1.9 | Potential of Half Reactions#
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We have seen that for any redox reaction, a redox potential can be defined in relation to Gibb’s free energy of reaction
While we could tabulate the potential of every possible redox reaction, just as we saw in thermochemistry with heats of reaction, we can greatly reduce the number of reaction we need to consider by measuring potentials with respect to a standard reference electrode.
For example in the zinc-copper cell, if we allow no current to flow between the electrodes (i.e. open circuit), the potential of the zinc electrode is measured to be
We could also measure the thermodynamic redox potential of a zinc-tin cell to be 0.62 V. And we could also measure the thermodynamic redox potential of a copper-tin. However, we can simply calculate the redox potential for the copper-tin cell from the known redox potentials of the zinc-copper and zinc-tin.
By expressing these electrochemical cells balanced redox reactions, using Hess’s law, the two reactions can be combined to give the net redox reaction for the copper-tin cell.
Here, we have effectively used zinc as a reference electrode. More specifically, we have use the half reaction
Standard Hydrogen Electrode#
In practice, Zn is not used as a common reference electrode. Instead, the standard hydrogen electrode (SHE) is the most common reference electrode. Tables of the potentials of half reactions are usually given with respect to the SHE unless otherwise specified.
The SHE (Fig. 13) is a platinum electrode coated with platinum black and immersed in a solution of 1 M HCl. Hydrogen gas is bubbled through the solution at 1 atm pressure. The half reaction for the SHE electrode is:
In the standard hydrogen electrode the Pt electrode does not participate in the net reaction as either a reactant or product. Platinum only mediate the transfer of electrons to solution where hydronium ions are reduced to form hydrogen gas dissolved in solution. So long as the concentration of
Fig. 13 The net transfer of charge as electrons and ios within the galvanic zinc-copper cell with a zinc sulfate electrolyte at the anode and a copper sulfate electrolyte at the cathode. Electrons flow from the anode to the cathode through the external circuit. Negatively charged, sulfate ions (SO42–) flow from the cathode to the anode through the electrolyte.#
Determining the Voltage of an Electrochemical Cell#
The open circuit potential of a cell,
This expression is merely a shorthand of Hess’s Law that allows use to combine multiple reactions in a system to determine the net reaction. NOTE: Unlike in thermochemistry where
Applying the formula above, the voltage of the zinc-copper cell is:
In the case of the zinc half reaction,
Standard Conditions for Electrochemical Cells
The standard conditions for electrochemical cells are:
All ions are 1 M aqueous solutions.
All gases are at 1 atm pressure.
The temperature is 25°C (298 K).
Solids are pure and in their standard state.
The potential of the standard hydrogen electrode is 0.00 V. All other potentials are measured relative to the SHE.
Spontaneity of Electrochemical Cells#
The spontaneity of a redox reaction can be determined by the sign of the cell potential.
Galvanic cells are electrochemical cells in which a spontaneous redox reaction will occur. That is
Fig. 14 The electrolysis of water to produce hydrogen and oxygen gas. The electrolysis of water is a non-spontaneous redox reaction that can be driven by applying an external source of electrical potential. Protons are reduced at the cathode to form hydrogen gas while hydroxide ions are oxidized at the anode to form oxygen gas. An external voltage source is required to drive reaction and evolve the gases at the cathode and anode.#
Electrolytic cells are cells in which the redox reaction is not spontaneous redox . That is
Table of Standard Reduction Potentials#
The potentials of half reactions are usually tabulated as standard reduction potentials, Fig. 15. When doing so the reactants contain the oxidant (species being reduced by accepting electrons). The products of the half reactions in Fig. 15 are chemical reductants if these half reactions are reversed then the standard oxidation potential of these chemical reductants is simply the multiplicative inverse of the listed reduction potential.
Fig. 15 The standard reduction potentials for half reactions tabulated with respect to the SHE. Chemical oxidants are listed as reactants (species that accept electrons) and chemical reductants are listed as products (species that donate electrons). When the potentials listed from most positive to most negative from top to bottom, the oxidants (reactants) are decreasing in strength and the reductants (products) are increasing in strength.#