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General Chemistry Principles, Patterns, and Applications, 2011

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phenomenon of electrolysis <strong>and</strong> laid the foundations of electrochemistry. In fact, most of the specialized<br />

terms introduced in this chapter (electrode, anode, cathode, <strong>and</strong> so forth) are due to Faraday. In addition,<br />

he discovered benzene <strong>and</strong> invented the system of oxidation state numbers that we use today. Faraday is<br />

probably best known for “The Chemical History of a C<strong>and</strong>le,” a series of public lectures on the chemistry<br />

<strong>and</strong> physics of flames.<br />

The maximum amount of work that can be produced by an electrochemical cell (wmax) is equal to the<br />

product of the cell potential (Ecell) <strong>and</strong> the total charge transferred during the reaction (nF):<br />

Equation 19.50<br />

w max = nFE cell<br />

Work is expressed as a negative number because work is being done by a system (an electrochemical cell<br />

with a positive potential) on its surroundings.<br />

As you learned in Chapter 18 "Chemical Thermodynamics", the change in free energy (ΔG) is also a<br />

measure of the maximum amount of work that can be performed during a chemical process (ΔG = wmax).<br />

Consequently, there must be a relationship between the potential of an electrochemical cell <strong>and</strong> ΔG, the<br />

most important thermodynamic quantity discussed in Chapter 18 "Chemical Thermodynamics". This<br />

relationship is as follows:<br />

Equation 19.51<br />

ΔG = −nFE cell<br />

A spontaneous redox reaction is therefore characterized by a negative value of ΔG <strong>and</strong> apositive value<br />

of Ecell, consistent with our earlier discussions. When both reactants <strong>and</strong> products are in their st<strong>and</strong>ard<br />

states, the relationship between ΔG° <strong>and</strong> E°cell is as follows:<br />

Equation 19.52<br />

ΔG° = −nFE° cell<br />

Note the Pattern<br />

A spontaneous redox reaction is characterized by a negative value of ΔG°, which corresponds to a positive<br />

value of E°cell.<br />

E X A M P L E 7<br />

Suppose you want to prepare elemental bromine from bromide using the dichromate ion as an oxidant.<br />

Using the data in Table 19.2 "St<strong>and</strong>ard Potentials for Selected Reduction Half-Reactions at 25°C", calculate<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

1756

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