IB Chemistry HLTopic 6 — Electron TransferPaper 1 & 2Core idea~9 min read
The Hydrogen Electrode
You can never measure the voltage of one half-cell on its own. A voltmeter has two leads, so it can only ever tell you the difference between two things. Chemists got around this by picking one half-cell, calling it zero, and measuring everything else against it. That half-cell is the standard hydrogen electrode.
📚 What you need to know
A single electrode potential cannot be measured. Only the difference between two half-cells can.
The standard hydrogen electrode (SHE) is the agreed reference. It is given the value Eθ = 0.00 V by agreement, not by measurement.
Standard conditions for it: hydrogen gas at 100 kPa, H+(aq) at 1.00 mol dm−3, temperature 298 K, and an inert platinum electrode.
Its half-equation is 2H+(aq) + 2e− ⇌ H2(g).
The SHE is always drawn and written on the left, so the voltmeter reading is the other half-cell’s Eθ directly.
All the values in Section 19 of the data booklet are standard reduction potentials — every half-equation is written as a gain of electrons.
The SHE is awkward to use in a real lab, so once a few values are known, other half-cells are used as secondary references.
Why you cannot measure one half-cell
Dip a zinc rod into zinc sulfate solution and something real happens: zinc atoms leave the rod as Zn2+ ions and dump electrons behind, so the rod builds up a negative charge. There is a genuine potential difference between the rod and the solution.
The problem is getting at it. To read a voltage you must touch both leads of the voltmeter to something. One lead goes on the zinc rod — fine. But the moment you dip the other lead into the solution, that lead becomes a second electrode with its own potential, and now you are reading two things at once.
Think about altitude. You can say a mountain is 3000 m high, but 3000 m above what? We all agreed on sea level. Sea level is not “no height” in any deep sense — it is just the line everyone decided to call zero. The hydrogen electrode is chemistry’s sea level.
What the standard hydrogen electrode actually is
It is a half-cell built out of hydrogen gas and hydrogen ions, with a piece of platinum acting as the metal that carries electrons in and out.
The platinum does not react — it is there purely as a landing strip where hydrogen gas and hydrogen ions can swap electrons.
Why platinum, and why roughened?
Inert. Platinum will not be oxidised by the acid, so it adds nothing of its own to the potential.
Conducting. Electrons need a route in and out to the external circuit.
Rough surface. The platinum is coated with a fine black layer of platinum. More surface area means the H+/H2 equilibrium settles quickly, so the reading is steady.
The reference half-equation
2H+(aq) + 2e− ⇌ H2(g) Eθ = 0.00 V
Using it to measure another half-cell
Connect the SHE to the half-cell you want to measure, join the solutions with a salt bridge, and read the voltmeter. Because the left-hand value is zero, whatever the meter says is the standard electrode potential of the other half-cell.
Zinc comes out negative, so electrons push from the zinc towards the hydrogen electrode. A negative Eθ means “loses electrons more readily than hydrogen does”.
🧩 Reading a hydrogen-electrode cell
Put the SHE on the left. That is the convention, and it is what makes the maths easy.
Read the meter. EMF = E(right) − E(left), and E(left) = 0, so EMF = E(right).
Check the sign. If the unknown electrode is the negative terminal, its Eθ is negative.
Write it as a reduction. Data booklet values always show electrons being gained, e.g. Zn2+ + 2e− ⇌ Zn.
Why nobody actually uses one
The SHE is the definition, not the everyday tool. In a real lab it is a nuisance:
The H+/H2 reaction is slow to reach equilibrium, so readings drift.
Holding hydrogen at exactly 100 kPa needs a gas cylinder and a regulator — not portable.
Hydrogen is flammable, and the platinum is expensive and easily poisoned by impurities.
The workaround: once a half-cell has been measured against the SHE, it can be used as a stand-in reference. A silver/silver chloride electrode is the usual choice — small, stable and no gas cylinder required. Its value is still traceable back to hydrogen.
Worked examples
WORKED EXAMPLE
A standard hydrogen electrode is connected to a standard nickel half-cell. The voltmeter reads 0.26 V and the nickel is the negative terminal. Find Eθ for Ni2+/Ni.
Set the cell up the standard waySHE on the left, so E(left) = 0.00 V. Nickel on the right.Apply the ruleEMF = E(right) − E(left) = E(Ni) − 0.00Fix the sign from the polarityNickel is negative, so it is pushing electrons out. Its value must be below zero.Eθ(Ni2+/Ni) = −0.26 Vthe size comes from the meter, the sign comes from which terminal is negative
WORKED EXAMPLE
Write the conventional cell diagram for a standard hydrogen electrode joined to a standard silver half-cell, Eθ(Ag+/Ag) = +0.80 V.
Start on the left with the hydrogen electrodePlatinum, then the gas, then the ions. Single lines are phase boundaries.Pt(s) | H2(g) | H+(aq)Double line for the salt bridge, then the right-hand cellHighest oxidation state sits next to the salt bridge on both sides.Pt(s) | H2(g) | H+(aq) || Ag+(aq) | Ag(s) Eθ = +0.80 Vpositive value, so silver is the positive terminal and gains electrons
WORKED EXAMPLE
In a cell made from a SHE and an unknown metal half-cell, the hydrogen electrode is the positive terminal and the reading is 0.44 V. What is Eθ of the unknown, and what happens at each electrode?
Work out the sign firstHydrogen is positive, so the unknown metal is negative — it loses electrons more readily than hydrogen.Eθ(unknown) = −0.44 VDecide the electrode reactionsNegative electrode = oxidation. Positive electrode = reduction.M(s) → M2+(aq) + 2e− and 2H+(aq) + 2e− → H2(g)−0.44 V — the metal is oxidised, hydrogen gas is producedthat value belongs to iron: Fe2+ + 2e− ⇌ Fe is −0.45 V in the booklet
💡 Exam tip
If a question asks you to state the conditions for the SHE, give all four: 100 kPa H2, 1.00 mol dm−3 H+, 298 K, platinum electrode. Dropping the temperature is the usual lost mark.
Say the value is defined or assigned as 0.00 V. Saying it “was measured as zero” is wrong and examiners notice.
Always write half-equations from the booklet as reductions, with the electrons on the left. Flip them only when you build an overall equation.
“Inert” earns the mark for platinum — but add “provides a surface for the reaction” if the question is worth two.
Negative Eθ means better at losing electrons than hydrogen. Positive means better at gaining them.
If the question says a cell is at non-standard conditions, you cannot use booklet values directly — say so rather than guessing.
⚠ Common mix-up
“The hydrogen electrode has no potential.” It has one. We just cannot measure it, so we call it zero and work from there.
Forgetting the sign. A meter shows 0.76 V; whether that is +0.76 or −0.76 depends entirely on which electrode is negative.
Putting the SHE on the right. Then EMF = 0 − E(other), and every sign flips. Keep it on the left.
Thinking platinum takes part. It carries electrons and nothing else. It is not oxidised and it is not in the half-equation.
Confusing 100 kPa with 101.3 kPa. The IB uses 100 kPa for standard state.
Writing the SHE half-equation with one electron. It is 2H+ + 2e− ⇌ H2, because hydrogen gas is diatomic.
Up next: Standard Cell Potentials — now that every half-cell has a number, you can put any two together and work out the voltage, and predict whether the reaction goes at all.
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