Name: redox potential shifts in ladder diagrams
Uploaded: 2024-04-04T10:51:15.000Z
Duration: 1 min 30 s
Channel: JoVE

Ladder diagrams representing redox equilibria use electrochemical potential, E, as their scale.
Consider the half-cell reaction between Fe3+ and an electron to yield Fe2+. At equilibrium, E equals its standard state potential, &#160;Eo of +0.771V.
The concentration of Fe3+ is higher for more positive potentials than Eo, while Fe2+ dominates at potentials more negative than Eo.
Now, consider the half-cell reaction between Sn4+ and an electron to yield Sn2+ with Eo of +0.154V. Above this point, Sn4+ dominates, whereas Sn2+ is the dominant species below this point.&#160;
Adding excess Sn2+ to the system reduces Fe3+, changing the solution potential to +0.154V.
Generally, the electrochemical potential varies with the pH of the solution.
For instance, the ladder diagram of the half-cell reaction between UO22+ and two electrons to give U4+ shows that at pH zero, E equals +0.327V.
If the pH of the solution changes, the E value changes, affecting the concentration of the predominant species.

ladder diagrams: redox equilibria

Ladder Diagrams: Redox Equilibria

Ladder diagrams are useful tools for understanding redox equilibrium reactions, especially the effects of concentration changes on the electrochemical potential of the reaction. The vertical axis in the redox ladder diagrams represents the electrochemical potential, E. The area of predominance is demarcated using the Nernst equation.
Consider the Fe3+/Fe2+ half-reaction, which has a standard-state potential of +0.771 V. At potentials more positive than +0.771 V, Fe3+ predominates, whereas Fe2+ predominates at potentials more negative than +0.771 V. When the Fe3+/Fe2+ half-reaction is coupled with the Sn4+/Sn2+ reaction, the concentration of Fe3+ can be reduced by adding Sn2+ to excess. In this case, the potential of the resulting solution approaches +0.154 V down to +0.771 V, and Fe2+ and Sn4+ predominate.
To understand the interdependence between change in solution pH and electrochemical potential, consider the example of UO22+/U4+ half-reaction, whose electrochemical potential varies with the pH of the solution. As the pH of the solution decreases, the electrochemical potential increases, changing the dominant species from U4+ to UO22+.

Ladder diagrams representing redox equilibria use electrochemical potential, E, as their scale.
Consider the half-cell reaction between Fe3+ and an ...

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Chemical Equilibria

KEY TERMS AND CONCEPTS

redox potential shifts in ladder diagrams

Ladder diagrams show redox equilibrium using electrochemical potential, E, as the vertical scale. They help students see how changes in concentration affect which species is most stable. The areas of predominance are set with the Nernst equation.

A useful example is the Fe3+/Fe2+ half-reaction, which has a standard-state potential of +0.771 V. At potentials more positive than +0.771 V, Fe3+ predominates. At potentials more negative than +0.771 V, Fe2+ predominates.

Ladder diagrams also show how one redox reaction can shift another. When the Fe3+/Fe2+ half-reaction is coupled with the Sn4+/Sn2+ reaction, adding excess Sn2+ reduces the concentration of Fe3+. In that situation, the solution potential moves from +0.154 V down to +0.771 V, and Fe2+ and Sn4+ predominate.

The same idea applies when pH changes the electrochemical potential. In the UO2 2+/U4+ half-reaction, the potential depends on the pH of the solution. As pH decreases, the electrochemical potential increases, and the dominant species changes from U4+ to UO2 2+.

Redox Potential Shifts in Ladder Diagrams

redox-potential-shifts-in-ladder-diagrams

ionic strength: overview

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Ionic strength describes the total electrolyte concentration in a solution. It is a quantitative measure of how many ions are present and how strongly they contribute to the solution. The concept was introduced in 1921 by Gilbert N. Lewis and Merle Randall while they were describing the activity coefficient of strong electrolytes. To calculate ionic strength, written as I or μ, both cations and anions are included. The concentration of each ion matters, but the charge number z matters even...

ionic strength: effects on chemical equilibria

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Chemical Potential in Solution Activity

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Activity coefficient in ionic solutions describes how much a species behaves like its measured concentration. Activity is the effective concentration of a solute in solution. It equals the molar concentration multiplied by the activity coefficient, which is a unitless number. The activity coefficient depends on the total ionic strength of the solution. It shows how far a solution is from ideal behavior. When ionic strength is very low, the activity coefficient of an ionic species is close to...

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Equilibrium Calculations with Mass and Charge

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How Ionic Strength Changes pH

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Using Ladder Diagrams to Predict pH

Ladder diagrams help show how acid-base equilibria change with pH. They are graphical tools that make the chemistry of a system easier to read during analysis. In a ladder diagram, the vertical axis shows pH. Horizontal bars, or steps, place the pKa values in the system.

Ladder diagrams show redox equilibrium using electrochemical potential, E, as the vertical scale. They help students see how changes in concentration affect which species is most stable. The areas of predominance are set with the Nernst equation. A useful example is the Fe3+/Fe2+ half-reaction, which has a standard-state potential of +0.771 V. At potentials more positive than +0.771 V, Fe3+ predominates. At potentials more negative than +0.771 V, Fe2+ predominates. Ladder diagrams also show...

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Reading Metal-Ligand Ladder Diagrams

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How Solubility Reaches Equilibrium

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Stepwise metal-ligand complexation forms coordination compounds when metal ions and ligands share electrons through dative, or coordinate covalent, bonds. In these reactions, the metal ion binds to one or more ligands until its coordination number is reached. This number tells how many bonds form between the metal ion and the ligands. In aqueous solution, many metal ions are already surrounded by water molecules. These species are called aqua complexes. Complexation reactions are described by...

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Chelating Ligands and Metal Complex Stability

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redox equilibria: overview

Redox Electron Transfer and Half-Reactions

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Direct, Back, and Displacement Titrations

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Four Reaction Types in Titrimetric Analysis

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key terms and concepts

Learn how ladder diagrams use electrochemical potential, the Nernst equation, and pH to show when Fe3+, Fe2+, UO2 2+, or U4+ predominate.

Redox Potential Shifts in Ladder Diagrams | JoVE K12

analytical chemistry

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Redox Potential Shifts in Ladder Diagrams

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