8.2 Classifying Composition, Decomposition, and Combustion Reactions
8.2 Classifying Composition, Decomposition, and Combustion Reactions
Learning Objectives
By the end of this section, you will be able to:
- Identify composition, decomposition, and combustion reactions.
- Predict the products of a combustion reaction.
Three classifications of chemical reactions will be reviewed in this section (composition, decomposition and combustion), while two additional chemical reactions (single and double displacement) will be explored in the proceeding section. Predicting the products in some of them may be difficult, but the reactions are still easy to recognize.
Watch Type of Chemical Reaction (5 mins)
Video source: Ysci. (2017, December 7). Types of chemical reactions [Video]. YouTube.
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 8.2a
Problems
Identify each equation as a composition reaction, a decomposition reaction, or neither.
- Fe2O3 + 3SO3 → Fe2(SO4)3
- NaCl + AgNO3 → AgCl + NaNO3
- (NH4)2Cr2O7 → Cr2O3 + 4H2O + N2
Solutions
- In this equation, two substances combine to make a single substance. This is a composition reaction.
- 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.
- A single substance reacts to make multiple substances. This is a decomposition reaction.
Exercise 8.2a
Identify the equation as a composition reaction, a decomposition reaction, or neither.
C3H8 → C3H4 + 2H2
Check Your Answer[1]
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,