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 reactionA chemical reaction in which a single substance is produced from multiple reactants. (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 reactionA chemical reaction in which a single substance becomes more than one substance. 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, in the decomposition of sodium hydrogen carbonate (also known as sodium bicarbonate),
2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(ℓ)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.
Identify each equation as a composition reaction, a decomposition reaction, or neither.
Solution
Test Yourself
Identify the equation as a composition reaction, a decomposition reaction, or neither.
C3H8 → C3H4 + 2H2Answer
decomposition
A combustion reactionA chemical reaction in which a reactant combines with oxygen to produce oxides of all other elements as products. 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 + 2H2OKerosene can be approximated with the formula C12H26, and its combustion equation is
2C12H26 + 37O2 → 24CO2 + 26H2OSometimes 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 + 3H2OIf 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 + 2N2Complete and balance each combustion equation.
Solution
The products of the reaction are CO2 and H2O, so our unbalanced equation is
C3H8 + O2 → CO2 + H2OBalancing (and you may have to go back and forth a few times to balance this), we get
C3H8 + 5O2 → 3CO2 + 4H2OThe nitrogen atoms in ammonia will react to make N2, while the hydrogen atoms will react with O2 to make H2O:
NH3 + O2 → N2 + H2OTo balance this equation without fractions (which is the convention), we get
4NH3 + 3O2 → 2N2 + 6H2OTest Yourself
Complete and balance the combustion equation for cyclopropanol, C3H6O.
Answer
C3H6O + 4O2 → 3CO2 + 3H2O
Which is a composition reaction and which is not?
Which is a composition reaction and which is not?
Which is a composition reaction and which is not?
Which is a composition reaction and which is not?
Which is a decomposition reaction and which is not?
Which is a decomposition reaction and which is not?
Which is a decomposition reaction and which is not?
Which is a decomposition reaction and which is not?
Which is a combustion reaction and which is not?
Which is a combustion reaction and which is not?
Which is a combustion reaction and which is not?
Which is a combustion reaction and which is not?
Is it possible for a composition reaction to also be a combustion reaction? Give an example to support your case.
Is it possible for a decomposition reaction to also be a combustion reaction? Give an example to support your case.
Complete and balance each combustion equation.
Complete and balance each combustion equation.
Yes; 2H2 + O2 → 2H2O (answers will vary)