ChemistryChapter 2: Electrochemistry
2.4.4 Method to Measure the Electrode Potentials
The main idea is to construct an electrochemical cell with the half-cell of interest and the Standard Hydrogen Electrode.
Explanation level
As we learned in the previous section, the absolute potential of a single electrode cannot be measured directly. To find the electrode potential of any half-cell, it must be connected to a reference electrode with a known, stable potential. The Standard Hydrogen Electrode (SHE) is the primary reference electrode used for this purpose. By measuring the cell potential of a galvanic cell made from the unknown electrode and the SHE, we can calculate the standard electrode potential of the unknown half-cell.
Main Idea
The main idea is to construct an electrochemical cell with the half-cell of interest and the Standard Hydrogen Electrode. The voltmeter measures the total cell potential (E0cell). Since the potential of the SHE is defined as exactly 0.00 V, the measured cell potential directly gives us the standard electrode potential of the unknown half-cell, or allows us to calculate it using the formula E0cell = E0cathode - E0anode.
Core Concept
The core concept is that the standard electrode potential is a relative measurement. It is always measured against a common reference point (the SHE). The cell potential measured by the voltmeter is the difference between the reduction potential of the cathode and the reduction potential of the anode. By knowing the potential of the SHE (0.00 V) and observing which electrode acts as the anode and which acts as the cathode, we can determine the standard reduction potential of the unknown electrode.
Detailed Explanation
The textbook provides a detailed example to explain this method: the determination of the standard electrode potential of the Zn^{2+}/Zn electrode.
Step 1: Setup the Electrochemical Cell
An electrochemical cell is set up in which a zinc rod is dipped in a 1 M zinc sulphate (ZnSO_4) solution. This half-cell is combined with a Standard Hydrogen Electrode (SHE) through a salt bridge.
- Left side (Anode): Zinc electrode in 1 M ZnSO_4 solution.
- Right side (Cathode): SHE (platinum electrode, 1 M H^+, 1 atm H_2).
- External circuit: A voltmeter is connected between the two electrodes.
- Salt bridge: Connects the two solutions, allowing ion flow and maintaining electrical neutrality.
Step 2: Observe the Voltmeter and Determine the Direction of Electron Flow
When the circuit is completed, the voltmeter shows a reading. The deflection of the voltmeter indicates that current is flowing from the zinc electrode to the hydrogen electrode, or that electrons are moving from the zinc rod to the hydrogen electrode.
- The zinc electrode is losing electrons (undergoing oxidation), so it acts as the anode.
- The SHE is gaining electrons (undergoing reduction), so it acts as the cathode.
Step 3: Read the Cell Potential
In this case, the reading (E0cell) by the voltmeter is 0.76 V.
The cell can be represented as: Zn_{(s)} + 2H^+_{(aq)} \longrightarrow Zn^{2+}_{(aq)} + H_{2(g)}
Step 4: Calculate the Standard Electrode Potential
Using the formula for cell potential: E0cell = E0cathode - E0anode We know: - E0cell = 0.76 V - E0cathode = E0SHE = 0.00 V (since SHE is the cathode)
Substituting the values: 0.76 = 0.00 - E0anode E0anode = -0.76 V
Conclusion
- The oxidation potential of zinc is +0.76 V.
- The reduction potential of zinc is -0.76 V (since oxidation potential is the negative of reduction potential).
This value (-0.76 V) is the standard reduction potential of the Zn^{2+}/Zn half-cell, which is the value listed in the electrochemical series.
General Procedure for Measuring Standard Electrode Potentials
The same method can be used to measure the standard electrode potential of any half-cell. The general steps are:
1. Set up a galvanic cell with the half-cell of interest and the Standard Hydrogen Electrode (SHE), connected by a salt bridge. 2. Measure the cell potential (E0cell) using a voltmeter. 3. Determine which electrode is the anode and which is the cathode based on the direction of electron flow (or the sign of the cell potential). 4. Calculate the standard electrode potential of the unknown half-cell using the formula: E0cell = E0cathode - E0anode - If the unknown electrode is the cathode, then E0unknown = E0cell (since E0anode is the SHE = 0.00 V). - If the unknown electrode is the anode, then E0unknown = -E0cell (since E0cathode is the SHE = 0.00 V).
Example: Measurement of Standard Electrode Potential of Cu^{2+}/Cu
- Copper is the cathode (reduction occurs).
- SHE is the anode (oxidation occurs).
