Learning Objectives To classify a given chemical reaction into a variety of types. Although there are untold millions of possible chemical reactions, most can be classified into a small number of general reaction types. Classifying reactions has two purposes: it helps us to recognize similarities among them, and it enables us to predict the products of certain reactions. A particular reaction may fall into more than one of the categories that we will define in this book. Combination (composition) Reactions A combination (composition) reaction is a chemical reaction that makes a single substance from two or more reactants. There may be more than one molecule of product in the balanced chemical equation, but there is only one substance produced. For example, the equation \[4Fe + 3O_2 \rightarrow 2Fe_2O_3 \label{Eq1}\] is a combination reaction that produces Fe 2 O 3 from its constituent elements—Fe and O 2 . Combination reactions do not have to combine elements, however. The chemical equation \[Fe_2O_3 + 3SO_3 \rightarrow Fe_2(SO_4)_3 \label{Eq2}\] shows a combination reaction in which Fe 2 O 3 combines with three molecules of SO 3 to make Fe 2 (SO 4 ) 3 . Example \(\PageIndex{1}\) Which equations are combination reactions? Co(s) + Cl 2 (g) → CoCl 2 (s) CO(g) + Cl 2 (g) → COCl 2 (g) N 2 H 4 (ℓ) + O 2 (g) → N 2 (g) + 2H 2 O(ℓ) Solution This is a combination reaction. This is a combination reaction. (The compound COCl 2 is called phosgene and, in the past, was used as a gassing agent in chemical warfare.) This is not a combination reaction. Exercise \(\PageIndex{1}\) Which equations are combination reactions? P 4 (s) + 6Cl 2 (g) → 4PCl 3 (g) SO 3 (ℓ) + H 2 O(ℓ) → H 2 SO 4 (ℓ) NaOH(s) + HCl(g) → NaCl(s) + H 2 O(ℓ) Decomposition Reactions A decomposition reaction is the reverse of a combination reaction. In a decomposition reaction, a single substance is converted into two or more products. There may be more than one molecule of the reactant, but there is only one substance initially. For example, the equation \[2NaHCO_{3(s)} \rightarrow Na_2CO{3(s)} + CO_{2(g)} + H_2O_{(ℓ)} \label{Eq3}\] is a decomposition reaction that occurs when NaHCO 3 is exposed to heat. Another example is the decomposition of KClO 3 : \[2KClO_{3(s)} \rightarrow 2KCl_{(s)} + 3O_{2(g)} \label{Eq4}\] This reaction was once commonly used to generate small amounts of oxygen in the chemistry lab. The decomposition reaction of NaHCO 3 is the reaction that occurs when baking soda is poured on a small kitchen fire. The intent is that the H 2 O and CO 2 produced by the decomposition will smother the flames. Combustion Reactions A combustion reaction occurs when a substance combines with molecular oxygen to make oxygen-containing compounds of other elements in the reaction. One example is the burning of acetylene (\(C_2H_2\)) in torches: \[2C_2H_2 + 5O_2 \rightarrow 4CO_2 + 2H_2O \label{Eq5}\] Oxygen (in its elemental form) is a crucial reactant in combustion reactions, and it is also present in the products. Energy in the form of heat is usually given off as a product in a combustion reaction as well. Example \(\PageIndex{2}\) Identify each type of reaction. 2K(s) + S(s) + 2O 2 (g) → K 2 SO 4 (s) (NH 4 ) 2 Cr 2 O 7 (s) → N 2 (g) + Cr 2 O 3 (s) + 4H 2 O(ℓ) CH 4 (g) + 2O 2 (g) → CO 2 (g) + 2H 2 O(ℓ) Solution Multiple reactants are combining to make a single product, so this reaction is a combination reaction. A single substance reacts to make several products, so we have a decomposition reaction. Oxygen reacts with a compound to make carbon dioxide (an oxide of carbon) and water (an oxide of hydrogen). This is a combustion reaction. Exercise \(\PageIndex{2}\) Identify each type of reaction. C 2 H 5 OH + 3O 2 → 2CO 2 + 3H 2 O 2Ca(s) + O 2 (g) → 2CaO(s) CaCO 3 (s) → CaO(s) + CO 2 (g) Summary There are several recognizable types of chemical reactions: combination, decomposition, and combustion reactions are examples. Concept Review Exercises What is the difference between a combination reaction and a combustion reaction? Give the distinguishing characteristic(s) of a decomposition reaction. How do we recognize a combustion reaction? Answers A combination reaction produces a certain substance; a combustion reaction is a vigorous reaction, usually a combination with oxygen, that is accompanied by the production of light and/or heat. In a decomposition reaction, a single substance reacts to make multiple substances as products. A combustion reaction is typically a vigorous reaction accompanied by light and/or heat, usually because of reaction with oxygen. Exercises Identify each type of reaction. C 6 H 5 CH 3 + 9O 2 → 7CO 2 + 4H 2 O 2NaHCO 3 → Na 2 CO 3 + H 2 O + CO 2 C + 2H 2 → CH 4 Identify each type of reaction. P 4 O 10 + 6H 2 O → 4H 3 PO 4 FeO + SO 3 → FeSO 4 CaCO 3 (s) → CO 2 (g) + CaO(s) Identify each type of reaction. 2NH 4 NO 3 (s) → 2N 2 (g) + 4H 2 O(g) + O 2 (g) Hg(ℓ) + ½O 2 (g) → HgO(s) CH 2 CH 2 (g) + Br 2 (ℓ) → CH 2 BrCH 2 Br Identify each type of reaction. Ti(s) + O 2 (g) → TiO 2 (s) H 2 SO 3 (aq) → H 2 O(ℓ) + SO 2 (g) 3O 2 (g) → 2O 3 (g) Answers combustion decomposition combination decomposition combustion or combination combination