8.4.1 - Standard Electrode Potential (E°)
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Introduction to Standard Electrode Potential
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Today, we are discussing the standard electrode potential, often denoted as E°. This term indicates the strength of a half-cell to be reduced, measured relative to the Standard Hydrogen Electrode, or SHE. Can anyone tell me what the potential of the SHE is?
Isn't it 0.00 V?
Correct! The SHE is defined as 0.00 V. This reference helps us gauge how other electrodes perform. Now, why do you think we can't measure the absolute potential of a single electrode?
Because there’s no single standard to compare it to?
Exactly! That's why we use the SHE as our baseline. Remember, the greater the positive E°, the stronger the oxidizing agent. Let's keep these concepts in mind as we dive deeper.
How is Standard Electrode Potential Measured?
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To measure the standard electrode potential of any half-cell, we connect it to the SHE. The voltage we measure reveals its E°. If the reaction causes the SHE to be oxidized, what do we get?
A positive E°.
Precisely! Conversely, if the SHE is reduced, the E° is negative. Can someone think of an example where we would measure E°?
In a galvanic cell, right? Like measuring zinc and copper half-cells.
Correct! That's a perfect example. These measures lead us into understanding cell potentials...
Understanding E° Values and Their Significance
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When we talk about standard electrode potentials, what does a higher E° value suggest about a substance's behavior?
It implies that the substance will more likely be reduced?
Exactly! A more positive E° means it’s a stronger oxidizing agent. So, what do you think a negative E° value indicates?
That it would act as a stronger reducing agent?
Good job! So, when we calculate the E°_cell for a galvanic cell, how do we summarize our findings?
By using the formula: E°_cell = E°_reduction (cathode) - E°_reduction (anode).
Right! And what does a positive E°_cell indicate?
It confirms a spontaneous reaction!
Great! Remember this as it’s crucial for understanding galvanic cells.
Introduction & Overview
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Quick Overview
Standard
The standard electrode potential (E°) is the voltage of a half-cell compared to the standard hydrogen electrode (SHE), which is defined as 0.00 V. E° values indicate the tendency of a species to gain electrons (be reduced) or lose electrons (be oxidized), allowing the calculation of the cell potential (E°_cell) in galvanic cells to determine spontaneity.
Detailed
Standard Electrode Potential (E°)
Standard electrode potential (E°) is an essential concept in electrochemistry, representing the inherent tendency of a chemical species to gain electrons (be reduced), measured relative to a standard reference, the Standard Hydrogen Electrode (SHE), defined at 0.00 V. The SHE consists of platinum dipped in a 1.0 mol dm⁻³ H⁺ solution, with hydrogen gas at 100 kPa bubbling over it.
To measure the E° of any half-cell, it is connected to the SHE, and the resulting potential is noted. A positive E° indicates that the half-cell is a stronger oxidizing agent, meaning it preferentially gains electrons, whereas a negative E° indicates a stronger reducing agent, which tends to lose electrons. The cell potential (E°_cell) of a galvanic cell can be calculated from the E° values of the anode and cathode using the formula:
E°_cell = E°_reduction (cathode) - E°_reduction (anode)
A greater E°_cell indicates a spontaneous reaction, essential for creating and understanding practical applications in electrochemistry, like batteries.
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Definition of Standard Electrode Potential (E°)
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Chapter Content
Standard Electrode Potential (E°): It is impossible to measure the absolute potential of a single electrode. Therefore, electrode potentials are measured relative to a standard reference electrode: the Standard Hydrogen Electrode (SHE).
Detailed Explanation
Standard Electrode Potential (E°) refers to a way of measuring the potential difference between two electrodes in an electrochemical cell. Since the absolute potential of a single electrode cannot be measured directly, all electrode potentials are compared to a reference point known as the Standard Hydrogen Electrode (SHE). This helps establish a common measuring standard across various reactions.
