3.2: Classifying Reactions by What Atoms Do
- Page ID
- 158416
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List and define the 5 types of reactions based on the rearrangement of atoms
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Match given reactions to their respective category of reaction type
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Relate combustion reactions of hydrocarbons to climate change
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Relate decomposition reactions to production of hydrogen fuel cells.
Some Basic Types of Chemical Reactions
We are going to identify 5 basic "types" of chemical reactions based similar patterns of rearrangement of atoms after the completion of the reaction. This allows us to use different techniques when we balance them. In the next section when we balance a reaction, the first thing we will ask is what type of reaction it is, and then we will use the technique that is appropriate for that type reaction. There are other ways of classifying reactions that may actually tell us more about the chemistry. We will discuss this alternative way of classification later in this chapter. The 5 categories of reactions based on how atoms rearrange themselves are:
- Formation Reactions (synthesis reactions)
- Decomposition Reactions
- Single Displacement Reactions
- Double Displacement Reaction
- Combustion Reactions
The following YouTube gives an atomic scale visualization of these 5 types of Reactions
Video 3.2a: 1'36" YouTube showing atomic level animation of the 5 basic types of reactions
1. Formation or Synthesis Reactions
H2(g) + Cl2(g) ® 2HCl(g)
As the name implies, two or more chemicals form another chemical, Here, HCl is being formed from hydrogen and chlorine. Typically, two smaller molecules combine to make a larger molecule.
2. Decomposition Reaction
CaCO3(s) --> CaCO(s) + CO2(g)
As the name implies, something decomposes into something else. Here Calcium Carbonate decomposes to calcium oxide and carbon dioxide. Typically, one larger molecule breaks down into smaller ones.
3. Single Displacement Reactions
Fe(s) + CuSO4(aq) --> FeSO4(aq) + Cu(s)
F2(g) + BaCl2(aq) -> BaF2(g) + Cl2(g)
In a single displacement reaction something that is "pure" displaces something in a compound. As a rule of thumb, we say metals form cations and nonmetals from anions. So a metal can exist in two forms, as a pure metal, or as a cation (in an ionic compound). Likewise, a nonmetal can exist as something pure, or as an anion (in an ionic compound). In the first of the above two equations, the metal iron displaces the cation Copper(II) and we call that a metal single displacement reaction, while in the second, the fluorine displaces the anion chloride, and so we call that a nonmetal single displacement reaction. Note, sometimes these are called single replacement reactions.
Metal Single Displacement Reactions
Fe(s) + AuSO4(aq) --> FeSO4(aq) + Au(s)
Some metals prefer to be ions, and other prefer to be pure. This can be understood by comparing iron ore with gold ore. Iron (pure) spontaneously rusts and iron ore that is mined must consists of compounds like hematite (Fe2O3) and magnetite (Fe3O4), while gold, a noble metal, does not tend to form ionic compounds, and so pure gold can be found in gold ore. So the above reaction is logical, in the sense that iron as a pure metal is not stable, while gold is.
fig.3.2a hematite ore on left (image from flickr) and gold on right (image from wikicommons).
Since the above reaction proceeds, we can correctly predict that:
FeSO4(aq) + Au(s) --> No Reaction
Nonmetal Single Displacement Reactions
F2(g) + BaCl2(aq) -> BaF2(g) + Cl2(g)
Single Displacement Reactions are actually a subset of another class (type) of reactions called Redox or Oxidation-Reduction Reactions, where electrons are transferred between species. We will cover those in section 3.6 of this chapter, and at the time come up with a way to predict if a single displacement reaction will occur, or if there is no reaction.
4. Double Displacement Reactions
PbNO3(aq) + NaS2(aq) ® PbS(s) + NaNO3(aq)
In a double displacement reaction, the ions of two ionic compounds swap counter ions.
Video 3.2b: 22 "In the above video lead(II)nitrate reacts with sodium sulfide to form the black lead(II)sulfide and sodium nitrate.
We will look at these in great detail in section 3.4.
5. Combustion Reaction
In a combustion reaction a compound reacts with oxygen and releases energy. Carbon dioxide and water are also released if the compound contains carbon.
CH4 + 2O2 --> CO2 + 2H2O + energy
Today's society uses fossil fuel as an energy source and the carbon dioxide emission is a concern to many scientists. The following youtube video was created by the EPA to describe the consequences of our economic reliance on fossil fuels. You may wish to check the link at the end of the video,
www.epa.gov/climatechange/students .
Video 3.2c: 2'37" youtube posted by EPA
Do combustion reactions always produce carbon dioxide and water?
No there are other types of combustion reactions, like the combustion of hydrogen and magnesium.
2H2 + O2 --> 2H2O + energy
2Mg + O2 --> 2MgO + energy
The hydrogen economy is considered as an alternative to fossil fuels and an area of intense current chemistry research. The concept is simple, one can use electricity generated by methods like solar, hydroelectric, wind, geothermal to split water into hydrogen and oxygen. Then you can transport the hydrogen to a new location and recombine them to produce energy where you need it, and produce no carbon dioxide. There are obvious challenges, like the storage of the hydrogen. As we shall learn later this semester, energy is the capacity to do work and transfer heat. So any energy lost as heat can not be used for work. You have probably heard of fuel cells, and they operate like a battery that consumes fuel, and can be more efficient than a combustion engine because they do not produce as much heat. In the following video the person uses a reversible fuel cell kit to demonstrate the concept of a hydrogen economy. Simply speaking, you split water into hydrogen and oxygen when you are near a power source (solar, geothermal, wind, hydroelectric) then you recombine them where you want to use the energy, and in the process, you produce no carbon dioxide.
Video 3.2d 4'18" Showing basic concepts behind hydrogen economy as an alternative to the fossil fuel economy.
Exercise \(\PageIndex{1}\)
Identify the type equations each of the following represent:
a. AlCl3(aq) + Na2CO3(aq) ® Al2(CO3)3(s) + NaCl(aq)
b. Cl2 + P ® PCl3
c. C8H18 + O2 ® CO2 + H2O
d. Li2S + Br2 ® 2LiBr + S
e. H2CO3 --> H2O + CO2
f. Ba(C2H3O2)2(aq) + 2HNO3(aq) ® Ba(NO3)2(aq) + 2HC2H3O2(aq)
g. H2S(g) ® H2(g) + S(s)
h. Pb + 2CuCl ® PbCl2 + Cu
i. CH2CO(g) + 2 O2(g) ® 2 CO2(g) + H2O(g)
j. HCl + NaOH --> NaCl + H2O
- Answer
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a. Double Displacement
b. Formation/synthesis
c. Combustion
d. Single Displacement
e. Decomposition
f. Double Displacement
g. Decomposition
h. Single Displacement
i. Combustion
j. Double Displacement
Contributors
- Bob Belford (UALR) and November Palmer (UALR)
- Modified by Ronia Kattoum (UA of Little Rock)