Reduction Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Reduction.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

The gain of electrons by an atom, ion, or molecule during a chemical reaction, resulting in a decrease in its oxidation state.

Grabbing electrons. The charge gets 'reduced' (becomes more negative).

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Reduction starts by assigning oxidation and reduction, then traces electrons through the wire and ions through solution.

Common stuck point: Students often know a formula related to reduction but skip the recognition step: Am I tracking oxidation, reduction, electron flow, ions, electrodes, and whether the cell is spontaneous or driven? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I tracking oxidation, reduction, electron flow, ions, electrodes, and whether the cell is spontaneous or driven?

Worked Examples

Example 1

easy
In the reaction 2Mg+O2→2MgO2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}, identify which species is reduced.

Answer

O2 is reduced (0 → -2)\text{O}_2\text{ is reduced (0 → -2)}

First step

1
Oxygen goes from oxidation state 00 (in O2\text{O}_2) to −2-2 (in MgO).

Full solution

  1. 2
    A decrease in oxidation number means the species has gained electrons.
  2. 3
    Therefore, O2\text{O}_2 is reduced: O2+4e−→2O2−\text{O}_2 + 4e^- \rightarrow 2\text{O}^{2-}.
Reduction is the gain of electrons (RIG in OIL RIG). The substance that is reduced is the oxidizing agent because it takes electrons from the other reactant.

Example 2

medium
In a blast furnace, iron ore is reduced: Fe2O3+3CO→2Fe+3CO2\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2. Identify the reducing agent.

Example 3

medium
Write the reduction half-reaction for Cr2O72−→2Cr3+\text{Cr}_2\text{O}_7^{2-} \rightarrow 2\text{Cr}^{3+} in acidic solution.

Example 4

medium
Identify the species reduced in Fe2O3+3CO→2Fe+3CO2\text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2.

Example 5

medium
In acidic solution, write the reduction half-reaction for MnO4−→Mn2+\text{MnO}_4^- \rightarrow \text{Mn}^{2+}.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy
In the reaction Cu2++2e−→Cu\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}, is copper being oxidized or reduced?

Example 2

medium
In the reaction CuO+H2→Cu+H2O\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}, which substance is reduced?

Example 3

easy
Define reduction in terms of electrons.

Example 4

easy
Does reduction increase or decrease the oxidation state of a species?

Example 5

easy
In Cu2++2e−→Cu\text{Cu}^{2+}+2e^-\rightarrow\text{Cu}, is copper oxidized or reduced?

Example 6

easy
Is reduction always accompanied by oxidation in a chemical reaction?

Example 7

easy
The species that gets reduced acts as the oxidizing agent or the reducing agent?

Example 8

easy
OIL RIG: what does 'RIG' stand for?

Example 9

easy
In 2Na+Cl2→2NaCl2\text{Na}+\text{Cl}_2\rightarrow2\text{NaCl}, which element is reduced?

Example 10

easy
When Fe3+\text{Fe}^{3+} becomes Fe2+\text{Fe}^{2+}, is iron oxidized or reduced?

Example 11

medium
In Zn+Cu2+→Zn2++Cu\text{Zn}+\text{Cu}^{2+}\rightarrow\text{Zn}^{2+}+\text{Cu}, which species is reduced and is it the oxidizing agent?

Example 12

medium
Write the reduction half-reaction for Ag+\text{Ag}^+ forming silver metal.

Example 13

medium
In MnO4−→Mn2+\text{MnO}_4^-\rightarrow\text{Mn}^{2+}, Mn goes from +7+7 to +2+2. How many electrons are gained, and is this reduction?

Example 14

medium
In Cl2+2Br−→2Cl−+Br2\text{Cl}_2+2\text{Br}^-\rightarrow2\text{Cl}^-+\text{Br}_2, which species is reduced?

Example 15

medium
Hydrogen in H2\text{H}_2 (state 0) becomes hydride H−\text{H}^- in NaH\text{NaH}. Is hydrogen reduced? By how many electrons per atom?

Example 16

medium
In 2Mg+O2→2MgO2\text{Mg}+\text{O}_2\rightarrow2\text{MgO}, write oxygen's reduction half-reaction.

Example 17

medium
Why is the reduced species called the oxidizing agent rather than the reducing agent?

Example 18

medium
In 2Al+3Cl2→2AlCl32\text{Al}+3\text{Cl}_2\rightarrow2\text{AlCl}_3, which element is reduced and what is its oxidation-state change?

Example 19

medium
In Cu2++2e−→Cu\text{Cu}^{2+}+2e^-\rightarrow\text{Cu}, how many electrons are gained per copper, and what is the oxidation-state change?