Examples & Analogies
Think of the SHE as a 'zero' baseline on a scale. Just as we might measure our weight compared to a baseline of zero, we use the SHE as a reference point for measuring how 'heavy' or 'light' (the potential) other electrodes are in comparison.
Standard Hydrogen Electrode (SHE) Details
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The SHE consists of a platinum electrode immersed in a 1.0 mol dm⁻³ solution of H⁺ ions, with hydrogen gas at 100 kPa bubbling over the electrode, all at 298 K. The standard electrode potential of the SHE is arbitrarily defined as exactly 0.00 V.
● Standard Hydrogen Electrode (SHE) half-reaction: 2H⁺(aq, 1M) + 2e⁻ ⇌ H₂(g, 100 kPa) E° = 0.00 V
Detailed Explanation
The Standard Hydrogen Electrode (SHE) is a specific setup used for measuring electrode potentials. It involves a platinum electrode in a solution containing hydrogen ions (H⁺) at a concentration of 1.0 mol per liter, with hydrogen gas at a pressure of 100 kPa. The potential of this standard electrode is defined as 0.00 volts, setting a baseline for all other electrodes to compare against. The half-reaction associated with SHE shows how hydrogen ions can gain electrons to form hydrogen gas.
Examples & Analogies
Imagine the SHE as the key benchmark in a race. Just like runners might measure their times against a stopping point (the benchmark), electrode potentials are measured against the SHE. If an electrode has a positive potential compared to the SHE, it indicates a greater 'tendency' to gain electrons, just like a faster runner might have a better time against the benchmark.
Measuring Standard Electrode Potentials
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To measure the standard electrode potential of a half-cell, it is connected to a SHE. The voltage measured by the voltmeter is the standard electrode potential (E°) of that specific half-cell.
● If the reaction with SHE causes the SHE to be oxidized (i.e., the other half-cell is reduced), the measured E° is positive.
● If the reaction with SHE causes the SHE to be reduced (i.e., the other half-cell is oxidized), the measured E° is negative.
Detailed Explanation
To find the standard electrode potential of a specific half-cell, you connect it to the SHE. The voltage that you read from the voltmeter when these two are linked tells you the electrode potential. If the half-cell causes the SHE (acting as an electrode) to lose electrons, that indicates a positive standard electrode potential. Conversely, if the half-cell causes the SHE to gain electrons, it results in a negative potential reading.
Examples & Analogies
Imagine you're using a treadmill that measures how fast you run compared to a stationary marker (the SHE). If you run faster, you get a positive reading, indicating good performance (positive electrode potential). If the treadmill runs slower than you, it means you're exerting less effort (negative electrode potential).
Significance of Standard Electrode Potential Values
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A table of standard electrode potentials (reduction potentials) lists various half-reactions in order of their tendency to be reduced.
● More positive E° value: Indicates a stronger tendency for the substance to be reduced (i.e., it is a stronger oxidizing agent).
● More negative E° value: Indicates a stronger tendency for the substance to be oxidized (i.e., it is a stronger reducing agent).
Detailed Explanation
The values listed in a standard electrode potential table help predict how likely a substance is to gain or lose electrons. A higher (more positive) value means a greater tendency for that substance to undergo reduction, making it a stronger oxidizing agent. Conversely, a lower (more negative) value suggests a higher likelihood of oxidation, identifying it as a stronger reducing agent.
Examples & Analogies
Think of these electrode potentials like popularity ratings. A popular person (positive E°) is more likely to be invited to social events (reduction) than someone who is less liked (negative E°), who's often left out (oxidation). The higher the potential, the more desirable they become in forming bonds.
Calculating Standard Cell Potential (E°_cell)
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The standard cell potential of a galvanic cell is the potential difference between the two half-cells when all components are in their standard states. It can be calculated from the standard electrode potentials of the two half-cells:
E°_cell = E°_reduction (cathode) - E°_reduction (anode).
Alternatively, one can reverse the sign of the anode's reduction potential to make it an oxidation potential and then add the two potentials: E°_cell = E°_reduction (cathode) + E°_oxidation (anode).
