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Encyclopedia of Physical Science and Technology EN005F-213 June 15, 2001 20:32

350 Electron Transfer Reactions

FIGURE 1 Oxidation–reduction: electron transfer. (a) Very pure zinc in very pure sulfuric acid gives no reaction
although the potential for hydrogen (H 2 ) gas evolution is very favorable. (b) When the zinc is impure or has strains
in the surface, brisk evolution of hydrogen occurs, or (c) if the zinc is connected by a wire to pure copper that is also
immersed in the sulfuric acid solution, brisk evolution of hydrogen occurs at the copper surface (2H + 2e → H 2 )
+
−
and the zinc “dissolves” (Zn → Zn 2+ + 2e ). This arrangement constitutes a primary cell, or battery, which generates
−
electric current as electrons pass from the zinc anode, where oxidation of zinc occurs, to the copper cathode, where
reduction of hydrogen ions occurs. (d) Meters inserted in the circuit will measure current flow (amperes) or voltage
(0.76 V). The connecting wire and copper cathode provide a favorable path for electron transfer from zinc atoms to
hydrogen ions.

the potential to donate electrons to hydrogen ions. In the sequences of steps involving electron transfer and forma-
circuit in Fig. 1, positive cations are repelled by the zinc tion of hydrogen molecules (H 2 ) cannot be made on the
anode, which suggests that the surface of the zinc in con- evidence provided here for the nature of a charge barrier
tact with the acid becomes positively charged, which cre- to electron transfer at a metal surface.
ates a barrier to orbital contact with positive hydrogen
ions in solution. No such barrier exists at copper, and the
conduction band in the metallic wire provides an avenue 3. Reductions of Magnesium Anodes
for electrons to travel from the zinc surface to the copper
The normal electrochemical process observed at magne-
surface. Ultrapure aluminum exhibits similar inertia when
immersed in very pure sulfuric acid, which suggests a very sium anodes is the oxidation of magnesium metal (Mg →
−
Mg 2+ + 2e ). However, organic oxidizing agents have
high potential barrier for direct transfer of electrons from
been observed to be reduced during the electrolytic oxida-
an electropositive metal to hydrogen ions.
tion of magnesium in a manner analogous to reduction by
Grignard reagents (e.g., RMgBr). Such reductions have
been detected some distance from the electrode in flow
2. Removal of the Barrier
systems and are attributed to the production of monova-
Both aluminum and zinc react directly with strongly basic lent Mg at the electrode. This ion has an extraordinarily
+
solutions to give hydrogen gas: high potential for electron transfer to form the very sta-
2+
ble Mg , yet it is repelled by the anode before it can
− −
2Al + 2OH + 6H 2 O → 2Al(OH) + 3H 2
4 donate its remaining valence electron to the electrical cir-
cuit and must find a chemical electron acceptor in the
and
solution:
−
Zn + 2OH + 2H 2 O → Zn(OH) 2− + H 2 .
4
Evidently the negative hydroxide ion (OH ) can approach
−
the positive metal surface and provide a path for electrons
to reach protons bound to water. A choice among possible