Redox Equilibria: Variable Oxidation States, Electrode Potentials, Electrochemical Series

Edited by Jamie (ScienceAid Editor), Taylor (ScienceAid Editor), vcdanht

Ad

Variable Oxidation States

On of the properties of transition metals is their variable oxidation states. This is crucial for looking at their reactions; the rules for working out the oxidation number is exactly the same in transition metals. The oxidation state of a metal can change in redox reactions. We represent this change of individual elements in half equations. Have a look at the example below.

Was this helpful? Yes | No| I need help
half equations

When combining half equations; once you have them both (one for reduction and one for oxidation), you must make the number of electrons in each the same, then it is a simple matter of combining the two.

Was this helpful? Yes | No| I need help
how to combine two half equations

Electrode Potentials

When a zinc wire is placed in a solution of zinc ions, an equilibrium of electrons leaving and joining zinc is set up. Zn (s) ==>> Zn2+ + 2e- By convention however, half equations are written as reductions, therefore the above equilibrium will be reversed. This also means that any enthalpy change will also be reversed.

Was this helpful? Yes | No| I need help

If the equilibrium is more to the right and the zinc dissolves, a negative charge will build up. This arrangement is called a half cell, and if we could measure the charge, it would indicate how readily electrons are released and hence how good a reducing agent the metal is. This is the electrode potential.

Was this helpful? Yes | No| I need help

It is possible however to form a circuit and measure the potential difference using a voltmeter. In order to do this, two half cells are made and connected via a salt bridge. A simple salt bridge would be filter paper soaked in salt solution. A wire cannot be used to complete the circuit because that would create another half cell.

Was this helpful? Yes | No| I need help
diagram of two half cells connected by a salt bridge

Below the diagram is written the standard representation of cells. The single lines represent a change in phase and the double lines (sometimes dashed) represent the salt bridge. It can also be written if the oxidation state of a species changes, in this case a comma separates them. Or, if a platinum electrode is used because the particular element does not exist has a solid (as is the case with the standard hydrogen electrode) it will be written as below.

Was this helpful? Yes | No| I need help

Pt | H2 (aq) | 2H+ (aq) || ...

In order to compare the electrode potential of different metals, it is important to use a standard cell to connect to other half cells to. For this, we use the hydrogen electrode.

the hydrogen electrode

Hydrogen is bubbled in to the solution and the black platinum wire is used to conduct electricity to the voltmeter. The potential of the hydrogen electrode is defined as zero, so when it is connected to another half cell the voltage, known as the electromotive force (e.m.f or E) can be measured - this is the electrode potential. However, the hydrogen electrode is difficult to use so it is much easier to use a secondary standard such as the calomel electrode or silver/silver chloride. These are calibrate against the hydrogen electrode. To calculate the standard E (Eθ) the second cell must be in standard conditions too: ions at 1 moldm-3, 298K.

Was this helpful? Yes | No| I need help

Electrochemical Series

A more positive Eθ means the metal is a strong oxidizing agent (a substance oxidizing another and itself reduced), and negative are strong reducing agents.

A list of electrode potentials is called an electrochemical series and these can be used to predict the direction of a redox reaction. It is possible to calculate the feasibility of a redox reaction using their E values the calculations involved are outlined below.

Was this helpful? Yes | No| I need help

calculating the feasibility of a reaction using the e values

Referencing this Article

If you need to reference this article in your work, you can copy-paste the following depending on your required format:

APA (American Psychological Association)
Redox Equilibria: Variable Oxidation States, Electrode Potentials, Electrochemical Series. (2017). In ScienceAid. Retrieved Jul 24, 2017, from https://scienceaid.net/chemistry/physical/redeq.html

Was this helpful? Yes | No| I need help

MLA (Modern Language Association) "Redox Equilibria: Variable Oxidation States, Electrode Potentials, Electrochemical Series." ScienceAid, scienceaid.net/chemistry/physical/redeq.html Accessed 24 Jul 2017.

Was this helpful? Yes | No| I need help

Chicago / Turabian ScienceAid.net. "Redox Equilibria: Variable Oxidation States, Electrode Potentials, Electrochemical Series." Accessed Jul 24, 2017. https://scienceaid.net/chemistry/physical/redeq.html.

Was this helpful? Yes | No| I need help

If you have problems with any of the steps in this article, please ask a question for more help, or post in the comments section below.

Comments

ScienceAid welcomes all comments. If you do not want to be anonymous, register or log in. It is free.

Article Info

Categories : Physical

Recent edits by: Taylor (ScienceAid Editor), Jamie (ScienceAid Editor)

Share this Article:

Thanks to all authors for creating a page that has been read 125 times.

Do you have a question not answered in this article?
Click here to ask one of the writers of this article
x

Thank Our Volunteer Authors.

Would you like to give back to the community by fixing a spelling mistake? Yes | No