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The Aqueous Chemistry of the Elements$
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George K. Schweitzer and Lester L. Pesterfield

Print publication date: 2010

Print ISBN-13: 9780195393354

Published to Oxford Scholarship Online: November 2020

DOI: 10.1093/oso/9780195393354.001.0001

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Reactions and Applications

Reactions and Applications

(p.47) 3 Reactions and Applications
The Aqueous Chemistry of the Elements

George K. Schweitzer

Lester L. Pesterfield

Oxford University Press

E–pH diagrams involve two types of reactions: (1) Non-redox full reactions and (2) Redox half-reactions. Non-redox full reactions are exemplified by ones such as Mn(OH)2 + 2H+ → Mn+2 + 2HOH. This is not a redox (reduction–oxidation) reaction, since there are no changes in oxidation numbers of the elements. Such reactions are reflected as vertical lines on E–pH diagrams. An example of a redox half-reaction is 2e− + Mn+2 → Mn. As can be seen, this is a redox reaction, since electrons appear in the equation, and there is an oxidation number change (II to 0 for Mn). Such reactions are represented by horizontal or sloped lines in an E–pH diagram. In order to write complete reactions in which oxidation numbers change, two half-reactions must be combined. One half reaction will represent a reduction (2e− + Mn+2 → Mn) and the other will represent an oxidation (Mg → Mg+2 + 2e−). These half-reactions are combined such that the electrons cancel out and a complete redox equation is obtained (Mn+2 + Mg →Mn + Mg+2). Each of the two half-reactions has an E value, and the E value of the resulting complete redox equation is obtained by the difference in the E values of the contributing half-reactions. E–pH diagrams may be employed to predict non-redox full reactions and complete redox reactions and to ascertain E values of the latter. This will be the subject matter of the next few sections. Dashed lines in every E–pH diagram represent the E values for changes in HOH-related species (HOH, H+, H2, O2, and implicitly OH−). The upper dashed sloped line represents the reaction 4e− + 4H+ + O2 → 2HOH and is described by the equation E = 1.23−0.059 pH. The lower dashed sloped line represents the reaction 2e− + 2H+ → H2 and is described by the equation E= 0.00−0.059 pH. Figure 3.1 shows the E–pH diagram for HOH, and Figure 3.2 shows the E–pH diagram for Mg with all soluble species at 1.00 M except H+. The solid horizontal line represents the reaction 2e− + Mg+2 → Mg, and the equation for the line is E = −2.36 + 0.030 log [Mg+2].

Keywords:   Electron ladders

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