● Spontaneity: A positive E°_cell indicates a spontaneous reaction (voltaic cell). A negative E°_cell indicates a non-spontaneous reaction (electrolytic cell requires energy input).
Detailed Explanation
The standard cell potential (E°_cell) refers to the difference in potential between the two half-cells in an electrochemical cell at standard conditions. You can calculate it by subtracting the reduction potential of the anode from that of the cathode. If the result is positive, the reaction is spontaneous (like in a battery), while a negative value indicates the reaction will not occur without external energy (like in electrolysis).
Examples & Analogies
Consider E°_cell as similar to the gravitational pull in a waterfall. If the waters flow naturally downwards (positive E°_cell), they generate energy effortlessly (spontaneous). If you need to pump water upwards (negative E°_cell), energy must be supplied artificially (non-spontaneous) to move it uphill.
Application Example: Daniell Cell
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Example: Calculate E°_cell for the Daniell cell: Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s) Given: E°(Zn²⁺/Zn) = -0.76 V (Zinc half-cell potential) E°(Cu²⁺/Cu) = +0.34 V (Copper half-cell potential)
1. Identify cathode and anode: Cu²⁺ has a more positive E° (+0.34 V) than Zn²⁺ (-0.76 V). Therefore, Cu²⁺ will be reduced (cathode) and Zn will be oxidized (anode).
○ Cathode: Cu²⁺(aq) + 2e⁻ → Cu(s)
○ Anode: Zn(s) → Zn²⁺(aq) + 2e⁻
2. Calculate E°_cell: E°_cell = E°(cathode) - E°(anode) E°_cell = (+0.34 V) - (-0.76 V) = +1.10 V
The positive E°_cell of +1.10 V confirms that the Daniell cell is a spontaneous voltaic cell.
Detailed Explanation
In the example of the Daniell cell, you can determine the standard cell potential by identifying the cathode and anode based on their electrode potentials. Since copper has a higher potential than zinc, it acts as the cathode. By substituting the given values into the formula for E°_cell, you find a positive value of +1.10 V, indicating that the Daniell cell can operate spontaneously, producing electrical energy.
Examples & Analogies
Think of it as a tug-of-war between two teams. The team with a stronger pull (higher E°) can easily pull the weaker team (lower E°) towards their side. Here, copper pulls zinc, generating energy during the process, similar to how teams generate excitement in a game.
Key Concepts
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Standard Electrode Potential (E°): Indicates tendency of half-cell to be reduced, relative to SHE.
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Cathode and Anode: The roles of each electrode in a galvanic cell.
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Cell Potential (E°_cell): Calculated from E° values to determine spontaneity of reactions.
Examples & Applications
In a typical galvanic cell, such as the Daniell cell, measuring the E° of the zinc and copper half-cells provides insight into their tendency to participate in electron transfer reactions.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
Anode loses, cathode gains, electrons flow, it’s not a pain!
Stories
In a small town, there lived two friends, Anna and Cody. Anna was always positive, while Cody was known to be negative. Together, they made spark, always trading energy, hence creating a bond that no one could break. Thus, whenever Anna (the cathode) felt down, Cody (the anode) was there to lift her up, creating joy in their town - the electrochemical cell!
Memory Tools
RIG = Reduction Is Gain, to remember that when a substance is reduced, it gains electrons.
Acronyms
E° = Everything flows as Oxidation occurs and Reduction takes place.
Flash Cards
Glossary
- Standard Electrode Potential (E°)
A measure of the tendency of a half-cell to gain electrons, measured against the Standard Hydrogen Electrode.
- Standard Hydrogen Electrode (SHE)
A reference electrode with a potential defined to be 0.00 V, consisting of hydrogen gas bubbling over a platinum electrode in a solution of H⁺.
- Cathode
The electrode where reduction occurs, gaining electrons.
- Anode
The electrode where oxidation occurs, losing electrons.
- Cell Potential (E°_cell)
The voltage difference between the cathode and anode in an electrochemical cell.
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