Redox Reactions In Depth: Oxidation Number, Oxidizing/Reducing Agents

Identify which element’s charge increased. That atom has been oxidized. The atom which has been reduced will see a decrease in its charge.

  • Oxidized element: charge increases; also called the reducing agent
  • Reduced element: charge decreases; also called the oxidizing agent
  • The element which was oxidized will act as the reducing agent for the other element.
  • The element which was reduced will act as the oxidizing agent for the other element.

Let’s try an example:

Oxid1

Element Oxidization Number BEFORE Oxidization Number AFTER
Zn 0 +2
Cu +2 0

Q: Which element was oxidized and why?
A: Zinc because its charge increased from 0 to +2.

Q: Which element was reduced and why?
A: Copper because its charge decreased from +2 to 0.

Q: What is the oxidizing agent?
A: Copper was reduced, making it an oxidizing agent (it oxidized Zn).

Q: What is the reducing agent?
A: Zinc was oxidized, making it a reducing agent (it reduced Cu).

Q: How many electrons were exchanged?
A: Two electrons (because the difference between 0 and +2 is 2).

Example:

Oxid2

Element Oxidization Number BEFORE Oxidization Number AFTER
Mg 0 +2
H +1 0
Cl -1 -1

Related Byte: Redox Reaction Basics, Activity Series

Q: Which element was oxidized and why?
A: Magnesium because its charge increased from 0 to +2.

Q: Which element was reduced and why?
A: Hydrogen because its charge decreased from +1 to 0.

Q: Which element is the spectator ion and why?
A: Chlorine because the charge did not change. It was -1 before and after the reaction.

Q: What is the oxidizing agent?
A: H was reduced, making it an oxidizing agent (it oxidized Mg).

Q: What is the reducing agent?
A: Mg was oxidized, making it a reducing agent (it reduced H).

Q: How many electrons were exchanged?
A: Two electrons (because Mg lost 2 electrons and each of the 2 H gained one electron).

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