By: Dr Irannejad

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3 Decrease in the Gibbs Function as a Condition for Spontaneous Reaction

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7 Standard Gibbs Free-Energy Change for Chemical Reactions

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9 Calculation of Standard Change of Gibbs Free Energy for Chemical Reactions from Gibbs Free Energy of Formation

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11 Electrochemical Reactions, the Electrochemical Cell, and the Gibbs Free-Energy Change

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17 Interface Potential Difference and Half-Cell Potential (Ref 3, 6)

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23 The Generalized Cell Reaction

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32 The Nernst Equation: Effect of Concentration on Half-Cell Potential (Ref 3, 6)

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38 Half-Cell Reactions and Nernst-Equation Calculations

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48 Example 5: Cells with Complexing Agents.

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55 Electrochemical Cell Calculations in Relationship to Corrosion

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67  Example 7. Copper is generally considered to be corrosion resistant in nonoxidizing, deaerated acids. However, a recent publication reported measurable corrosion in HCl (m = 12,a =a =5) H+ Cl–. Consider this apparent dilemma.  First, consider that the only cathodic reaction is the evolution of hydrogen due to reduction of hydrogen ions, and show that copper should not corrode by calculating Ecell. Assume aCu2+ = 10–6 and PH2 = 1 atm. Cell reaction:

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69  Next, consider the suggestion that copper corrodes in the concentrated HCl because of the formation of a soluble chloride complex with an equilibrium constant for the reaction Cu2+ + 4Cl– = (CuCl4)2– of K = If a (CuCl4 ) 2− = 10–4, and the activity of the Cl– is that given above in the concentrated acid (aCl− = 5), calculate Ecell and determine whether corrosion will occur due to the formation of the complex ion. Cell reaction:

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71 Graphical Representation of Electrochemical Equilibrium: Pourbaix Diagrams Origin and Interpretation of Pourbaix Diagrams  The equilibrium electrochemistry of an element in aqueous solution can be represented graphically using coordinates of equilibrium half-cell potential, E′, and pH. These graphical representations, known as Pourbaix diagrams.  The objective of these diagrams is to provide a large amount of information in a convenient form for quick reference.

The coordinates are pH and electrode potential. the pH may be established by appropriate additions of an acid or base. To establish any predetermined electrode potential, the experimental arrangement shown in Fig is used. The components and their functions include: 72

 The aqueous solution of controlled pH. This solution may contain dissolved oxygen, or the container may be closed and an inert gas, such as N2 or He, bubbled through the solution to remove the oxygen present from contact with air  The working electrode, which is the electrode under study. It may be an active metal such as iron, with iron ions being exchanged between the electrode and the solution. This electrode may also be an inert metal, such as platinum, which supplies a conducting surface through which electrons pass to oxidize or reduce species in solution. 73

74  The auxiliary or counter electrode, usually platinum, against which the potential of the working electrode is established.  The reference electrode, against whose known half-cell potential the electrode potential of the working electrode is measured.

75  The electrometer or high impedance voltmeter, which is used to measure the potential of the working electrode relative to the reference electrode.  The potentiostat, which establishes the potential of the working electrode.

 In the following discussion of the Pourbaix diagram for the system iron/water (Fig. 2.11), it is convenient to consider that the potentials represented along the ordinate axis have been established by a potentiostat. 76

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82  Corrosion: In these regions of potential and pH, the iron should ultimately become virtually all ions in solution, and therefore, iron exposed at these conditions should corrode.  Passivation: In this region, the equilibrium state is one of oxide plus solution, meaningful only along a boundary such as Y in Fig. 2.16(a). These regions in Pourbaix diagrams would be more accurately identified as regions of “possible passivation.”

 The diagrams in Fig are taken from Pourbaix’s Atlas of Electrochemical Equilibria in Aqueous Solutions as representative of how regions of immunity, corrosion, and passivation can be identified. 83

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Refer to points A through E as indicated on the Pourbaix  diagram (Fig. 2.18). The state of the system at each point and the change in state when going from one point to another are to be interpreted in next page slide: 90

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