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:Oxidation 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 oxidation 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→2MgO, identify which species is oxidized.Which species is oxidized in this reaction?
Answer
Mg is oxidized (0 → +2)
First step
1
Mg goes from oxidation state 0 (element) to +2 (in MgO).
Full solution
2
An increase in oxidation number means the species has lost electrons.
3
Therefore, Mg is oxidized: Mg→Mg2++2e−.
Oxidation is the loss of electrons (OIL RIG: Oxidation Is Loss). The substance that is oxidized serves as the reducing agent because it provides electrons to the other reactant.
Example 2
medium
In the reaction Zn+Cu2+→Zn2++Cu, write the oxidation and reduction half-reactions.
Example 3
medium
In Fe+CuSO4→FeSO4+Cu, write the oxidation half-reaction.
Example 4
medium
Show how a galvanic cell with Zn anode and Cu cathode uses oxidation.
Example 5
hard
Balance the oxidation of I− to IO3− in acidic solution.
Practice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
easy
Is iron rusting an example of oxidation? Explain.
Example 2
medium
In the reaction CuO+H2→Cu+H2O, which substance is oxidized?
Example 3
easy
Define oxidation in terms of electrons.
Example 4
easy
Does oxidation increase or decrease the oxidation state of a species?
Example 5
easy
In Zn→Zn2++2e−, is zinc oxidized or reduced?
Example 6
easy
Is oxidation always accompanied by reduction in a chemical reaction?
Example 7
easy
The species that gets oxidized acts as the oxidizing agent or the reducing agent?
Example 8
easy
What is the oxidation state of an element in its pure form, like Fe metal?
Example 9
easy
Originally 'oxidation' meant gaining oxygen. Is the modern definition broader or narrower?
Example 10
easy
When magnesium burns to form MgO, Mg goes from 0 to +2. Is it oxidized?
Example 11
medium
In 2Na+Cl2→2NaCl, identify which element is oxidized.Identify which element is oxidized.
Example 12
medium
In Zn+Cu2+→Zn2++Cu, which species is oxidized and is it the reducing agent?
Example 13
medium
Write the oxidation half-reaction for iron forming Fe3+.
Example 14
medium
Carbon in CH4 (−4) becomes CO2 (+4). How many electrons does each carbon lose?
Example 15
medium
Why is burning hydrogen (2H2+O2→2H2O) an oxidation of hydrogen?
Example 16
medium
In 2Mg+O2→2MgO, write Mg's oxidation half-reaction.
Example 17
medium
In Cl2+2Br−→2Cl−+Br2, which species is oxidized?
Example 18
medium
In Fe2+→Fe3++e−, is iron oxidized, and how many electrons are lost?
Example 19
medium
In 2Al+3Cl2→2AlCl3, which element is oxidized and what is its oxidation-state change?
Example 20
challenge
In MnO4−→Mn2+, Mn changes oxidation state. Determine whether Mn is oxidized or reduced and the electrons involved.
Example 21
challenge
Balance the electrons: Al→Al3++3e− and Cu2++2e−→Cu are combined. Find the whole-number coefficients.Find the whole-number coefficients that balance the combined half-reactions.
Example 22
challenge
In the combustion C+O2→CO2, identify the oxidized element, the reduced element, and the oxidizing agent.Identify the oxidized element, reduced element, and oxidizing agent.
Example 23
easy
When Mg becomes Mg2+, how many electrons does each magnesium atom lose?
Example 24
easy
Iron rusting: 4Fe+3O2→2Fe2O3. What is iron's change in oxidation number?What is iron's change in oxidation number in this reaction?
Example 25
easy
True or false: oxidation must always involve oxygen gas.
Example 26
easy
Write the oxidation half-reaction for K becoming K+.
Example 27
easy
Ca→Ca2++2e−. Is this oxidation or reduction?
Example 28
medium
In Sn+2HCl→SnCl2+H2, identify the oxidized species.
Example 29
medium
In CH4+2O2→CO2+2H2O, identify the element that is oxidized.
Example 30
medium
Cu→Cu2++2e− has E∘=−0.34 V. What does the negative value mean for spontaneity as written?
Example 31
medium
Write the oxidation half-reaction for sulfide ion forming elemental sulfur.
Example 32
medium
In 2H2S+O2→2S+2H2O, which atoms are oxidized?Which atoms are oxidized in this reaction?
Example 33
medium
In ethanol combustion C2H5OH+3O2→2CO2+3H2O, is carbon oxidized? Justify with oxidation numbers.
Example 34
medium
Which is the stronger reducing agent under standard conditions, Zn or Cu? Why?
Example 35
medium
In 2Ag++Cu→2Ag+Cu2+, write the oxidation half-reaction and the number of electrons transferred per formula unit.
Example 36
medium
Is the bleaching of a stain by hydrogen peroxide a kind of oxidation?
Example 37
medium
In Sn2+→Sn4++2e−, identify oxidation and the change in oxidation number.
Example 38
hard
Balance the oxidation half-reaction Mn2+→MnO4− in acidic solution.
Example 39
hard
In 5C2O42−+2MnO4−+16H+→10CO2+2Mn2++8H2O, identify the species oxidized and the electrons lost per oxalate.
Example 40
hard
Combine Al→Al3++3e− with Fe3++3e−→Fe to balance the redox.
Example 41
hard
Combine Mg→Mg2++2e− and Al3++3e−→Al into a balanced overall equation.
Example 42
challenge
Balance the oxidation of ethanol to acetic acid in acidic solution: C2H5OH→CH3COOH.