18 Composition, Decomposition, and Combustion Reactions

Learning Objectives

  1. Recognize composition, decomposition, and combustion reactions.
  2. Predict the products of a combustion reaction.

Three classifications of chemical reactions will be reviewed in this section. Predicting the products in some of them may be difficult, but the reactions are still easy to recognize.

A composition reaction (sometimes also called a combination reaction or a synthesis reaction) produces a single substance from multiple reactants. A single substance as a product is the key characteristic of the composition reaction. There may be a coefficient other than one for the substance, but if the reaction has only a single substance as a product, it can be called a composition reaction. In the reaction

2H2(g) + O2(g) → 2H2O(ℓ)

water is produced from hydrogen and oxygen. Although there are two molecules of water being produced, there is only one substance—water—as a product. So this is a composition reaction.

A decomposition reaction starts from a single substance and produces more than one substance; that is, it decomposes. One substance as a reactant and more than one substance as the products is the key characteristic of a decomposition reaction. For example, if we look at the decomposition of sodium hydrogen carbonate (also known as sodium bicarbonate):

2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(ℓ)

We can see that sodium carbonate, carbon dioxide, and water are produced from the single substance sodium hydrogen carbonate.

Composition and decomposition reactions are difficult to predict; however, they should be easy to recognize.

Example 4.9

Problems

Identify each equation as a composition reaction, a decomposition reaction, or neither.

  1. Fe2O3 + 3SO3 → Fe2(SO4)3
  2. NaCl + AgNO3 → AgCl + NaNO3
  3. (NH4)2Cr2O7 → Cr2O3 + 4H2O + N2

Solutions

  1. In this equation, two substances combine to make a single substance. This is a composition reaction.
  2. Two different substances react to make two new substances. This does not fit the definition of either a composition reaction or a decomposition reaction, so it is neither. In fact, you may recognize this as a double-replacement reaction.
  3. A single substance reacts to make multiple substances. This is a decomposition reaction.

Test Yourself

Identify the equation as a composition reaction, a decomposition reaction, or neither.

C3H8 → C3H4 + 2H2

Answer

Decomposition.

A combustion reaction occurs when a reactant combines with oxygen, many times from the atmosphere, to produce oxides of all other elements as products; any nitrogen in the reactant is converted to elemental nitrogen, N2. Many reactants, called fuels, contain mostly carbon and hydrogen atoms, reacting with oxygen to produce CO2 and H2O. For example, the balanced chemical equation for the combustion of methane, CH4, is as follows:

CH4 + 2O2 → CO2 + 2H2O

Kerosene can be approximated with the formula C12H26, and its combustion equation is

2C12H26 + 37O2 → 24CO2 + 26H2O

Sometimes fuels contain oxygen atoms, which must be counted when balancing the chemical equation. One common fuel is ethanol, C2H5OH, whose combustion equation is

C2H5OH + 3O2 → 2CO2 + 3H2O

If nitrogen is present in the original fuel, it is converted to N2, not to a nitrogen-oxygen compound. Thus, for the combustion of the fuel dinitroethylene, whose formula is C2H2N2O4, we have

2C2H2N2O4 + O2 → 4CO2 + 2H2O + 2N2

Example 4.10

Problems

Complete and balance each combustion equation.

  1. the combustion of propane, C3H8
  2. the combustion of ammonia, NH3

Solutions

  1. The products of the reaction are CO2 and H2O, so our unbalanced equation is C3H8 + O2 → CO2 + H2O. Balancing (and you may have to go back and forth a few times to balance this), we get C3H8 + 5O2 → 3CO2 + 4H2O.
  2. The nitrogen atoms in ammonia will react to make N2, while the hydrogen atoms will react with O2 to make H2O, thus NH3 + O2 → N2 + H2O. To balance this equation without fractions (which is the convention), we get 4NH3 + 3O2 → 2N2 + 6H2O.

Test Yourself

Complete and balance the combustion equation for cyclopropanol, C3H6O.

Answer

C3H6O + 4O2 → 3CO2 + 3H2O

 

Purple flowers grow tall beside a propane tank standing on grass.
Propane is a fuel used to provide heat for some homes. Propane is stored in large tanks like that shown here.

Key Takeaways

  • A composition reaction produces a single substance from multiple reactants.
  • A decomposition reaction produces multiple products from a single reactant.
  • Combustion reactions are the combination of some compound with oxygen to make oxides of the other elements as products (although nitrogen atoms react to make N2).

Exercises

Questions

  1. Which is a composition reaction and which is not?
    1. NaCl + AgNO3 → AgCl + NaNO3
    2. CaO + CO2 → CaCO3
  2. Which is a composition reaction and which is not?
    1. H2 + Cl2 → 2HCl
    2. 2HBr + Cl2 → 2HCl + Br2
  3. Which is a composition reaction and which is not?
    1. 2SO2 + O2 → 2SO3
    2. 6C + 3H2 → C6H6
  4. Which is a composition reaction and which is not?
    1. 4Na + 2C + 3O2 → 2Na2CO3
    2. Na2CO3 → Na2O + CO2
  5. Which is a decomposition reaction and which is not?
    1. HCl + NaOH → NaCl + H2O
    2. CaCO3 → CaO + CO2
  6. Which is a decomposition reaction and which is not?
    1. 3O2 → 2O3
    2. 2KClO3 → 2KCl + 3O2
  7. Which is a decomposition reaction and which is not?
    1. Na2O + CO2 → Na2CO3
    2. H2SO3 → H2O + SO2
  8. Which is a decomposition reaction and which is not?
    1. 2C7H5N3O6 → 3N2 + 5H2O + 7CO + 7C
    2. C6H12O6 + 6O2 → 6CO2 + 6H2O
  9. Which is a combustion reaction and which is not?
    1. C6H12O6 + 6O2 → 6CO2 + 6H2O
    2. 2Fe2S3 + 9O2 → 2Fe2O3 + 6SO2
  10. Which is a combustion reaction and which is not?
    1. CH4 + 2F2 → CF4 + 2H2
    2. 2H2 + O2 → 2H2O
  11. Which is a combustion reaction and which is not?
    1. P4 + 5O2 → 2P2O5
    2. 2Al2S3 + 9O2 → 2Al2O3 + 6SO2
  12. Which is a combustion reaction and which is not?
    1. C2H4 + O2 → C2H4O2
    2. C2H4 + Cl2 → C2H4Cl2
  13. Is it possible for a composition reaction to also be a combustion reaction? Give an example to support your case.
  14. Is it possible for a decomposition reaction to also be a combustion reaction? Give an example to support your case.
  15. Complete and balance each combustion equation.
    1. C4H9OH + O2 → ?
    2. CH3NO2 + O2 → ?
  16. Complete and balance each combustion equation.
    1. B2H6 + O2 → ? (The oxide of boron formed is B2O3.)
    2. Al2S3 + O2 → ? (The oxide of sulfur formed is SO2.)
    3. Al2S3 + O2 → ? (The oxide of sulfur formed is SO3.)

Answers

    1. not composition
    2. composition
    1. composition
    2. composition
    1. not decomposition
    2. decomposition
    1. not decomposition
    2. decomposition
    1. combustion
    2. combustion
    1. combustion
    2. combustion
  1. Yes; 2H2 + O2 → 2H2O (answers will vary)
    1. C4H9OH + 6O2 → 4CO2 + 5H2O
    2. 4CH3NO2 + 3O2 → 4CO2 + 6H2O + 2N2

Media Attributions

Example 4.10

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