Example 20

challenge
In Cr2O72−→Cr3+\text{Cr}_2\text{O}_7^{2-}\rightarrow\text{Cr}^{3+}, find Cr's oxidation state in the dichromate ion and the electrons gained per Cr.

Example 21

challenge
Combine Fe2+→Fe3++e−\text{Fe}^{2+}\rightarrow\text{Fe}^{3+}+e^- with Cl2+2e−→2Cl−\text{Cl}_2+2e^-\rightarrow2\text{Cl}^-. Give the balanced overall equation.

Example 22

challenge
In the cell reaction Zn+Cu2+→Zn2++Cu\text{Zn}+\text{Cu}^{2+}\rightarrow\text{Zn}^{2+}+\text{Cu}, identify the reduction half-reaction and at which electrode (anode/cathode) it occurs.

Example 23

easy
Write the reduction half-reaction for Cu2+^{2+} becoming Cu metal.

Example 24

easy
In Br2+2e−→2Br−\text{Br}_2 + 2e^- \rightarrow 2\text{Br}^-, what is the change in oxidation number of Br?

Example 25

easy
In 2H++2e−→H22\text{H}^+ + 2e^- \rightarrow \text{H}_2, is hydrogen oxidized or reduced?

Example 26

easy
In Fe3++e−→Fe2+\text{Fe}^{3+} + e^- \rightarrow \text{Fe}^{2+}, how many electrons are gained per Fe?

Example 27

medium
In Cl2+2NaBr→2NaCl+Br2\text{Cl}_2 + 2\text{NaBr} \rightarrow 2\text{NaCl} + \text{Br}_2, which element is reduced?

Example 28

medium
Write the reduction half-reaction for nitrate to NO in acidic solution: NO3−→NO\text{NO}_3^- \rightarrow \text{NO}.

Example 29

medium
In 2Al+3Cu2+→2Al3++3Cu2\text{Al} + 3\text{Cu}^{2+} \rightarrow 2\text{Al}^{3+} + 3\text{Cu}, how many electrons are gained per Cu2+^{2+}?

Example 30

medium
Write the reduction half-reaction for Cu2++e−\text{Cu}^{2+} + \text{e}^- in steps for one electron at a time: how would it actually be balanced?

Example 31

medium
In the electrolysis of molten NaCl, what is reduced at the cathode?

Example 32

medium
In Sn4++2e−→Sn2+\text{Sn}^{4+} + 2e^- \rightarrow \text{Sn}^{2+}, what is the change in oxidation number?

Example 33

medium
In thermite 2Al+Fe2O3→2Fe+Al2O32\text{Al} + \text{Fe}_2\text{O}_3 \rightarrow 2\text{Fe} + \text{Al}_2\text{O}_3, which is reduced?

Example 34

medium
In H2+F2→2HF\text{H}_2 + \text{F}_2 \rightarrow 2\text{HF}, which is reduced?

Example 35

medium
In Pb2++2e−→Pb\text{Pb}^{2+} + 2e^- \rightarrow \text{Pb}, how many moles of electrons are needed per mole of Pb2+^{2+} reduced?

Example 36

hard
Balance the reduction half-reaction ClO3−→Cl−\text{ClO}_3^- \rightarrow \text{Cl}^- in acidic solution.

Example 37

hard
How many coulombs are needed to reduce 0.10 mol of Cu2+^{2+} to Cu? (F=96,500 C/molF = 96{,}500\,\text{C/mol})

Example 38

hard
In O2+4H++4e−→2H2O\text{O}_2 + 4\text{H}^+ + 4e^- \rightarrow 2\text{H}_2\text{O}, what is the oxidation-state change of O per atom?

Example 39

hard
In 2Cu+→Cu+Cu2+2\text{Cu}^+ \rightarrow \text{Cu} + \text{Cu}^{2+}, identify which Cu+^+ is reduced.

Example 40

hard
In a daniell cell with overall Zn+Cu2+→Zn2++Cu\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}, write the half-reaction at the cathode.

Example 41

hard
In Cr2O72−→2Cr3+\text{Cr}_2\text{O}_7^{2-} \rightarrow 2\text{Cr}^{3+}, how many moles of electrons are needed per mole of dichromate?

Example 42

challenge
How many grams of Al are deposited at a cathode by 0.30 mol of electrons? (Al3++3e−→Al\text{Al}^{3+}+3e^-\rightarrow\text{Al}, M=27)
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Background Knowledge

These ideas may be useful before you work through the harder examples.

electronion