Decomposition Reaction Examples in Chemistry

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

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

Concept Recap

A chemical reaction in which a single compound breaks down into two or more simpler substances (elements or simpler compounds), typically requiring an energy input.

The reverse of synthesis โ€” taking apart a complex structure into simpler pieces.

Read the full concept explanation โ†’

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: Decomposition Reaction starts by comparing the reactant-product pattern, charges, states, and conserved atoms.

Common stuck point: Students often know a formula related to decomposition reaction but skip the recognition step: Does the balanced equation match a recognizable pattern of reactants, products, ions, oxygen, or electron transfer? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Does the balanced equation match a recognizable pattern of reactants, products, ions, oxygen, or electron transfer?

Worked Examples

Example 1

easy
Write the balanced equation for the decomposition of water by electrolysis.

Answer

2H2Oโ†’electricity2H2+O22\text{H}_2\text{O} \xrightarrow{\text{electricity}} 2\text{H}_2 + \text{O}_2

First step

1
Decomposition: one compound breaks into two or more simpler substances (ABโ†’A+B\text{AB} \rightarrow \text{A} + \text{B}).

Full solution

  1. 2
    H2Oโ†’H2+O2\text{H}_2\text{O} \rightarrow \text{H}_2 + \text{O}_2 (unbalanced).
  2. 3
    Balanced: 2H2Oโ†’2H2+O22\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2. Check: H 4=4, O 2=2. โœ“
Decomposition reactions are the reverse of synthesis reactions. Many require energy input (heat, electricity, or light) to proceed, making them typically endothermic.

Example 2

medium
When heated, calcium carbonate decomposes: CaCO3โ†’CaO+CO2\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2. Classify this reaction and explain why heat is needed.

Example 3

medium
How many grams of O2\text{O}_2 are produced when 4.90 g KClO3\text{KClO}_3 (M=122.5M=122.5) fully decomposes? (2KClO3โ†’2KCl+3O22\text{KClO}_3 \rightarrow 2\text{KCl} + 3\text{O}_2)

Example 4

medium
Explain why H2O2\text{H}_2\text{O}_2 decomposition can be classified as both a decomposition and a redox reaction.

Example 5

hard
A 1.00 g sample of NaHCO3_3 (M=84M=84) decomposes by 2NaHCO3โ†’Na2CO3+H2O+CO22\text{NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2. Find the mass of Na2_2CO3_3 (M=106M=106) produced.

Practice Problems

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

Example 1

easy
Classify: 2KClO3โ†’ฮ”2KCl+3O22\text{KClO}_3 \xrightarrow{\Delta} 2\text{KCl} + 3\text{O}_2.

Example 2

hard
Hydrogen peroxide (H2O2\text{H}_2\text{O}_2) slowly decomposes into water and oxygen gas. Write the balanced equation. Adding manganese dioxide (MnO2\text{MnO}_2) speeds up the decomposition dramatically โ€” what role does MnO2\text{MnO}_2 play, and is it consumed?

Example 3

easy
Classify: 2H2O2โ†’2H2O+O22H_2O_2 \rightarrow 2H_2O + O_2. What reaction type?

Example 4

easy
Is CaCO3โ†’CaO+CO2CaCO_3 \rightarrow CaO + CO_2 a decomposition reaction?

Example 5

easy
Balance: H2Oโ†’H2+O2H_2O \rightarrow H_2 + O_2 (electrolysis).

Example 6

easy
Which is decomposition? (a) 2KClO3โ†’2KCl+3O22KClO_3 \rightarrow 2KCl + 3O_2 (b) C+O2โ†’CO2C + O_2 \rightarrow CO_2

Example 7

easy
Most decomposition reactions require what input to proceed?

Example 8

easy
A single compound breaks into two elements. Name the reaction type.

Example 9

easy
Complete and identify: 2HgOโ†’2HgO \rightarrow ? (decomposition)

Example 10

easy
Is 2NaClโ†’2Na+Cl22NaCl \rightarrow 2Na + Cl_2 a decomposition?

Example 11

medium
Balance the decomposition: KClO3โ†’KCl+O2KClO_3 \rightarrow KCl + O_2.

Example 12

medium
Balance: Al2O3โ†’Al+O2Al_2O_3 \rightarrow Al + O_2 (electrolysis).

Example 13

medium
Classify 2NaHCO3โ†’Na2CO3+H2O+CO22NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2 and check balance.

Example 14

medium
Predict products of heating Ca(OH)2Ca(OH)_2 (decomposition into oxide and water).

Example 15

medium
Why does NaCl need electrolysis while H2O2H_2O_2 decomposes easily?

Example 16

medium
Balance: NH4NO3โ†’N2O+H2ONH_4NO_3 \rightarrow N_2O + H_2O.

