6.2.12Additional lessons
Standard Cell Potentials — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 143 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 5Calculating E° cell
The standard cell potential is the voltage when both half-cells are at standard conditions.
Cathode
- Cathode The half-cell with the more positive E° is reduced, so it is the cathode of the voltaic cell.
Anode
- Anode The half-cell with the less positive E° is oxidised, so it is the anode of the voltaic cell.
Formula
- Formula E°cell = E°(reduction half-cell) − E°(oxidation half-cell), using the values as listed for reduction.
No scaling
- No scaling Do not multiply E° by the stoichiometric coefficients, because potential does not depend on the amount of substance.
Section 2 of 5Predicting Spontaneity
1
Write both relevant half-equations as reductions and look up the E° value of each.
2
Choose the reaction in which the more positive half-cell is reduced and the other is oxidised.
3
Subtract the less positive E° from the more positive E° to find E°cell.
4
A positive E°cell means the reaction is feasible under standard conditions, and a negative value means it is not.
5
Reverse the direction if E°cell is negative, because the reverse reaction would have a positive value.
Section 3 of 5Limits of Predictions
Limits of Predictions
Rate
- Rate A positive E°cell says a reaction is possible but nothing about how fast it goes, as activation energy may be high.
Concentration
- Concentration Real concentrations and temperatures differ from standard conditions, which can change the voltage.
Typical example
- Typical example Zn + Cu²⁺ → Zn²⁺ + Cu has E°cell = +0.34 − (−0.76) = +1.10 V, so it is spontaneous.
Cell notation
- Cell notation The anode is written on the left and the cathode on the right, as in Zn | Zn²⁺ || Cu²⁺ | Cu.
| Couple | E° / V |
|---|---|
| Mg²⁺/Mg | −2.37 |
| Zn²⁺/Zn | −0.76 |
| Cu²⁺/Cu | +0.34 |
| Ag⁺/Ag | +0.80 |
Section 4 of 5Worked Example
Question Calculate E°cell for Cu(s) + 2Ag⁺(aq) → Cu²⁺(aq) + 2Ag(s), given E°(Ag⁺/Ag) = +0.80 V and E°(Cu²⁺/Cu) = +0.34 V, and state whether the reaction is spontaneous.
- Silver ions are reduced, so Ag⁺/Ag is the cathode with E° = +0.80 V.
- Copper is oxidised, so Cu²⁺/Cu is the anode with E° = +0.34 V.
- E°cell = +0.80 − (+0.34) = +0.46 V, and the silver coefficient of 2 does not change this value.
Answer
- E°cell = +0.46 V;
- positive, so the reaction is spontaneous under standard conditions.
Section 5 of 5Exam Checklist & Tips
6.2.12 – Key Points for the Exam
- E°cell The potential difference of a cell under standard conditions, in volts.
- Spontaneous A reaction that is thermodynamically feasible without continuous energy input.
- Cathode The electrode where reduction occurs in the cell.
- Anode The electrode where oxidation occurs in the cell.
Common Mistakes
- Doubling E° when the half-equation is doubled; potentials are intensive.
- Subtracting in the wrong order and getting a negative value for a feasible cell.
- Assuming a positive E°cell means the reaction is fast.
- Using oxidation potentials from the table without reversing their sign.
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.