Kaplan MCAT General Chemistry Review

A reduction potential is measured in volts (V) and defined relative to the standard hydrogen electrode (SHE), which is arbitrarily given a potential of 0.00 volts. The species in a reaction that will be oxidized or reduced can be determined from the reduction potential of each species, defined as the tendency of a species to acquire electrons and be reduced. Each species has its own intrinsic reduction potential; the more positive the potential, the greater the species’ tendency to be reduced. Standard reduction potential, (E_red^o), is measured under standard conditions: 25°C, 1 M concentration for each ion participating in the reaction, a partial pressure of 1 atm for each gas that is part of the reaction, and metals in their pure state. The relative reactivities of different half-cells can be compared to predict the direction of electron flow. A higher E_red^omeans a greater relative tendency for reduction to occur, while a lower E_red^omeans a greater relative tendency for oxidation to occur.

Key Concept

A reduction potential is exactly what it sounds like. It tells us how likely a compound is to be reduced. The higher the value, the more likely it is to be reduced.

In galvanic cells, the electrode species with the higher reduction potential is the cathode, and the electrode species with the lower reduction potential is the anode. Since the species that has a stronger tendency to gain electrons is actually gaining electrons, the redox reaction is spontaneous, and the G is negative, as we’ve seen. In electrolytic cells, the electrode species with the higher reduction potential is “forced” (by the external voltage source) to be oxidized and is, therefore, the anode. The electrode species with the lower reduction potential is “forced” to be reduced and is, therefore, the cathode. Since the movement of electrons is in the direction against the “tendency” of the respective electrochemical species, the redox reaction is nonspontaneous, and the G is positive.

Example: Given the following half-reactions and E° values, determine which species would be oxidized and which would be reduced.

Ag⁺ + e^- Ag (s) E° = + 0.8 V

Tl⁺ + e^- Tl (s) E° = -0.34 V

Solution: Ag⁺ would be reduced to Ag(s) and Tl(s) would be oxidized to Tl⁺, because Ag⁺ has the higher E°. Therefore, the reaction equation would be

Ag⁺ + Tl (s) Tl⁺ + Ag (s)

which is the sum of the two spontaneous half-reactions.

It should be noted that reduction and oxidation are opposite processes. Therefore, to obtain the oxidation potential of a given half-reaction, the reduction half-reaction and the sign of the reduction potential are both reversed. For instance, from the example above, the oxidation half-reaction and oxidation potential of Tl(s) are

Tl (s) Tl⁺ + e^-         E° = + 0.34 V

THE ELECTROMOTIVE FORCE

Standard reduction potentials are also used to calculate the standard electromotive force (emf or E°_cell) of a reaction, the difference in potential between two half-cells at standard conditions. The emf of a reaction is determined by adding the standard reduction potential of the reduced species and the standard oxidation potential of the oxidized species. When adding standard potentials, do not multiply them by the number of moles oxidized or reduced.

emf ° = E°_cath - E°_anode = E°_red + E°_ox

where E°_ox is the oxidation potential of the anode, which is the negative of the reduction potential. The standard emf of a galvanic cell is positive, while the standard emf of an electrolytic cell is negative.

Example: Given that the standard reduction potentials for Sm³⁺ and [RhCl₆]^3- are -2.41 V and +0.44 V, respectively, calculate the emf of the following reaction:

Sm³⁺ + Rh + 6 Cl^- [RhCl₆]^3- + Sm

Solution: First, determine the oxidation and reduction half-reactions. As written, the Rh is oxidized, and the Sm³⁺ is reduced. Thus, the Sm³⁺ reduction potential is used as is, while the reverse reaction for Rh, [RhCl₆]^3- Rh + 6 Cl^-, applies and the oxidation potential of [RhCl₆]^3- must be used. Then, using the equation given, the emf can be calculated to be (-2.41 V) + (-0.44 V) = -2.85 V. The cell is thus electrolytic as written. From this result, it is evident that the reaction would proceed spontaneously to the left, in which case the Sm would be oxidized while [RhCl₆]^3- would be reduced.

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