Types of Chemical Reactions

Types of Chemical Reactions

Overview

  • Chemical reactions can be classified into categories to predict their products.
  • Five basic types of reactions:
    • Synthesis
    • Decomposition
    • Single-displacement
    • Double-displacement
    • Combustion reactions

Synthesis Reactions

  • Also known as composition reactions, two or more substances combine to form a new compound.
  • General equation: A + X \rightarrow AX
    • A and X can be elements or compounds.
    • AX is a compound.

Reactions of Elements with Oxygen and Sulfur

  • Combination of an element with oxygen produces an oxide.
  • Almost all metals react with oxygen to form oxides.
    • Example: 2Mg(s) + O_2(g) \rightarrow 2MgO(s)
  • Group 1 metals form oxides with the formula M_2O.
    • Example: Li_2O
  • Group 2 elements react similarly, forming oxides with the formula MO, where M represents the metal.
  • Group 1 and Group 2 elements react with sulfur, forming sulfides with the formulas M_2S and MS, respectively.
    • Example: 8Ba(s) + S_8(s) \rightarrow 8BaS(s)
    • Example: 16Rb(s) + S8(s) \rightarrow 8Rb2S(s)
  • Nonmetals also undergo synthesis reactions with oxygen to form oxides.
    • Example: S8(s) + 8O2(g) \rightarrow 8SO_2(g)
    • Example: 2H2(g) + O2(g) \rightarrow 2H_2O(g)

Reactions of Metals with Halogens

  • Most metals react with Group 17 elements (halogens) to form ionic or covalent compounds.
  • Group 1 metals react with halogens to form ionic compounds with the formula MX, where M is the metal and X is the halogen.
    • Example: 2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)
  • Group 2 metals react with halogens to form ionic compounds with the formula MX_2.
    • Example: Mg(s) + F2(g) \rightarrow MgF2(s)
  • Fluorine is highly reactive and combines with almost all metals.

Synthesis Reactions with Oxides

  • Active metals (highly reactive metals) oxides react with water to produce metal hydroxides.
    • Example: CaO(s) + H2O(l) \rightarrow Ca(OH)2(s)
  • Many oxides of nonmetals in the upper right portion of the periodic table react with water to produce oxyacids.
    • Example: SO2(g) + H2O(l) \rightarrow H2SO3(aq)
  • Certain metal oxides and nonmetal oxides react to form salts.
    • Example: CaO(s) + SO2(g) \rightarrow CaSO3(s)

Decomposition Reactions

  • A single compound undergoes a reaction that produces two or more simpler substances.
  • Opposite of synthesis reactions.
  • General equation: AX \rightarrow A + X
    • AX is a compound.
    • A and X can be elements or compounds.

Decomposition of Binary Compounds

  • Decomposition of a substance by an electric current is called electrolysis.
    • Example: 2H2O(l) \xrightarrow{\text{electricity}} 2H2(g) + O_2(g)
  • Oxides of less-active metals (lower center of the periodic table) decompose into their elements when heated.

Decomposition of Metal Carbonates

  • Example: CaCO3(s) \xrightarrow{\Delta} CaO(s) + CO2(g)

Decomposition of Metal Hydroxides

  • Example: Ca(OH)2(s) \xrightarrow{\Delta} CaO(s) + H2O(g)

Decomposition of Metal Chlorates

  • Example: 2KClO3(s) \xrightarrow[MnO2]{\Delta} 2KCl(s) + 3O_2(g)

Decomposition of Acids

  • Certain acids decompose into nonmetal oxides and water.
  • Example: Carbonic acid is unstable and decomposes readily at room temperature to produce carbon dioxide and water.
    • H2CO3(aq) \rightarrow CO2(g) + H2O(l)

Single-Displacement Reactions

  • Also known as replacement reactions, one element replaces a similar element in a compound.
  • Many take place in aqueous solution.
  • General equations: A + BX \rightarrow AX + B or Y + BX \rightarrow BY + X
    • A, B, X, and Y are elements.
    • AX, BX, and BY are compounds.

