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Elements and ions differ in the ease with which they are reduced or oxidized. Reduction half-reactions for several ions and elements are listed in Table 14.3. After each equation is a number called the reduction potential (symbol, Eo), which compares the tendency for the listed reaction to occur with the tendency of aqueous hydrogen ion to be reduced:
2 H+ + 2 e- H2 E0 = 0.0 V
The reduction potential for the reduction of hydrogen ion to hydrogen has been arbitrarily assigned a value of 0.0 volts (V). A substance whose reduction potential is less than that of hydrogen ion (has a negative value) is reduced less easily than is hydrogen ion. A substance whose reduction potential is positive (is greater than that of a hydrogen ion) is more easily reduced than is hydrogen.
The reduction potentials listed were measured at 25°C and 1 atm pressure, using solutions in which one mole of reactant was dissolved in 1 L solution. At different concentrations, temperatures, or pressures, the values of the reduction potentials are slightly different.
Polyatomic ions (MnO4-, Cr2O72-, NO3-, and so on) are often involved in electron-transfer reactions. Reduction potentials for these ions are also shown in Table 14.3. Why do we show the whole ion instead of just the element? The element is not alone in solution but is part of the polyatomic ion. For example, in the permanganate ion, MnO4-, manganese has an oxidation number of +7. Manganese in this state is not an ion but part of a covalently bonded polyatomic ion. The half-reactions show only substances that actually exist in solution.Table 1. A partial list of reduction potentials at 25°C
|Oxidized form||Reduced form||E0 (volts)|
|Li+ + e-||Li||-3.05|
|K+ + e-||K||-2.93|
|Mg2+ + 2 e-||Mg||-2.38|
|Zn2+ + 2 e-||Zn||-0.76|
|Fe2+ + 2 e -||Fe||-0.44|
|2 H+ + 2 e-||H2||0.00|
|SO22- + 4 H+ + 2 e-||H2SO3 + H2O||0.20|
|Cu2+ + 2 e-||Cu||0.34|
|I2 + 2 e-||2 I-||0.54|
|Fe3+ + e-||Fe2+||0.77|
|Hg22+ + 2 e-||2 Hg||0.79|
|Ag+ + e-||Ag||0.80|
|NO3- + 4 H+ + 3 e-||NO + 2 H2O||0.96|
|Br2 + 2 e-||2 Br-||1.09|
|O2 + 4 H+ + 4 e-||2 H2O||1.23|
|Cr2O72- + 14 H+ + 6 e-||2 Cr3+ + 7 H2O||1.33|
|Cl2 + 2 e-||2 Cl-||1.36|
|MnO4- + 8 H+ + 5 e-||Mn2+ + 4 H2O||1.49|
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