The activity series lists elements in their order of strength as oxidizers or reducers. This is what determines if a reaction will or will not occur.
Example:
| Element | Oxidization Number BEFORE | Oxidization Number AFTER |
| Mg | 0 | +2 |
| Cl | -1 | -1 |
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: 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: Does this reaction make sense according to the activity series?
A: Yes; Since Mg is listed above H in the activity series, it makes a stronger reducer. Therefore, it makes sense that Mg would be the reducing agent.
Related Byte: Redox Reaction Basics; How To Determine Oxidation Number, Oxidizing and Reducing Agents
The Activity Series:
Example:
| Element | Oxidization Number BEFORE | Oxidization Number AFTER |
| Cu | 0 | +2 |
| Cl | -1 | -1 |
Q: Which element was oxidized and why?
A: Copper 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: What is the oxidizing agent?
A: H was reduced, making it an oxidizing agent (it oxidized Cu).
Q: What is the reducing agent?
A: Cu was oxidized, making it a reducing agent (it reduced H).
Q: Does this reaction make sense according to the activity series?
A: No; Since Cu is listed below H in the activity series, it means that hydrogen is actually the stronger reducer. Therefore, Cu could not possibly reduce H and this reaction would not take place.
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