What are OAs and RAs?

So far in this unit, we have explored oxidation numbers, half-reactions, and how to balance chemical equations in a new way.  Often it is said that in a half-reaction, the element is being oxidized or reduced; but scientists use the terms reducing agent (RA) and oxidizing agent (OA), which dates back to the early days of metallurgy.  A alchemist would say,

"to reduce a larger volume of iron (III) oxide to a smaller volume of pure iron,
a substance called a reducing agent was required, such as CO(g)."

They would say the substance is being reduced because the final metal (iron(III)) weighted less than the initial metal (iron oxide).

The chemical formula for the process of using carbon monoxide is,

In this chemical formula, the iron in iron oxide is being reduced (as the alchemists determined it was getting lighter) while the carbon in carbon monoxide is being oxidized (as it gains oxygen to become carbon dioxide). 

Using the above chemical formula, we can determine which is the OA and which is the RA based on the following definitions,

Oxidizing Agent (OA)

The substance in a redox reaction that causes reduction to happen to the species being oxidized.  Using what we know from lesson 1 and 2 in this unit, we can identify the OA as the ion gaining electrons.  In the above reaction, that would be the iron as shown using the iron (III) and iron (II) half-reactions,
Reducing Agent (RA)

The substance in the redox reaction that causes oxidization to happen in the species being oxidized. In the above reaction, we can identify the RA as the ion that loses electrons.  Since oxygen's charges balance in the reaction, and the carbon-containing molecules both sum to 0 net charge, we are forces to look at the ions that make up the molecules. In the above reaction, that would the carbon as shown using a carbon half reaction (based on what we know about the rules for oxidation numbers of oxygen),

Note: Carbon is often considered inert and will not undergo the above reaction unless forced to by another ion, such as oxygen.

Now, back then, oxygen was the only widely used oxidizing agent.  Since then scientists have discovered that other elements (i.e. halogens) can also oxidize and corrode metals.

One such example is when copper is introduced into a solution containing silver ion.  Given enough time, the copper will replace the silver ions.  Using only the ion that are involved and applying LEO and GER, the net ionic equation can be written as,

Notice that the OA arrow goes up and over and the RA arrow goes down and under.  This convention is what is commonly used and is recommended that you attempt to use this format.

Video Examples

Make sure to watch the videos below to learn about OAs and RAs in greater detail and see a few examples of typical questions you may see.  Making notes about the process to determine OAs and RAs is recommended.

If you still are not clear on what an oxidizing agents and reducing agents are, please ask your teacher.

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