- E0cell = 0.34 V
- E0cathode = E0Cu2+/Cu
- E0anode = E0SHE = 0.00 V
- 0.34 = E0Cu2+/Cu - 0.00 ⇒ E0Cu2+/Cu = +0.34 V
Intermediate Level
The measurement of electrode potentials is based on constructing a galvanic cell with the unknown half-cell and the SHE. The voltmeter measures the cell potential (E0cell). The direction of electron flow tells us which electrode is the anode (oxidation) and which is the cathode (reduction). Using the formula E0cell = E0cathode - E0anode and knowing that the SHE has a potential of 0.00 V, we can calculate the standard reduction potential of the unknown half-cell. The sign of the measured value indicates whether the unknown electrode is the cathode (positive) or the anode (negative).
Important Points
- - Method: Connect the unknown half-cell to the SHE and measure the cell potential with a voltmeter.
- - SHE: Reference electrode with a potential of 0.00 V.
- - Anode/Cathode: Determined by the direction of electron flow (or the sign of the measured potential).
- - Formula: E0cell = E0cathode - E0anode.
- - If the unknown is the cathode: E0unknown = E0cell (positive value).
- - If the unknown is the anode: E0unknown = -E0cell (negative value).
- - Result: The standard reduction potential of the unknown half-cell.
Remember
- - SHE is the reference (0.00 V).
- - Voltmeter measures E0cell.
- - Electron flow tells you the anode and cathode.
- - Use the formula: E0cell = E0cathode - E0anode.
- - If the unknown electrode is the cathode, its potential is positive.
- - If the unknown electrode is the anode, its potential is negative.
- - Example: Zn gives electrons to SHE → Zn is anode → E0 = -0.76 V.
Common Mistakes
- - Forgetting to determine which electrode is the anode and which is the cathode. The formula requires knowing this, as it affects the sign of the result.
- - Using the oxidation potential instead of the reduction potential. Standard tables provide reduction potentials. If the unknown electrode is the anode, you must take the negative of the measured cell potential to get the reduction potential.
- - Confusing the measured cell potential with the electrode potential. The voltmeter measures the cell potential (difference between two electrodes). You need to calculate the electrode potential of the unknown half-cell using the formula.
- - Assuming the SHE is always the cathode. It can be the anode if the other half-cell has a more positive reduction potential.
Quick Revision
To measure the standard electrode potential of a half-cell, it is connected to the Standard Hydrogen Electrode (SHE) through a salt bridge to form a galvanic cell. The voltmeter measures the cell potential. By observing the direction of electron flow, we determine the anode (oxidation) and cathode (reduction). Using the formula E0cell = E0cathode - E0anode and knowing the SHE has a potential of 0.00 V, we can calculate the standard reduction potential of the unknown half-cell. If the unknown is the cathode, its potential equals the cell potential. If it is the anode, its potential is the negative of the cell potential.
60-Second Revision
To measure standard electrode potentials, connect the unknown half-cell to the SHE (0.00 V) to form a cell. The voltmeter gives the cell potential. The direction of electron flow identifies the anode (oxidation) and cathode (reduction). Use E0cell = E0cathode - E0anode. If the unknown is the cathode, its potential is positive; if it is the anode, its potential is negative. The result is the standard reduction potential of the unknown half-cell.
Frequently Asked Questions (FAQs)
Q1. How can the standard electrode potential of a half-cell be measured? Answer: The standard electrode potential of a half-cell is measured by connecting it to the Standard Hydrogen Electrode (SHE) to form a galvanic cell. The cell potential is measured with a voltmeter. Using the formula E0cell = E0cathode - E0anode and knowing the SHE has a potential of 0.00 V, the potential of the unknown half-cell can be calculated.
Q2. What is the role of the voltmeter in measuring electrode potential? Answer: The voltmeter measures the cell potential (E0cell), which is the potential difference between the two electrodes of the galvanic cell. This measured value is used to calculate the standard electrode potential of the unknown half-cell.
Q3. How do you determine which electrode is the anode and which is the cathode? Answer: The direction of electron flow in the external circuit indicates the anode and cathode. Electrons always flow from the anode (where oxidation occurs) to the cathode (where reduction occurs). The electrode from which electrons flow is the anode, and the one to which they flow is the cathode.
Q4. Why is the standard hydrogen electrode used as a reference? Answer: The SHE is used as a reference because its electrode potential is defined as exactly 0.00 V. This provides a common baseline for all measurements, allowing the relative potentials of different half-cells to be compared and tabulated.
