Redox reactions

Section 1. Definitions
An oxidation reaction is one that involves a gain in oxygen, gain in oxidation state, loss of hydrogen and/ or loss of electrons.

A reduction reaction is one that involves a loss in oxygen, decrease in oxidation state, gain in hydrogen and/ or gain in electrons.

An oxidizing agent is one that oxidizes another substance (and itself is being reduced).

A reducing agent is one that reduces another substance (and itself is being oxidized).

A redox reaction is one in which oxidation and reduction occurs.

Section 2. Oxidation number
The oxidation number of elements is 0.



When it exists as a compound...
  • oxygen has an oxidation number of -2 (except for hydrogen peroxide, where the oxidation state is -1)
  • hydrogen has an oxidation number of +1 (except when it exists as a hydride e.g. NaH, where its oxidation state is -1) 
  • for most compounds, the oxidation state of each element is usually the same as the charge of the ions they form.

Section 3. Oxidizing and Reducing Agents

Example 1: Acidified potassium manganate (VII) is an oxidising agent. When it oxidises another substance, it changes from purple to colourless.

Click the link here to watch the youtube video I found online. In this video, hydrogen peroxide is added to acidified potassium manganate solution. Purple potassium manganate (KMnO4) acts as a oxidising agent, and itself is being reduced to Mn2+, which is colourless. Hydrogen peroxide (H2O2) is being oxidized to form oxygen (that is why you notice a lot of bubbles being produced). Hydrogen peroxide is a reducing agent in this example, as it reduces potassium manganate to manganese(II).

Example 2: Acidified potassium dichromate (VI) is an oxidising agent. When it oxidises another substance, it changes from orange to green. 

Click the link here to watch the youtube video I found online. In this video, iron (II) sulfate solution is added to acidified potassium dichromate (VI) solution.

The orange acidified potassium dichromate (VI) (K2Cr2O7) acts as a oxidising agent, and itself is being reduced to Cr3+, which is green. Iron (II) sulfate, which is pale green, is being oxidised to iron (III) sulfate which is yellow or brown. You don't see any yellow or brown colour at the end of the reaction, as the colour intensity of Cr3+ "mask" away the yellow or brown colour of iron (III) sulfate.

Example 3: Potassium iodide is a reducing agent. When it reduces another substance, it changes from colourless to brown, as iodide is converted to iodine.

Click the link here to watch the youtube video I found online. In this video, potassium iodide and hydrogen peroxide are added to each other. Note that these two solutions are actually colourless. However, when they are added together, a brown solution is formed, due to the formation of iodine.

Hydrogen peroxide (H2O2)  acts as an oxidising agent, oxidising potassium iodide (KI) to iodine (I2). Hydrogen peroxide itself is reduced to water. As such, potassium iodide is the reducing agent here.

Example 4:
Note that hydrogen peroxide acts as a reducing agent in example 1, and as a oxidizing agent in example 3.  This depends on the type of reactant it reacts with.

Hydrogen peroxide can also undergo disproportionation. A disproportionation reaction is one in which a substance is being oxidized and reduced simultaneously. In the presence of manganese(IV) oxide (or MnO2), hydrogen peroxide is oxidized to oxygen and reduced to water simultaneously.

Click there link here to watch the youtube video I found online for the disproportionation of hydrogen peroxide to water and oxygen.

hydrogen peroxide ---> water + oxygen

2H2O2 ---> 2H2O + O2

Related Google Apps
Redox

Questions
- Quizzes and Tests on Redox Reactions

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