The equilibrium potential of an electrode reaction, such as that between a metal and its ions, under standard conditions, relative to the standard hydrogen electrode at 0 V. The more negative it is, the more easily the metal is oxidized.
Japanese name: 標準電極電位
Standard electrode potential appears in 1 practice question across 1 topic, including Corrosion and surface treatment.
The standard electrode potential is the equilibrium potential of the electrode reaction Mⁿ⁺ + ne⁻ ⇄ M under standard conditions, such as 25 °C and unit ion activity, expressed relative to the standard hydrogen electrode (2H⁺ + 2e⁻ ⇄ H₂) at 0 V. For example, Cu²⁺/Cu is +0.34 V, Fe²⁺/Fe is −0.44 V and Zn²⁺/Zn is −0.76 V. Active metals with more negative values tend to give up electrons and become cations readily, while noble metals with more positive values tend to resist ionization. The emf of a cell combining two electrodes is the potential of the noble side minus that of the active side.
For a metal to corrode, a cathodic reaction must be paired with its dissolution. In neutral water this is oxygen reduction and in acidic solutions hydrogen evolution; if the metal's potential is below the potential of those reactions, corrosion is thermodynamically possible. Gold and platinum hardly corrode because their potentials are very noble. Zinc and magnesium, on the other hand, with active potentials, are used as sacrificial anodes to protect steel. Which metal dissolves when dissimilar metals are in contact can also be judged from their relative potentials.
The standard electrode potential is determined by equilibrium thermodynamics and tells you nothing about the rate of corrosion. Aluminum, at −1.66 V, and titanium, at −1.63 V, are very active, yet they resist real environments well thanks to their dense surface oxide films. Also, real solutions differ from standard conditions in ion concentration and temperature, and the potential changes according to the Nernst equation. When evaluating combinations of dissimilar metals, it is therefore common to use the ranking of corrosion potentials measured in real environments such as seawater (the galvanic series).
Answer: C. Zinc
The standard electrode potential expresses the equilibrium between a metal and its ions relative to the standard hydrogen electrode at 0 V. The more negative (more active) the value, the stronger the tendency of the metal to give up electrons and become a cation, so of the three, zinc at −0.76 V is the most easily oxidized and copper at +0.34 V is the least (the most noble). Choosing copper reads the sign the wrong way round. Iron (−0.44 V) is more noble than zinc, so it is not as easily oxidized. Note, however, that the standard electrode potential indicates a tendency, not the actual rate of corrosion. Aluminum, for example, is very active at −1.66 V, but resists the atmosphere well thanks to its dense surface oxide film. To evaluate metal combinations in real environments, the ranking of corrosion potentials measured in seawater (the galvanic series) is used.
Metals whose electrode potentials lie on the negative side are called active (base), and those on the positive side noble. The more active a metal, the more readily it ionizes, and when dissimilar metals are in contact, the more active one dissolves.
The order of ionization tendency (the activity series) roughly corresponds to arranging standard electrode potentials from low to high. You can think of the standard electrode potential as expressing it numerically.
A dense, stable oxide film forms on its surface and prevents further reaction. The standard electrode potential shows only a tendency and does not include differences in rate caused by films.
The standard electrode potential expresses the equilibrium between a metal and its ions relative to the standard hydrogen electrode at 0 V.
At 25 °C, the standard electrode potentials are +0.34 V for…Faraday's law of electrolysis Galvanic corrosion Local cell Passivity Pitting corrosion Sacrificial anode Stress corrosion cracking