Q5. In the measurement of the standard electrode potential of zinc, why is the measured value -0.76 V? Answer: The measured cell potential is 0.76 V, and the zinc electrode acts as the anode (oxidation). Since the SHE is the cathode (0.00 V), using the formula E0cell = E0cathode - E0anode, we get 0.76 = 0.00 - E0anode, so E0anode = -0.76 V. This is the standard reduction potential of zinc.
Q5. What if the unknown electrode is the cathode? Answer: If the unknown electrode is the cathode (reduction occurs there), then its potential is positive. The cell potential measured would be equal to the standard reduction potential of the unknown electrode because the anode is the SHE (0.00 V). For example, the copper electrode has a standard reduction potential of +0.34 V when measured against the SHE.
Q6. How is the standard electrode potential of copper determined using the SHE? Answer: A copper electrode in 1 M CuSO_4 solution is connected to the SHE. The voltmeter reads 0.34 V, and electrons flow from the SHE to the copper electrode, meaning copper is the cathode. Since E0cell = E0cathode - E0anode, and the SHE is the anode (0.00 V), E0cathode = +0.34 V.
Q7. What are the standard conditions required for measuring electrode potentials? Answer: The standard conditions are: 1 mol dm⁻³ concentration for solutions, 1 atm pressure for gases, and a temperature of 25°C (298 K). Under these conditions, the measured potentials are called standard electrode potentials (E0).
Q8. Why is it difficult to measure the potential of a single electrode directly? Answer: It is impossible to measure the potential of a single electrode because a half-cell reaction cannot occur without a simultaneous reaction in another half-cell. The potential is always a difference between two electrodes. The SHE provides a reference point to measure this difference.
Multiple Choice Questions (MCQs)
- Q1. How is the standard electrode potential of an unknown half-cell measured?
- A. By measuring its potential directly with a voltmeter.
- B. By connecting it to the SHE and measuring the cell potential.
- C. By measuring its resistance with an ohmmeter.
- D. By measuring its temperature.
- Correct Answer: B
- Explanation: The standard electrode potential is measured by connecting the half-cell to the SHE and using a voltmeter to measure the cell potential.
- Q2. In the measurement of the zinc electrode potential, which electrode acts as the anode?
- A. Zinc
- B. SHE
- C. Both
- D. Neither
- Correct Answer: A
- Explanation: Zinc has a more negative reduction potential than the SHE, so it is oxidized and acts as the anode.
- Q3. What is the reading of the voltmeter when a zinc electrode is connected to the SHE?
- A. 0.00 V
- B. 0.34 V
- C. 0.76 V
- D. 1.10 V
- Correct Answer: C
- Explanation: The standard electrode potential of zinc is -0.76 V, and the measured cell potential is 0.76 V.
- Q4. If the unknown electrode is the cathode, its standard reduction potential is:
- A. Negative
- B. Positive
- C. Zero
- D. Equal to the SHE potential
- Correct Answer: B
- Explanation: If the unknown electrode is the cathode, it has a more positive reduction potential than the SHE, so its potential is positive.
- Q5. Which electrode is used as the reference in measuring standard electrode potentials?
- A. Copper electrode
- B. Zinc electrode
- C. Standard Hydrogen Electrode (SHE)
- D. Silver electrode
- Correct Answer: C
- Explanation: The SHE is the universal reference electrode with a defined potential of 0.00 V.
- Q6. In the formula E0cell = E0cathode - E0anode, what is the value of E0SHE?
- A. 1.00 V
- B. 0.00 V
- C. -1.00 V
- D. 0.76 V
- Correct Answer: B
- Explanation: The SHE is defined as having a standard electrode potential of exactly 0.00 V.
- Q7. What does the direction of electron flow in the external circuit indicate?
- A. The temperature of the cell.
- B. The concentration of the solution.
- C. The anode and cathode of the cell.
- D. The pressure of the gases.
- Correct Answer: C
- Explanation: Electrons flow from the anode (where oxidation occurs) to the cathode (where reduction occurs).
- Q8. If a copper electrode is connected to the SHE, which electrode is the cathode?
- A. Copper
- B. SHE
- C. Both
- D. Neither
- Correct Answer: A
- Explanation: Copper has a more positive reduction potential (+0.34 V) than the SHE (0.00 V), so it is reduced and acts as the cathode.
