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Balancing Redox Reactions: Oxidation Number Change Method

Explains how to balance redox reactions by observing the change in oxidation state.

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Credit: Asim18
Source: http://commons.wikimedia.org/wiki/File:02_-_Single_Energizer_Battery.jpg
License: CC BY-NC 3.0

When you hear the term “battery”, what do you think of? Most of us probably do not call to mind our car batteries since we don’t normally see them. More likely, we think of the small AA or AAA batteries that power flashlights, digital cameras, and other similar devices. We can buy these batteries by the package at the local grocery store, hardware store, or drug store. Most of us keep a bunch of batteries around “just in case”. Some powerful (sorry!) chemistry in such a small package.

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  • We can divide batteries into dry cell and wet call categories. The car battery is perhaps the best-known example of a wet cell battery. This battery contains sulfuric acid as the electrolyte in the system which provides the acid conditions for the redox reactions and ions for charge balance. Another type of wet cell battery is the nickel-cadmium battery. This battery is an alkaline battery since it uses potassium hydroxide as the electrolyte. The nickel and cadmium transfer electrons from cadmium to nickel during discharge.
  • Credit: Sean MacEntee
    Source: http://www.flickr.com/photos/smemon/8150852640/
    License: CC BY-NC 3.0

    A typical car battery with a positive and a negative terminal. One of the terminals is covered by the red cap to prevent accidents [Figure2]

  • Many dry cell batteries are composed of a central graphite rod in a mixture of ammonium chloride and manganese dioxide. The ammonium ion can donate a proton to the system, so this battery would be functioning under acid conditions. Zinc metal surrounds the mixture and serves to donate electrons to both the ammonium and manganese components of the system.
  • The alkaline battery has a similar construction, but uses potassium hydroxide as the electrolyte. Similar chemistry occurs with metallic zinc being oxidized and the manganese being reduced. Alkaline batteries in general have greater power output and longer shelf lives than other dry cell batteries.
  • Watch a video about batteries at the link below:


Show What You Know

Use the links below to learn more about redox reactions in basic solutions. Then answer the following questions.

  1. What are the two poles in a wet cell battery?
  2. What are typical poles for a lithium battery?
  3. In an alkaline battery, what is the oxidation number of the Zn before and after the reaction?
  4. What is the oxidation number for the manganese in the same system before and after the reaction?
  5. Which battery performs better in extreme cold?

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    Image Attributions

    1. [1]^ Credit: Asim18; Source: http://commons.wikimedia.org/wiki/File:02_-_Single_Energizer_Battery.jpg; License: CC BY-NC 3.0
    2. [2]^ Credit: Sean MacEntee; Source: http://www.flickr.com/photos/smemon/8150852640/; License: CC BY-NC 3.0

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