Example 17

medium
Distinguish: CaCO3โ†’CaO+CO2CaCO_3 \rightarrow CaO + CO_2 vs Zn+CuSO4โ†’ZnSO4+CuZn + CuSO_4 \rightarrow ZnSO_4 + Cu.

Example 18

medium
How many mol O2O_2 form from 4 mol KClO3KClO_3 decomposing? (2KClO3โ†’2KCl+3O22KClO_3 \rightarrow 2KCl + 3O_2)

Example 19

medium
Classify 2KNO3โ†’2KNO2+O22KNO_3 \rightarrow 2KNO_2 + O_2 and verify it is balanced.

Example 20

challenge
Heating 10.0 g CaCO3CaCO_3 (M=100) fully. Mass of CO2CO_2 released? (C=12,O=16)

Example 21

challenge
A 245 g sample of KClO3KClO_3 (M=122.5) decomposes. Volume of O2O_2 at STP (22.4 L/mol)?

Example 22

challenge
Decomposing Ag2OAg_2O gives 0.040 mol Ag. How many mol O2O_2 formed? (2Ag2Oโ†’4Ag+O22Ag_2O \rightarrow 4Ag + O_2)

Example 23

easy
Classify: 2H2Oโ†’2H2+O22\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2.

Example 24

easy
Balance: HgOโ†’Hg+O2\text{HgO} \rightarrow \text{Hg} + \text{O}_2.

Example 25

easy
Is the heating of MgCO3\text{MgCO}_3 to give MgO+CO2\text{MgO} + \text{CO}_2 a decomposition?

Example 26

easy
Which energy source is used to decompose water industrially?

Example 27

easy
Complete: 2NaN3โ†’2\text{NaN}_3 \rightarrow ? (airbag reaction)

Example 28

medium
Balance: Fe(OH)3โ†’Fe2O3+H2O\text{Fe(OH)}_3 \rightarrow \text{Fe}_2\text{O}_3 + \text{H}_2\text{O}.

Example 29

medium
Predict products: thermal decomposition of Cu(OH)2\text{Cu(OH)}_2.

Example 30

medium
Balance: NH3โ†’N2+H2\text{NH}_3 \rightarrow \text{N}_2 + \text{H}_2.

Example 31

medium
How many moles of CO2_2 form from 0.50 mol CaCO3\text{CaCO}_3 decomposing?

Example 32

medium
Balance: N2O5โ†’NO2+O2\text{N}_2\text{O}_5 \rightarrow \text{NO}_2 + \text{O}_2.

Example 33

medium
A catalyst is added to a decomposition. Does it change the products or the rate?

Example 34

medium
Decomposing 1 mol Al2O3\text{Al}_2\text{O}_3 gives how many mol Al? (2Al2O3โ†’4Al+3O22\text{Al}_2\text{O}_3 \rightarrow 4\text{Al} + 3\text{O}_2)

Example 35

medium
Why must thermal decomposition of NaCl use very high temperatures or electrolysis?

Example 36

hard
24.5 g KClO3\text{KClO}_3 (M=122.5M=122.5) decomposes completely. Volume of O2_2 at STP (22.4 L/mol)?

Example 37

hard
Decomposition of 50.0 g CaCO3\text{CaCO}_3 (M=100M=100) yields how many grams of CaO (M=56M=56)?

Example 38

hard
In the decomposition 2H2O2โ†’2H2O+O22\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2, find the mass of O2_2 from 68.0 g H2_2O2_2 (M=34M=34).

Example 39

hard
Balance: Pb(NO3)2โ†’PbO+NO2+O2\text{Pb(NO}_3\text{)}_2 \rightarrow \text{PbO} + \text{NO}_2 + \text{O}_2.

Example 40

hard
Distinguish decomposition from a combustion reaction using 2H2O2โ†’2H2O+O22\text{H}_2\text{O}_2 \to 2\text{H}_2\text{O} + \text{O}_2 and CH4+2O2โ†’CO2+2H2O\text{CH}_4 + 2\text{O}_2 \to \text{CO}_2 + 2\text{H}_2\text{O}.

Example 41

challenge
NH4NO3\text{NH}_4\text{NO}_3 (M=80M=80) decomposes to N2_2O (M=44M=44) and water. If 16.0 g of NH4_4NO3_3 decomposes, what volume of N2_2O at STP forms? (22.4 L/mol)

Example 42

challenge
A mixture of KClO3_3 and KCl weighing 5.00 g is heated until decomposition is complete and the mass drops by 1.44 g (O2_2 lost, M=32M=32). What was the mass percent of KClO3_3 (M=122.5M=122.5) in the original mixture?
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Background Knowledge

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

chemical reactionsynthesis reaction