- Q9. The standard electrode potential of an unknown half-cell is measured by connecting it to:
- A. A voltmeter directly.
- B. An ammeter.
- C. The standard hydrogen electrode.
- D. A barometer.
- Correct Answer: C
- Explanation: The unknown half-cell is connected to the SHE, which serves as the reference electrode for the measurement.
- Q10. In the measurement of the zinc electrode potential, what is the standard reduction potential of zinc?
- A. +0.76 V
- B. -0.76 V
- C. +0.34 V
- D. -0.34 V
- Correct Answer: B
- Explanation: The cell potential is 0.76 V, and since zinc is the anode, its reduction potential is -0.76 V.
Multiple Choice Questions (MCQs)
- Q1. What is the standard potential of the SHE used as a reference?
- A. 0.00 V
- B. 1.00 V
- C. -1.00 V
- D. 0.76 V
- Correct Answer: A
- Explanation: The SHE is defined as having a standard electrode potential of 0.00 V.
- Q2. When a zinc electrode is connected to the SHE, which electrode is the anode?
- A. Zinc
- B. SHE
- C. Both
- D. Neither
- Correct Answer: A
- Explanation: Zinc is oxidized and acts as the anode because it has a more negative reduction potential than the SHE.
- Q3. What is the cell potential when a zinc electrode is connected to the SHE?
- A. 0.00 V
- B. 0.34 V
- C. 0.76 V
- D. 1.10 V
- Correct Answer: C
- Explanation: The voltmeter reads 0.76 V, which is the cell potential of the zinc-SHE cell.
- Q4. If a copper electrode is connected to the SHE, what is the cell potential?
- A. 0.00 V
- B. 0.34 V
- C. 0.76 V
- D. 1.10 V
- Correct Answer: B
- Explanation: The standard reduction potential of copper is +0.34 V, so the cell potential is 0.34 V.
Q5. The standard electrode potential of an unknown half-cell is equal to the cell potential if the unknown electrode is the: A. Anode B. Cathode C. Salt bridge D. Voltmeter Correct Answer: B Explanation: If the unknown electrode is the cathode, the SHE is the anode (0.00 V), so E0cell = E0cathode - 0.00 = E0cathode.
- Q6. In the formula E0cell = E0cathode - E0anode, if the SHE is the cathode, what is its value?
- A. 0.00 V
- B. 1.00 V
- C. -1.00 V
- D. 0.76 V
- Correct Answer: A
- Explanation: The standard potential of the SHE is always 0.00 V.
- Q7. Which instrument is used to measure the cell potential in the electrode potential measurement?
- A. Ammeter
- B. Voltmeter
- C. Thermometer
- D. Barometer
- Correct Answer: B
- Explanation: A voltmeter measures the potential difference (voltage) between the two electrodes.
- Q8. If the standard electrode potential of zinc is -0.76 V, what is its oxidation potential?
- A. -0.76 V
- B. +0.76 V
- C. 0.00 V
- D. +1.10 V
- Correct Answer: B
- Explanation: The oxidation potential is the negative of the reduction potential. So, -(-0.76) = +0.76 V.
- Q9. What is the role of the salt bridge in the measurement of electrode potential?
- A. It allows electrons to flow.
- B. It maintains electrical neutrality and completes the circuit.
- C. It provides a source of ions for the reaction.
- D. It measures the potential.
- Correct Answer: B
- Explanation: The salt bridge allows ions to flow between the two half-cells, maintaining electrical neutrality and completing the circuit.
- Q10. The standard electrode potential of an unknown half-cell is measured by comparing it to:
- A. A copper electrode
- B. A zinc electrode
- C. The standard hydrogen electrode
- D. A silver electrode
- Correct Answer: C
- Explanation: The SHE is the primary reference electrode used to measure the standard electrode potentials of other half-cells.
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Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.2.4.4 Method to Measure the Electrode Potentials
Sticky Notes · Quick Revision
Key Point 1
- Measure electrode potential: Connect to SHE, read voltmeter.
Key Point 2
- **SHE = 0.00 V.**
Key Point 3
- Electron flow: From anode to cathode.
Key Point 4
- Formula: E0cell = E0cathode - E0anode.
Key Point 5
- If unknown is cathode: E0unknown = +E0cell.
Key Point 6
- If unknown is anode: E0unknown = -E0cell.
Key Point 7
- Example: Zn (E0 = -0.76 V), Cu (E0 = +0.34 V).
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