Chapter 11 Electron Transfer and Electrochemistry
c) Will A reduce B to B
2-;
i.e.
, is A + B
→
A
2+ + B
2- spontaneous?
Example 11.1c
Example 11.1d
A
B
2+A
2-B
Reactants
Products
Reactants
Products
A
2B
2+
4+A
2B
The empty orbital on B is lower in energy
than the high-energy electrons of A (green
circle), so the electrons transfer spontaneously
from A to B. B is a strong enough oxidant
to oxidize A to A
2+. Alternatively, A is a strong enough reductant to reduce B to B
2-.
d) Will B
2+ oxidize A to A
4+;
i.e
., is A + 2B
2+^
→
A
4+ + 2B spontaneous?
Both valence orbitals are empty in the B
2+ ion, and they are lower in energy than all of the
valence electrons on A, so all four
electrons transfer spontaneously. One B
2+ would
accept the high-energy pair (A + B
2+^
→
A
2+ + B), and a second B
2+ would accept the low-
energy pair (A
2+ + B
2+^
→
A
2+ + B). Thus, B
2+ is a strong enough oxidizing agent to oxidize
A to A
4+.
The number of electrons gained during the
reduction, the number of electrons lost
during the oxidation, and the number of electr
ons transferred during the redox reaction are
all the same number, n. For example, the reaction between Cu
2+ and Fe involves a gain of
two electrons by Cu
2+, a loss of two electrons by Fe, and is a two-electron transfer from Fe
to Cu
2+;
i.e.
, n = 2 electrons. To determine the number of electrons transferred,
- Identify the oxidant and reductant. 2. Determine the oxidation st
ate changes of the atoms invo
lved in the oxidation and
reduction.
- Multiply the oxidation state change of the atoms involved in
the oxidation or reduction by
the number of atoms involved in the proc
ess to obtain n, the number of electrons
transferred.
Redox reactions often involve ions that are not involved in the electron transfer, and
the species that are involved can be polyatomic ions or molecules with several elements, so the identities of the oxidant and reductant
are not always obvious. Although, the change
in oxidation number is attributed to a single atom in the molecule or ion, the entire ion or molecule is considered to be the oxidizing
or reducing agent. The following guidelines can
be helpful in identifying the atoms whose oxidation states* are changing:
* Oxidation states were discussed
in Section 4.4.A review of that
material would help your understanding here.
a. Atoms appearing in elements are
always being oxidized or reduced.
b. O and H often appear as H
O, OH 2
1-, and/or H
1+, but they are not involved in the electron
transfer unless they are elemental (O
or H 2
), hydride (H 2
1-), or peroxide (O
2- 2
).
c. Most transition elements can exist in severa
l oxidation states, so they are involved in
many redox reactions.
d. The central atoms of polyatomic ions fr
equently undergo oxidation or reduction.