Categorizing Chemical Reactions

The next topic for this lesson is categorizing chemical reactions on the basis of what happens to the elements in the reaction as represented by the chemical equation The categories we will use for this lesson are decomposition, combination, single replacement (or single displacement), double displacement (or double replacement), and combustion. If chemistry were taught in grade school these different categories of reactions would probably be called something like "take apart," "put together," "push out," and "trade partners" reactions, plus combustion in its own special category. Of course, you need to learn the chemical terminology, but these simpler phrases may help you remember what happens in each of these core reaction types.

Decomposition

One of the reactions you observed in this lesson was the decomposition of water to form hydrogen and oxygen. Look at this equation and note what happens to the elements involved in this reaction:

2 H₂O → 2 H₂ + O₂

Water is a compound containing the elements hydrogen and oxygen bonded together in a way that we will study soon. In this reaction those elements are taken apart from one another. When you look at the chemical formulas in an equation and see that a compound is being taken apart, you are looking at a decomposition reaction.

The products of a decomposition reaction are not necessarily going to be elements. This equation shows the decomposition of sugar (C₆H₁₂O₆) into carbon and water.

C₆H₁₂O₆ → 6 C + 6 H₂O

The defining feature of a decomposition reaction is that it has only one reactant, always a compound, which forms multiple products (elements, compounds, or both).

Combination

The reverse of a decomposition reaction is a combination reaction. These are reactions where two (or more, but usually just two) reactants come together to form a single product. They are literally the reverse of decompositions, in that if you flip the products and reactants around the arrow, you get a combination.

2 H₂ + O₂ → 2 H₂O

As before, it is not necessary that the reactants be elements. A combination reaction of water and sulfur trioxide, two compounds, is shown below.

H₂O + SO₃ → H₂SO₄

Combination reactions also go by a couple other names; you may hear them referred to as "composition" or "synthesis" reactions. These are all terms for the same idea.

Single Replacement

Let's look at the reaction of zinc and copper (II) chloride below.

Zn + CuCl₂ → Cu + ZnCl₂

The zinc is replacing the copper in the compound. This is called a single replacement or single displacement reaction. (Either name is fine; however, do not shorten it to "single" - you must have either "replacement" or "displacement" with "single.")

The key idea this time is that a compound containing some element "A" is reacting with another element "B" with the result being that A is pushed out of the compound and B is pushed in it.

The reaction of Cl₂ with NaBr is another illustration of this type of reaction.

2 NaBr + Cl₂ → 2 NaCl + Br₂

Double Replacement

Our next type of reaction involves two replacements so they are called double replacement or double displacement reactions. (Again, either name is fine, but don't shorten it to just "double.")

AgNO₃ + HCl → AgCl + HNO₃

In this equation the silver from the first compound displaces the hydrogen in the second compound, while at the same time that hydrogen is taking the place of the silver in the first compound. This could just as easily be viewed as the NO₃ in the first compound and the Cl in the second compound displacing one another. So make note that what is being displaced need not be a simple element.

In the next equation you should be able to see that the iron and hydrogen trade places, or the oxygen and chlorine trade places.

Fe₂O₃ + 6 HCl → 2 FeCl₃ + 3 H₂O

The core idea of double replacement is that, instead of an element reacting with a compound, the reaction is between two compounds, and two elements are exchanged between them.

Combustion

Combustion reactions are a very specific breed of reaction. They always involve a hydrocarbon, meaning a compound of C and H (though O may also be present in the hydrocarbon). That compound always reacts with oxygen gas (its formula is O₂) to form two and only two products: CO₂ and H₂O.

2 C₂H₆ + 7 O₂ → 4 CO₂ + 6 H₂O

This highly consistent pattern makes combustion reactions fairly easy to spot, although they can sometimes be confused with single-replacement or double-replacement reactions.

Practice

Take a moment now to practice categorizing reactions (and balancing them) by working on the following equations. After you have done that, check your answers below. The blanks are to remind you to balance the equations.

Practice Categorizing (and Balancing) Equations:

a. _ Na + _ O₂ → _ Na₂O

b. _ Fe + _ Br₂ → _ FeBr₃

c. _ CaO → _ Ca + _ O₂

d. _ K₂SO₄ + _ CaCl₂ → _ KCl + _ CaSO₄

e. _ Al(NO₃)₃ + _ NaOH → _ Al(OH)₃ + _ NaNO₃

Answers:

a. 2 Na + O₂ → Na₂O combination

b. 2 Fe + 3 Br₂ → 2 FeBr₃ combination

c. 2 CaO → 2 Ca + O₂ decomposition

d. K₂SO₄ + CaCl₂ → 2 KCl + CaSO₄ double replacement

e. Al(NO₃)₃ + 3 NaOH → Al(OH)₃ + 3 NaNO₃ double replacement