1. Standard Hydrogen Electrode (SHE)
An individual half-cell cannot have its potential measured in isolation because oxidation and reduction must occur together. To establish an absolute electrochemical scale, scientists defined an arbitrary zero-point: the Standard Hydrogen Electrode (SHE), which is assigned an electrode potential of 0.00 V at all temperatures.
- Hydrogen gas pressure: 100 kPa (1 bar).
- Hydrogen ion concentration: 1.00 mol dm-3 H+(aq) (e.g. 1.0 M HCl or 0.5 M H2SO4).
- Temperature: 298 K (25 degrees C).
- Electrode: Platinum black (platinised platinum) foil.
Role of the Platinum Electrode
Platinum is chemically inert and will not take part in the redox reaction. Coated with finely divided platinum black, it provides a large catalytic surface area to facilitate the rapid attainment of dynamic equilibrium between hydrogen gas and aqueous protons:
2H+(aq) + 2e- <=> H2(g) [E_std = 0.00 V]
2. Conventional IUPAC Cell Notation
By international agreement (IUPAC), electrochemical cells are depicted on paper using a standardised shorthand notation without drawing complete glassware diagrams:
- Phase boundary (|): Represented by a single vertical line between different physical states (e.g. solid metal and aqueous solution).
- Salt bridge (||): Represented by a double vertical line in the center.
- Left half-cell: By convention, the negative electrode (oxidation / anode) is written on the left.
- Right half-cell: The positive electrode (reduction / cathode) is written on the right.
- Electrons flow: From left to right through the external circuit wire.
For example, the Daniell cell consisting of zinc and copper half-cells is written as:
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
If a half-cell involves only aqueous species (such as Fe3+/Fe2+), an inert platinum electrode is included, with species in the same phase separated by a comma:
Pt(s) | H2(g) | H+(aq) || Fe3+(aq), Fe2+(aq) | Pt(s)
3. Electrochemical Cell Setup Diagram
The diagram below displays the standard experimental arrangement measuring the standard electrode potential of the copper half-cell against the SHE:
Function of the Salt Bridge
A salt bridge connects the two solutions to complete the electrical circuit while keeping the solutions physically separate. Usually prepared by soaking filter paper in saturated potassium nitrate (KNO3), it allows the migration of ions (K+ towards cathode; NO3- towards anode) to prevent the build-up of net charge in either beaker, which would halt current flow.
4. Cell EMF & Feasibility Predictions
The standard electromotive force (EMF) of a cell, E_cell, is calculated from standard electrode potentials (always tabulated as reduction potentials):
E_cell = E_right - E_left = E_reduction - E_oxidation
E_cell > 0.00 V
(Linked to Gibbs free energy: delta G = -n * F * E_cell, where n is moles of electrons and F is Faraday constant).
An exam question may note that a reaction with E_cell > 0 does not visibly occur in the laboratory. The reasons are:
1. Kinetic stability: The activation energy (Ea) is too high at room temperature.
2. Non-standard conditions: Concentrations deviate from 1.0 mol dm-3 or pressure deviates from 100 kPa, shifting equilibrium potentials.
5. Commercial Batteries & Fuel Cells
1. Primary (Non-Rechargeable) Cells
Standard zinc-carbon and alkaline cells. The chemical reactions cannot be reversed by passing an electric current because the electrode reactions are irreversible and components are consumed.
2. Secondary (Rechargeable) Lithium-Ion Cells
Used widely in mobile phones, laptops, and electric vehicles. Consists of a lithium cobalt oxide (LiCoO2) positive electrode, a graphite negative electrode intercalated with lithium atoms, and a non-aqueous organic solvent electrolyte containing lithium salts.
- Discharging (at negative electrode): Li_x C6 -> x Li+ + x e- + C6
- Discharging (at positive electrode): Li_(1-x)CoO2 + x Li+ + x e- -> LiCoO2
- Recharging: Applying external potential forces Li+ ions back through electrolyte to the graphite matrix.
