Chemical Reactions Study Guide

Chemical Reactions

9.1 Types of Chemical Reactions

What is a Chemical Reaction?

• Definition: A process whereby one or more substances (reactants) are converted into one or more different substances (products). -

• Key Points:
    - Reactants: The substances that are converted.
    - Products: The new substances that are formed.
    - Substances can be either elements or compounds.

Five Major Types of Chemical Reactions

1. Combination Reaction
      - Definition: Two or more substances combine to form a single product.
      - Example:
        $A + B <br>ightarrow AB$
      - Additional information regarding the state of each chemical is often included in the equation (gas, liquid, solid, solution).

2. Decomposition Reaction
      - Definition: A single compound decomposes into two or more elements or compounds.
      - Example:
        - Water decomposes into hydrogen and oxygen:
          $H_2O <br>ightarrow H_2 + O_2$
        - Calcium carbonate decomposes into calcium oxide and carbon dioxide:
          $CaCO_{3} <br>ightarrow CaO + CO_2$
        - Sulfurous acid decomposes into water and sulfur dioxide:
          $H_2SO_3 <br>ightarrow H_2O + SO_2$
        - Ammonium hydroxide decomposes into water and ammonia:
          $NH_4OH <br>ightarrow H_2O + NH_3$

3. Displacement Reaction
      - Definition: An atom (or a molecule) displaces an atom (or a group of atoms) in another compound.
      - Examples:
        - Example 1: Fe displaces Cu from CuSO4 to form FeSO4
          $Fe(s) + CuSO_4(aq) <br>ightarrow Cu(s) + FeSO_4(aq)$
        - Example 2: Cl2 displaces I from NiI2 to form NiCl2
          $Cl_2(aq) + NiI_2(aq) <br>ightarrow I_2(aq) + NiCl_2(aq)$

4. Exchange Reaction
      - Definition: Two substances exchange parts with one another to form two new substances.
      - Examples:
        - Sodium chloride and silver nitrate exchange ions to form sodium nitrate and silver chloride:
          $NaCl + AgNO_3 <br>ightarrow NaNO_3 + AgCl$
        - Hydrochloric acid and sodium hydroxide exchange ions to form sodium chloride and water:
          $HCl + NaOH <br>ightarrow NaCl + H_2O$

5. Combustion Reaction
      - Definition: A chemical reaction between a substance and oxygen that produces heat and light (flame).
      - Examples:
        - Methane reacts with oxygen to form carbon dioxide and water:
          $CH_4 + 2O_2 <br>ightarrow CO_2 + 2H_2O$
        - Hydrogen reacts with oxygen to form water:
          $2H_2 + O_2 <br>ightarrow 2H_2O$
        - Octane reacts with oxygen to form carbon dioxide and water:
          $C_8H_{18} + 12.5O_2 <br>ightarrow 8CO_2 + 9H_2O$

9.2 Redox and Non-Redox Chemical Reactions

Definition of Redox Reaction

• A redox (reduction-oxidation) reaction involves a transfer of electrons from one reactant to another.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Mnemonic: "OIL-RIG" (Oxidation Is Loss, Reduction Is Gain).

Identifying Redox Reactions

1. Look for changes in oxidation numbers of elements involved; a change indicates a redox reaction.

2. If there is no change, it is a non-redox reaction.

3. Example: The decomposition of calcium carbonate is a non-redox reaction because oxidation numbers do not change.

Common Redox Reactions in Everyday Life

1. Photosynthesis:
      $6CO_2 + 6H_2O + ext{light} <br>ightarrow C_6H_{12}O_6 + 6O_2$
      - CO2 is reduced to glucose while water is oxidized to oxygen.
    

2. Respiration:
      $C_6H_{12}O_6 + 6O_2 <br>ightarrow 6CO_2 + 6H_2O + ext{energy}$
      - Glucose is oxidized to carbon dioxide while oxygen is reduced to water.

3. Combustion:
      $C_3H_8 + 5O_2 <br>ightarrow 4H_2O + 3CO_2 + ext{energy}$
      - Propane is oxidized to carbon dioxide while oxygen is reduced to water.

4. Rusting:
      $Fe + O_2 + H_2O <br>ightarrow Fe_2O_3 \bullet x H_2O$
      - Iron is oxidized to ferric oxide hydrate while oxygen is reduced.

Redox Terminology

• Oxidizing Agent: A substance that accepts electrons and gets reduced.

• Reducing Agent: A substance that donates electrons and gets oxidized.

9.3 Terminology Associated with Redox Processes

Oxidation Numbers

• Definition: The charge an atom appears to have when electrons are assigned to the more electronegative atom in a bond.

• Rules for Assigning Oxidation Numbers:
    1. In a pure element or homoatomic compound, the oxidation number is 0.
    2. In monatomic ions, the oxidation number equals the charge of the ion.
    3. In heteroatomic ionic compounds, the oxidation number of a monatomic ion equals its charge.
    4. In covalent compounds, the more electronegative element is assigned a negative oxidation number based on its valency.
    5. The sum of oxidation numbers must equal the charge on the polyatomic ion or zero for neutral compounds.

Example of Assigning Oxidation Numbers

• Example: In $KH_2SO_4$:
    - K = +1
    - H = +1
    - S = +6
    - O = -2 (4 atoms)
    - Total must resolve to 0.

Practice Assigning Oxidation Numbers

• Assign oxidation numbers to K in $KBr$, and H in $H_2O$.

• Example: For $KBr$:
    - K = +1, Br = -1

• Example: For $H_2O$:
    - H = +1, O = -2

Summary of Chemical Reactions

• Combination Reactions: Single product from multiple reactants.

• Decomposition Reactions: Multiple products from a single reactant.

• Displacement Reactions: An atom or molecule replaces atoms in a compound.

• Exchange Reactions: Two substances switch parts to form different products.

• Combustion Reactions: Substance reacts with oxygen producing heat and light.