6.2.11Additional lessons
The Hydrogen Electrode — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 142 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Standard Hydrogen Electrode
The hydrogen electrode is the reference against which all other electrode potentials are measured.
Construction
- Construction Hydrogen gas is bubbled over a platinum electrode coated with platinum black, which dips into an acid solution.
Conditions
- Conditions The acid has [H⁺] = 1.00 mol dm⁻³, the gas pressure is 100 kPa, and the temperature is 298 K.
Half-equation
- Half-equation 2H⁺(aq) + 2e⁻ ⇌ H₂(g), and its standard electrode potential is defined as exactly 0.00 V.
Platinum role
- Platinum role Platinum is inert and catalyses the reaction, while its rough surface adsorbs hydrogen gas.
Section 2 of 5Measuring Electrode Potentials
1
Connect the half-cell under test to the hydrogen electrode with a salt bridge and a high-resistance voltmeter.
2
Make sure all solutions are at 1.00 mol dm⁻³ and the temperature is 298 K, so that the potential is a standard value.
3
Read the voltage, which is the standard electrode potential E° of the half-cell because the reference is zero.
4
If the half-cell is the positive terminal its E° is positive, and if it is the negative terminal E° is negative.
5
Tabulate each value as a reduction half-equation so that the series can be compared directly.
Section 3 of 5Using the Values
Using the Values
Positive E°
- Positive E° The oxidised form is easily reduced, so it is a strong oxidising agent, for example Cl₂ with E° = +1.36 V.
Negative E°
- Negative E° The oxidised form is hard to reduce, so the metal is easily oxidised, for example Zn²⁺/Zn with E° = −0.76 V.
Ions in solution
- Ions in solution When no metal is present, such as Fe³⁺/Fe²⁺, an inert platinum wire is used to carry electrons.
Reference limits
- Reference limits Because the electrode needs hydrogen gas, other practical reference electrodes are often used.
| Half-cell | E° / V |
|---|---|
| Zn²⁺/Zn | −0.76 |
| H⁺/H₂ | 0.00 |
| Cu²⁺/Cu | +0.34 |
| Ag⁺/Ag | +0.80 |
Section 4 of 5Worked Example
Question
- A standard zinc half-cell is connected to a standard hydrogen electrode, and zinc is the negative terminal with a cell voltage of 0.76 V.
- State the standard electrode potential of Zn²⁺/Zn.
- The hydrogen electrode has E° = 0.00 V, so the whole cell voltage is due to the zinc half-cell.
- Zinc is the negative terminal, so the zinc half-cell is less able to attract electrons than the hydrogen electrode.
- Therefore the reduction potential of the zinc half-cell is negative.
Answer E°(Zn²⁺/Zn) = −0.76 V
Section 5 of 5Exam Checklist & Tips
6.2.11 – Key Points for the Exam
- Standard hydrogen electrode The reference half-cell with an electrode potential defined as 0.00 V.
- Standard electrode potential The voltage of a half-cell measured against the SHE under standard conditions.
- Standard conditions 1.00 mol dm⁻³ solutions, 100 kPa gas pressure and 298 K.
- Inert electrode An electrode such as platinum that conducts but does not react.
Common Mistakes
- Writing the electrode potential for an oxidation equation without changing its sign.
- Claiming the SHE potential is measured, when it is defined to be zero.
- Forgetting that 1.00 mol dm⁻³ H⁺ and 100 kPa H₂ are both required.
- Using a copper wire as the inert electrode in an iron(III)/iron(II) half-cell.
Last updated:
\n