Standard Hydrogen Electrode: MDCAT Chemistry notes
Standard Hydrogen Electrode for MDCAT: SHE construction, standard electrode potentials, the electrochemical series and predicting anode, cathode and emf.
The standard hydrogen electrode (SHE)
A single electrode potential cannot be measured on its own. It is measured against a reference, the SHE, whose potential is arbitrarily taken as 0.00 V at 298 K.
- A platinum foil coated with platinum black dips into 1 M $\mathrm{H^+}$ solution (e.g. 1 M HCl).
- Pure $\mathrm{H_2}$ gas at 1 atm is bubbled over the foil at 25 °C (298 K).
- Platinum is inert: it does not react. It conducts electrons, gives a large surface for adsorbing $\mathrm{H_2}$, and catalyses the equilibrium $\mathrm{2H^+ + 2e^- \rightleftharpoons H_2}$.
Standard electrode potential
When a metal is dipped into a solution of its own ions, an equilibrium is set up and a potential develops. The standard electrode potential $E^\circ$ is this potential measured under standard conditions (1 M ions, 1 atm gas, 298 K) with the SHE as the other electrode. The two half-cells are joined by a salt bridge and an external wire. The emf is read with a high-resistance voltmeter, which draws almost no current.
By convention, $E^\circ$ values are quoted as reduction potentials.
- Cu with SHE: $\mathrm{Cu^{2+} + 2e^- \to Cu}$, $E^\circ = +0.34$ V. Copper is the cathode, so $\mathrm{Cu^{2+}}$ is reduced and $\mathrm{H_2}$ is oxidized.
- Zn with SHE: $E^\circ = -0.76$ V. Zinc is the anode, so Zn is oxidized and $\mathrm{H^+}$ is reduced.
Electrochemical series (reduction potentials)
| Half-reaction | $E^\circ$ / V |
|---|---|
| $\mathrm{Li^+ + e^- \to Li}$ | −3.04 |
| $\mathrm{K^+ + e^- \to K}$ | −2.93 |
| $\mathrm{Mg^{2+} + 2e^- \to Mg}$ | −2.37 |
| $\mathrm{Zn^{2+} + 2e^- \to Zn}$ | −0.76 |
| $\mathrm{2H^+ + 2e^- \to H_2}$ | 0.00 |
| $\mathrm{Cu^{2+} + 2e^- \to Cu}$ | +0.34 |
| $\mathrm{Ag^+ + e^- \to Ag}$ | +0.80 |
| $\mathrm{F_2 + 2e^- \to 2F^-}$ | +2.87 |
- A higher (more positive) reduction potential means a stronger oxidizing agent. $\mathrm{F_2}$ is the strongest and has the highest value.
- A more negative value means a stronger reducing agent and a more reactive metal (Li, K).
- Cu, Ag and Au have positive reduction potentials, so they are not easily oxidized. They are the least reactive metals and do not displace hydrogen from dilute acids.
- When two electrodes are coupled, the one with the higher reduction potential is the cathode. So Zn acts as the cathode against Mg, because −0.76 > −2.37.
Key formulas
$$E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}$$Example: in the Zn–Cu cell, $E^\circ = 0.34 - (-0.76) = 1.10$ V. A positive cell emf means the reaction is spontaneous as written.
Related facts asked in this topic
- Strong electrolytes such as KI (an ionic salt) ionize completely. Acetic acid, carbonic acid and $\mathrm{NH_4OH}$ are weak electrolytes.
- The hydrogen–oxygen fuel cell produces electricity with water as the by-product, and astronauts use this water.
Common MDCAT traps
- Stronger oxidizing agent means greater reduction potential, not oxidation potential.
- The Pt in the SHE is not a salt bridge and supplies no voltage. It is an inert catalytic surface.
- When Cu is joined to the SHE, it is $\mathrm{Cu^{2+}}$ that is reduced, not Cu metal.
- Emf is measured with a voltmeter, not an ammeter or a galvanometer.
Quick revision
- $E^\circ$(SHE) = 0.00 V at 298 K, 1 atm $\mathrm{H_2}$, 1 M $\mathrm{H^+}$.
- $E^\circ(\mathrm{Cu^{2+}/Cu}) = +0.34$ V and $E^\circ(\mathrm{Zn^{2+}/Zn}) = -0.76$ V.
- Fluorine has the highest reduction potential and lithium the lowest.
- Higher $E^\circ$ means cathode; lower $E^\circ$ means anode.