General Chemistry/Redox Reactions/Electrochemistry
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[edit] Redox Reactions (review)
Redox (shorthand for reduction/oxidation reaction) describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed.
This can be either a simple redox process such as the oxidation of carbon to yield carbon dioxide, or the reduction of carbon by hydrogen to yield methane (CH4), or it can be a complex process such as the oxidation of sugar in the human body through a series of very complex electron transfer processes.
The term redox comes from the two concepts of reduction and oxidation. It can be explained in simple terms:
- Oxidation describes the loss of electrons by a molecule, atom, or ion
- Reduction describes the gain of electrons by a molecule, atom, or ion
However, these descriptions (though sufficient for many purposes) are not truly correct. Oxidation and reduction properly refer to a change in oxidation number—the actual transfer of electrons may never occur. Thus, oxidation is better defined as an increase in oxidation number, and reduction as a decrease in oxidation number. In practice, the transfer of electrons will always cause a change in oxidation number, but there are many reactions which are classed as "redox" even though no electron transfer occurs (such as those involving covalent bonds).
[edit] Electrochemistry
Electrochemical reactions occur when an external voltage is applied, or they generate a voltage themselves. The reactions that occur in an electric battery are electrochemical reactions.
- Three components of an electrochemical reaction
- A solution where redox reactions may occur (solutions are substances dissolved in liquid, usually water)
- A conductor for electrons to be transferred (such as a metal wire)
- A conductor for ions to be transferred (usually a salt bridge)
Most electrochemical reactions that produce voltage occur in two separate cells. One cell is home to the reduction half-reaction, and the other is home to the oxidation half-reaction. The cells are connected by a salt bridge that allows the ions to move from one cell to the other, so that the reaction can continue. They are also connected by a wire that lets the electrons flow from one cell to the other. If an electrical component, like a light bulb, is attached to the wire, it will receive power from the flowing electrons.
[edit] Voltage
Voltage is a measure of spontaneity of redox reactions, and it can be measured by a voltmeter. If the voltage of a reaction is positive, the reaction occurs spontaneously.
To compute the voltage of a redox equation, split the equation into its oxidation component and reduction component. Then, look up the voltages of each component on a standard electrode potential table. This table will list the voltage for the reduction equation. The oxidation reaction's voltage is negative of the corresponding reduction equation's voltage. To find the equation's voltage, add the standard voltages for each half reaction.
