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3.2: Types of Chemical Reactions

  • Page ID
    51197
  • learning objectives

    • List and define the 5 types of reactions based on the rearrangement of atoms
    • Match given reactions to their respective category of reaction type
    • Relate combustion reactions of hydrocarbons to climate change
    • 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 because they have characteristic equations that allow us to use different techniques when we balance them. That is, 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 to that type reaction. There are other ways of classifying reactions that may actually tell us more about the chemistry, like are electrons transferred between atoms when a reaction occurs, which we will discuss later in this Chapter, and are called oxidation-reduction (redox) reactions, or acid base reactions (which at this level, occur when protons are transferred between between reactants).

    1. Formation Reactions (synthesis reactions)
    2. Decomposition Reactions
    3. Single Displacement Reactions
    4. Double Displacement Reaction
    5. Combustion Reactions

    The following YouTube gives an atomic scale visualization of these 5 types of Reactions

    Video \(\PageIndex{1}\): 1'36" YouTube showing atomic level animation of the 5 basic types of reactions uploaded by kosasihiskandarsjah (https://youtu.be/tE4668aarck)

    Formation or Synthesis Reactions

    H2(g) + Cl2(g) ® 2HCl(g)

    As the name implies, two or more chemicals come together and form another chemical. Here, HCl is being formed from hydrogen and chlorine.

    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

    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.

    If you note, these reactions involve a change in the charge of the metal or nonmetal, and so they involve the transfer of electrons. We will pick these up again later in this Chapter when we get to section 3.6.2 and discuss these from the perspective of electron transfer.

    Metal

    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.

       
    Figure \(\PageIndex{1}\): 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

    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.

    Metal-Nonmetal

    A metal can also displace hydrogen from an acid or even from water (look at water as H-OH, where the anion is hydroxide). For example

    2Na(s) +2H2O(l) ->H2(g) + 2NaOH

    Here the pure sodium metal becomes a cation, while a hydrogen is pulled off of each water to from hydrogen gas (a diatomic), and so hydrogen is behaving similar to a cation on the reactant side, but it is not a cation, it is an acidic hydrogen atom in a covalent bond. These reactions can give off heat which can then combust the hydrogen, as seen in Video \(\PageIndex{2}\).

    Video \(\PageIndex{2}\). 1'09" YouTube uploaded by halosaqi showing alkali metals displacing hydrogen from water.

    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 \(\PageIndex{3}\): 22 " YouTube uploaded by Bob Belford showing lead(II)nitrate reacting with sodium sulfide to form the black lead(II)sulfide and sodium nitrate (https://youtu.be/X6BV8Av_UnU).

    We will look at these in great detail in section 3.4.

    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 fuels 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 \(\PageIndex{4}\): 2'37" youtube posted by EPA (https://youtu.be/ScX29WBJI3w)

    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 -->MgO + energy

     

     

    Video \(\PageIndex{5}\): RSC video of magnesium reacting with water from Peter Wothers lecture series (https://youtu.be/gAhsvOP5yaM)

     

    Video \(\PageIndex{6}\): RSC video of magnesium reacting with sand (silicon dioxide) from Peter Wothers lecture series (https://youtu.be/J60J-g4ifXg)

     

    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 hyrdogen 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 \(\PageIndex{5}\): 4'18" YouTube Uploaded by Andrew Copeland showing basic concepts behind hydrogen economy as an alternative to the fossil fuel economy (https://youtu.be/qZhhnBfXP7I).

    Practice Problems

    Exercise \(\PageIndex{1}\)

    Identify the types of reactions for each equation

    1. AlCl3(aq) + Na2CO3(aq) ® Al2(CO3)3(s) + NaCl(aq)
    2. C8H18 + O2 ® CO2 + H2O
    3. Li2S + Br2 ® 2LiBr + S
    4. H2CO3 --> H2O + CO2
    5. Ba(C2H3O2)2(aq) + 2HNO3(aq) ® Ba(NO3)2(aq) + 2HC2H3O2(aq)
    Answer a

    Double Displacement

    Answer b

    combustion

    Answer c

    Single Displacement (non metal)

    Answer d

    Decomposition

    Answer e

    Double Displacement (involving acid)

    Exercise \(\PageIndex{21}\)

    Identify the types of reactions for each equation

    1. H2S(g) ® H2(g) + S(s)
    2. Pb + 2CuCl ® PbCl2 + Cu
    3. CH2CO(g) + 2 O2(g) ® 2 CO2(g) + H2O(g)
    4. HCl + NaOH --> NaCl + H2O
    Answer a

    Decomposition

    Answer b

    Single Displacement (metal displacement)

    Answer c

    Combustion

    Answer d

    Double Displacement (neutralization)

    Contributors and Attributions

    Robert E. Belford (University of Arkansas Little Rock; Department of Chemistry). The breadth, depth and veracity of this work is the responsibility of Robert E. Belford, rebelford@ualr.edu. You should contact him if you have any concerns. This material has both original contributions, and content built upon prior contributions of the LibreTexts Community and other resources, including but not limited to:

    • November Palmer & Emily Chaote (UALR)
    • Ronia Kattoum (Learning Goals)
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