Lesson 3: 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 following categories and equations are also shown in ex. 17 in your workbook.)
The categories we will use for this lesson are decomposition, combination, single replacement (or single displacement), double displacement (or double replacement), and miscellaneous.
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. Of course, you need to learn the chemical terminology, but these simpler phrases may help you identify what seems to be happening in each of these reaction types.
Decomposition | Combination | Single Replacement
Double Replacement | Miscellaneous | Practice
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:
|
H2O |
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.
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C6H12O6 |
Still, this compound is being taken apart and this is a decomposition reaction.
By the way, none of the equations in this exercise are balanced so you should balance them as you work through the examples.
Combination
Another reaction we considered in this lesson was the combination of magnesium and oxygen to form the compound magnesium oxide which has the formula MgO:
|
Mg + O2 |
Notice that the elements are coming together in a combination reaction.
The reaction of hydrogen and oxygen to form water illustrates that combination reactions are just the opposite of decomposition reactions.
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H2 + O2 |
It is not necessary that the reactants be elements. It is possible for compounds to combine with one another or with elements in combination reactions.
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C6H12 + Br2 |
Single Replacement
Now let's look at a slightly more complicated type of reaction. In the reaction shown here.
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Zn + CuCl2 |
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.")
In the next equation chlorine is replacing bromine in the compound.
Cl2 + NaBr  NaCl + Br2
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In the next equation magnesium is replacing or displacing hydrogen in the compound.
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Mg + HCl |
Notice that, in each case, the reactants were an element and a compound and the products were an element and a compound. The element that was by itself becomes part of a compound and one of the elements that was in a compound comes out to be by itself. This is the essence of a single replacement reaction.
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 |
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.
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Fe2O3 + HCl |
Miscellaneous
There are other types of reactions besides these four categories. We will lump together all of those other types and categorize them as miscellaneous reactions. Two examples are the combustion of methane (natural gas) and propane.
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CH4 + O2 CO2 + H2O
|
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C3H8 + O2 CO2 + H2O
|
Note that these equations do not fit into any of the previous categories, even though they do have some similar features. They are somewhat like decomposition because the carbon and hydrogen are coming apart. They are somewhat like combination because the oxygen is bonding to carbon and to hydrogen. They are somewhat like single replacement because the oxygen is replacing the carbon in its bond with hydrogen and replacing the hydrogen in its bond with carbon. But despite those similarities, these reactions don't really fit into any of the other four categories.
Practice
Take a moment now to practice categorizing reactions (and balancing them) by working on the following equations (which are also shown in ex. 18 in your workbook). Note that a couple of these are word equations. You probably won't be able to balance those but you should be able to categorize them. After you have done that, check your answers below.
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
f. magnesium + zinc bromide zinc + magnesium bromide
g. sodium chloride sodium + chlorine
Answers:
Equations a and b are combination reactions, c is decomposition, d and e are double displacement, f is single replacement, and g is decomposition.
a. 4 Na + O2 2 Na2O
b. 2 Fe + 3 Br2 2 FeBr3
c. 2 CaO 2 Ca + O2
d. K2SO4 + CaCl2 2 KCl
+ CaSO4
e. Al(NO3)3 + 3 NaOH Al(OH)3 + 3 NaNO3
f. magnesium + zinc bromide zinc + magnesium bromide
( Mg + ZnBr2 Zn +
MgBr2 )
g. sodium chloride sodium + chlorine
( 2 NaCl 2 Na + Cl2 )
For additional practice try categorizing the reactions represented by the equations in example 16 in your workbook. If you have trouble with any of those, check with your instructor.