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4.20: Classifying Chemical Reactions: Additional Examples

  • Page ID
    217481
  • Learning Objectives
    • Classify a chemical reaction as a combination, a decomposition, a single replacement, a double replacement, or a combustion reaction.
    • Classify a half-reaction as an oxidation reaction or a reduction reaction.
    Example \(\PageIndex{1}\)

    Classify each of the following chemical reactions as a combination, a decomposition, a single replacement, a double replacement, or a combustion reaction.

    1. \(\ce{2 C_1_2H_2_6} \left( l \right) + \ce{37 O_2} \left( g \right) \rightarrow \ce{24 CO_2} \left( g \right) + \ce{26 H_2O} \left( g \right) + \ce{E}\)
    2. \(\ce{HC_2H_3O_2} \left( g \right) \rightarrow \ce{CH_4} \left( g \right) + \ce{CO_2} \left( g \right)\)
    3. \(\ce{Mg(OH)_2} \left( aq \right) + \ce{(NH_4)_2SO_4} \left( aq \right) \rightarrow \ce{2 NH_4OH} \left( aq \right) + \ce{MgSO_4} \left( s \right)\)
    4. \(\ce{2 Cr} \left( s \right) + \ce{3 Br_2} \left( g \right) \rightarrow \ce{2 CrBr_3} \left( s \right)\)
    5. \(\ce{F_2} \left( g \right) + \ce{2 LiBr} \left( aq \right) \rightarrow \ce{2 LiF} \left( aq \right) + \ce{Br_2} \left( l \right)\)

    Solutions

    1. This reaction is classified as a combustion because energy, "E," is generated as a product.  Furthermore, a combustion reaction requires that molecular oxygen, O2, participate in the reaction as a reactant and that carbon dioxide, CO2, and water, H2O, be generated as products.  All of these chemicals are present in the given equation.
    2. This reaction is classified as a decomposition because only a single molecule, hydrogen acetate, HC2H3O2, is present on the left side of the reaction arrow.  Note that this reactant contains an acetate ion, C2H3O2-1, which is a polyatomic anion.  These multi-atom ions generally bond and react as indivisible units.  However, the acetate ion is not present in either of the products that are generated during the course of this reaction.  Because a decomposition reaction requires that a reactant molecule be divided into simpler substances, polyatomic ions are unable to remain unified in this type of reaction.
    3. This reaction is classified as a double replacement because the cationic components of two compounds, the magnesium ion, Mg+2, and the ammonium ion, NH4+1, respectively, exchange positions.
    4. This reaction is classified as a combination because only a single compound, chromium (III) bromide, CrBr3, is present on the right side of the reaction arrow.  
    5. This reaction is classified as a single replacement because an elemental reactant, molecular fluorine, F2, and the non-metallic portion of a compound, bromine, Br, exchange positions.
    Exercise \(\PageIndex{1}\)

    Classify each of the following half-reactions as an oxidation reaction or a reduction reaction.

    1. \(\ce{Cl_2} + \ce{2 e^{−}} \rightarrow \ce{2 Cl^{−1}}\)
    2. \(\ce{Zn} \rightarrow \ce{Zn^{+2}} + \ce{2 e^{−}}\)
    Answer a
    Two electrons, e, are written on the left side of the reaction arrow, indicating that these electrons are inserted into, or gained by, molecular chlorine, Cl2.  Therefore, this reaction is classified as a reduction.
    Answer b
    Two electrons, e, are written on the right side of the reaction arrow, indicating that these electrons are released by, or lost from, zinc, Zn.  Therefore, this reaction is classified as an oxidation.
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