reactions. The cell consists of two electrodes, one where reduction occurs and another where oxidation occurs, separated by an electrolyte. The video uses the example of a copper-zinc voltaic cell to explain the concepts of redox reactions, reducing and oxidizing agents, and cell potential.
💡Standard Reduction Potential Table
The standard reduction potential table is a chart that lists the standard reduction potentials of various half-reactions, which are used to predict the spontaneity of redox reactions and to determine the relative strengths of oxidizing and reducing agents. The video explains how to use this table to calculate the cell potential and to understand the roles of oxidizing and reducing agents in a redox reaction.
Highlights

The transcript discusses Standard Reduction Potentials and their significance in understanding redox reactions.

All half-reactions in the table are written as reduction half-reactions, signifying gain of electrons as reduction.

The standard reduction potential for silver ions forming solid silver is +0.80 volts.

The standard hydrogen electrode serves as the reference value with a potential of zero volts.

Copper 2+ ions have a standard reduction potential of +0.34 volts, indicating a likelihood to be reduced.

Zinc 2+ ions have a standard reduction potential of -0.76 volts, making them less likely to be reduced compared to copper 2+ ions.

In a redox reaction involving copper and zinc, copper 2+ ions are reduced to solid copper, forming the reduction half-reaction.

The oxidation half-reaction involves solid zinc being oxidized into zinc 2+ ions, losing two electrons.

The standard oxidation potential for the zinc oxidation half-reaction is +0.76 volts, derived by reversing the sign of its standard reduction potential.

The overall redox reaction combines the reduction and oxidation half-reactions, with electrons canceling out.

The products of the redox reaction include solid copper and zinc 2+ ions in solution.

The spontaneous redox reaction between copper and zinc forms a voltaic cell, a topic discussed in previous videos.

The standard cell potential is calculated by adding the reduction potential and the oxidation potential of the half-reactions.

The standard cell potential for the copper-zinc redox reaction is +1.10 volts.

Zinc is the reducing agent in the copper-zinc redox reaction, despite being oxidized itself.

Copper 2+ ions act as the oxidizing agent, facilitating the oxidation of zinc.

Higher standard reduction potentials indicate a stronger oxidizing agent.

Lower standard reduction potentials suggest a stronger reducing agent, with a higher tendency to be oxidized.

Lithium has a very negative reduction potential, making it a strong reducing agent due to its high likelihood of being oxidized.

Transcripts

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