5.1.3Unit 5. Transition Metals and Organic Nitrogen Chemistry
Measuring Standard Electrode Potential — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 9 of 43 Unit quiz
Section 1 of 5The Standard Hydrogen Electrode
Electrode
- Electrode A piece of platinum coated with finely divided platinum black dips into the solution and provides a large surface for the reaction.
Gas
- Gas Hydrogen gas at 100 kPa is bubbled over the platinum, setting up the equilibrium 2H⁺ + 2e⁻ ⇌ H₂.
Solution
- Solution The electrode sits in acid containing hydrogen ions at 1.00 mol dm⁻³, for example hydrochloric acid.
Temperature
- Temperature The whole half-cell is held at 298 K, and by definition its potential is 0.00 V.
Section 2 of 5Building the Full Cell
1
Set up the half-cell to be measured with conditions at standard values: metal in 1.00 mol dm⁻³ solution of its own ions.
2
Connect the two half-cells with a salt bridge, such as filter paper soaked in potassium nitrate solution, so ions can move and the circuit is complete.
3
Connect the electrodes through a high-resistance voltmeter so that almost no current flows and the reading is the maximum potential difference.
4
Read the voltage, noting which electrode is positive, to give the magnitude and the sign of E⦵.
5
For a half-cell with two ions of the same element, such as Fe³⁺/Fe²⁺, use an inert platinum electrode with both ions at 1.00 mol dm⁻³.
Section 3 of 5Practical Points
Practical Points
Salt bridge
- Salt bridge It must not react with the solutions, which is why potassium nitrate is used instead of potassium chloride when silver ions are present.
High resistance
- High resistance A voltmeter that draws current would cause concentration changes and a voltage below the true value.
Half-cell types
- Half-cell types Metal in its ion, ion in a different oxidation state, and gas in its ion each need a particular electrode arrangement.
Practical limits
- Practical limits The hydrogen electrode is awkward to prepare, so more convenient reference electrodes are often used in the laboratory.
Section 4 of 5Worked Example
Question
- In a cell with a standard hydrogen electrode, a copper half-cell gives a reading of 0.34 V with the copper electrode positive.
- State E⦵ for Cu²⁺/Cu and the half-reaction occurring at the copper electrode.
- The copper electrode is positive so it is the site of reduction.
- The reading is relative to 0.00 V for the hydrogen electrode, so E⦵ is +0.34 V.
Answer
- E⦵(Cu²⁺/Cu) = +0.34 V;
- Cu²⁺ + 2e⁻ → Cu at the copper electrode
Section 5 of 5Exam Checklist & Tips
5.1.3 – Key Points for the Exam
- Standard hydrogen electrode The reference half-cell with E⦵ defined as 0.00 V.
- Salt bridge A connection that allows ion flow to complete the circuit.
- Platinum black Finely divided platinum giving a large surface area.
- High-resistance voltmeter A meter that draws negligible current.
Common Mistakes
- Using a wire instead of a salt bridge to join the solutions.
- Omitting platinum from a half-cell with only ions in solution.
- Using hydrogen at the wrong pressure and still calling the electrode standard.
- Forgetting that the voltmeter must have very high resistance.
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