Displacement of a Metal in a Compound by Another Metal

  • Example: Aluminum is more active than lead.
    • 2Al(s) + 3Pb(NO3)2(aq) \rightarrow 3Pb(s) + 2Al(NO3)3(aq)

Displacement of Hydrogen in Water by a Metal

  • Most-active metals (Group 1) react vigorously with water to produce metal hydroxides and hydrogen.
    • Example: 2Na(s) + 2H2O(l) \rightarrow 2NaOH(aq) + H2(g)
  • Less-active metals (iron) react with steam to form a metal oxide and hydrogen gas.
    • Example: 3Fe(s) + 4H2O(g) \rightarrow Fe3O4(s) + 4H2(g)

Displacement of Hydrogen in an Acid by a Metal

  • More-active metals react with acidic solutions (hydrochloric acid and dilute sulfuric acid), replacing hydrogen in the acid.
  • Reaction products are a metal compound (a salt) and hydrogen gas.
    • Example: Mg(s) + 2HCl(aq) \rightarrow H2(g) + MgCl2(aq)

Displacement of Halogens

  • Fluorine is the most-active halogen; can replace any other halogen in their compounds.
  • In Group 17, each element can replace any element below it, but not above it.
    • Example: F2(g) + 2NaCl(aq) \rightarrow 2NaF(aq) + Cl2(g)
    • Example: Cl2(g) + 2KBr(aq) \rightarrow 2KCl(aq) + Br2(l)
    • Example: Br_2(l) + KCl(aq) \rightarrow \text{no reaction}

Double-Displacement Reactions

  • Ions of two compounds exchange places in an aqueous solution to form two new compounds.
  • One compound formed is usually a precipitate, an insoluble gas that bubbles out of the solution, or a molecular compound (usually water).
  • The other compound is often soluble and remains dissolved in solution.
  • General equation: AX + BY \rightarrow AY + BX
    • A, X, B, and Y in the reactants represent ions.
    • AY and BX represent ionic or molecular compounds.

Formation of a Precipitate

  • Occurs when cations of one reactant combine with the anions of another reactant to form an insoluble or slightly soluble compound.
    • Example: 2KI(aq) + Pb(NO3)2(aq) \rightarrow PbI2(s) + 2KNO3(aq)
  • The precipitate forms due to strong attractive forces between the Pb^{2+} cations and the I^− anions.

Formation of a Gas

  • Example: FeS(s) + 2HCl(aq) \rightarrow H2S(g) + FeCl2(aq)

Formation of Water

  • Example: HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)

Combustion Reactions

  • A substance combines with oxygen, releasing a large amount of energy in the form of light and heat.
  • Example (combustion of hydrogen): 2H2(g) + O2(g) \rightarrow 2H_2O(g)
  • Example (combustion of propane): C3H8(g) + 5O2(g) \rightarrow 3CO2(g) + 4H_2O(g)

Identifying Reactions and Predicting Products

  1. Is there only one reactant?
    • If yes, you have a decomposition reaction.
      • A binary compound generally breaks into its elements.
      • A ternary compound breaks according to guidelines.
    • If no, go to step 2.
  2. Are the reactants two elements or two simple compounds?
    • If yes, you probably have a synthesis reaction.
      • If both reactants are elements, the product is a binary compound. Use expected charges to predict the formula.
      • If the reactants are compounds, the product will be a single ternary compound.
    • If no, go to step 3.
  3. Are the reactants oxygen and a hydrocarbon?
    • If yes, you have a combustion reaction.
      • The products are carbon dioxide and water.
    • If no, go to step 4.
  4. Are the reactants an element and a compound other than a hydrocarbon?
    • If yes, you probably have a displacement reaction.
      • Use the activity series to determine the activities of the elements.
      • If the more active element is already part of the compound, no reaction will occur.
      • Otherwise, the more active element will displace the less active element from the compound.
    • If no, go to step 5.
  5. Are the reactants two compounds composed of ions?
    • If yes, you probably have a double-displacement reaction.
      • Write formulas for the possible products by forming two new compounds from the ions available.
      • Determine if one of the possible products is a solid precipitate, a gas, or a molecular compound, such as water. If neither product qualifies, no reaction occurs.
    • If no, go back to step 1.