Who Gets Oxidized and Who Gets Reduced
At an open-air market, you see a vendor leave an iron piece out in the weather, and after a few days it is covered in rust. At another stall, a zinc nail is dropped into a blue solution and gradually becomes coated with a reddish powder. What do the two have in common? In both cases, electrons are changing owners. By the end of this page, you will be able to say who loses them, who gains them, and how to tell just by looking at the oxidation numbers.
Electrons Are Transferred
An oxidation-reduction (redox) reaction is one in which electrons are transferred from one species to another. What one loses, the other gains: electrons are neither destroyed nor created.
To keep track of them, we use the oxidation number: the charge an atom would have if all the electrons in its bonds went to the more electronegative element. If the oxidation number changes, electrons were transferred.
Why it increases when electrons are lost: electrons have a negative charge. If an atom loses one, it ends up with more positive charge (or less negative charge), so its number goes up. For example, $\text{Zn}$ (0) becomes $\text{Zn}^{2+}$ (+2): it lost 2 electrons.
How to Identify Who Is Oxidized and Who Is Reduced
The method is always the same: assign oxidation numbers to each element in the reactants and products, compare them, and see which went up and which went down.
This explains the nail: the zinc dissolves as $\text{Zn}^{2+}$ and the copper is deposited as the reddish powder.
When a zinc nail is dipped into a copper sulfate solution, the following reaction occurs: $$\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}$$ Assign…
When a zinc nail is dipped into a copper sulfate solution, the following reaction occurs:
$$\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}$$Assign the oxidation numbers of each species before and after, indicate which one is oxidized and which one is reduced, and identify the oxidizing agent and the reducing agent.
Oxidizing Agent and Reducing Agent
Each species that is oxidized or reduced has a "role" in the reaction:
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The reducing agent is the species that causes another to be reduced: it gives it electrons. That is why it is the one that is oxidized.
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The oxidizing agent is the species that causes another to be oxidized: it takes electrons from it. That is why it is the one that is reduced.
When Oxygen Is Involved: Burning Magnesium
The bright white light of some sparklers used at the posadas (traditional Christmas celebrations) comes from burning magnesium. The reaction is:
$$2\,\text{Mg} + \text{O}_2 \rightarrow 2\,\text{MgO}$$Here there are no loose ions, so we have to assign numbers using the rules.
Notice that the compound $\text{MgO}$ is made of $\text{Mg}^{2+}$ and $\text{O}^{2-}$ ions: magnesium gave its electrons to oxygen. That is why we say magnesium "was oxidized": because it lost electrons, and we now express it this way even when oxygen is not involved (as with Zn and Cu²⁺).
Magnesium ribbon burns in air with an intense white light according to the reaction $2\,\text{Mg} + \text{O}2 \rightarrow 2\,\text{MgO}$. Assign the oxidation numbers of…
Magnesium ribbon burns in air with an intense white light according to the reaction $2\,\text{Mg} + \text{O}_2 \rightarrow 2\,\text{MgO}$. Assign the oxidation numbers of all species, identify the oxidizing agent and the reducing agent, and explain why we say that the magnesium "was oxidized" even though the word oxidation does not always involve oxygen.
A Method for Any Reaction
Back to the beginning: in rust, iron loses electrons (is oxidized) and the oxygen in the air gains them (is reduced); with the zinc nail, zinc gives up electrons and copper receives them.
Summary
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A redox reaction is a transfer of electrons.
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Oxidation: loss of electrons, the oxidation number increases. Reduction: gain of electrons, the number decreases.
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They always happen together, and the electrons lost equal the electrons gained.
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The reducing agent is the one that is oxidized; the oxidizing agent is the one that is reduced.
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Oxidation does not always mean oxygen: what matters is the change in oxidation number.