3. Hydrogen-Oxygen Fuel Cells
A fuel cell operates continuously as long as fuel (hydrogen) and oxidant (oxygen) are supplied from external tanks. In an alkaline fuel cell using potassium hydroxide (KOH) electrolyte:
- Negative electrode (Anode): 2H2(g) + 4OH-(aq) -> 4H2O(l) + 4e- [E = -0.83 V]
- Positive electrode (Cathode): O2(g) + 2H2O(l) + 4e- -> 4OH-(aq) [E = +0.40 V]
- Overall reaction: 2H2(g) + O2(g) -> 2H2O(l) [E_cell = +1.23 V]
| Advantages of Hydrogen Fuel Cells | Disadvantages & Technical Challenges |
|---|---|
| Zero polluting tailpipe emissions: water is the sole product. | Hydrogen is flammable and explosive; requires high-pressure tanks. |
| Much higher thermal efficiency than internal combustion engines (60%+ vs 25%). | Most industrial hydrogen is generated by steam reforming fossil fuels (emitting CO2). |
| Continuous generation without needing hours to recharge like batteries. | Electrodes require expensive precious metal catalysts (platinum). |
6. Worked Calculations
Zn2+(aq) + 2e- <=> Zn(s) [E = -0.76 V]
Cu2+(aq) + 2e- <=> Cu(s) [E = +0.34 V]
Calculate the standard EMF of the cell and write the conventional cell representation.
Step 1: Identify oxidation and reduction:
The more negative potential (-0.76 V) oxidises (Zn -> Zn2+ + 2e-). The more positive potential (+0.34 V) reduces (Cu2+ + 2e- -> Cu).
Step 2: Calculate E_cell:
E_cell = E_reduction - E_oxidation = (+0.34) - (-0.76) = +1.10 V
Step 3: Write conventional representation:
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
Cu+(aq) + e- <=> Cu(s) [E = +0.52 V]
Cu2+(aq) + e- <=> Cu+(aq) [E = +0.15 V]
Step 1: Write disproportionation reaction:
2Cu+(aq) -> Cu2+(aq) + Cu(s)
Reduction half-cell: Cu+(aq) + e- -> Cu(s) [E = +0.52 V]
Oxidation half-cell: Cu+(aq) -> Cu2+(aq) + e- [E = +0.15 V]
Step 2: Calculate E_cell:
E_cell = E_reduction - E_oxidation = (+0.52) - (+0.15) = +0.37 V
Since E_cell > 0, the disproportionation of Cu+(aq) into Cu(s) and Cu2+(aq) is thermodynamically feasible.
Exam-Style Practice Questions
Test your understanding of these core syllabus concepts with targeted questions and detailed explanations.
Question 1: What is the primary function of the platinum black coating on the hydrogen electrode in the SHE?
Show Answer & Explanation
Correct Answer: B
Finely divided platinum black provides an expansive catalytic surface that allows the dynamic equilibrium between gaseous hydrogen and aqueous protons to establish rapidly.
Question 2: Why is a high-resistance voltmeter placed between the two half-cells when measuring standard electrode potential?
Show Answer & Explanation
Correct Answer: A
If significant current flowed, concentrations of ions in the half-cells would change, disrupting standard conditions. A high-resistance voltmeter measures the maximum potential difference without drawing current.
Question 3: Which species is the strongest oxidising agent among the following standard potentials: Ag+ (+0.80 V), Fe3+ (+0.77 V), Zn2+ (-0.76 V), Mg2+ (-2.37 V)?
Show Answer & Explanation
Correct Answer: D
The more positive the standard electrode potential, the greater the tendency to gain electrons (undergo reduction). Therefore, Ag+ (+0.80 V) is the strongest oxidising agent.
Question 4: What is the overall reaction occurring in a hydrogen-oxygen fuel cell?
Show Answer & Explanation
Correct Answer: A
The overall cell reaction in both alkaline and acidic fuel cells is the combination of hydrogen and oxygen gas to form pure water: 2H2(g) + O2(g) -> 2H2O(l).