Chemical Formulae and Equations

Chemical Language and Formulae

5.2.1 Chemical Formulae

• Purpose of Chemical Formulae:
    - Scientists globally communicate efficiently about chemical reactions using a common language: chemical symbols, formulae, and equations.
    - Chemical formulae serve as shorthand representations of molecules and compounds.

• Definition of Covalent Compounds:
    - Composed of non-metals chemically bonded together, typically forming small groups of atoms known as molecules.
    - Examples include water ($H_2O$) and carbon dioxide ($CO_2$).
  

• Naming Conventions:
    - The names of covalent compounds can give insights into their composition.
      - For instance:
        - Carbon monoxide has one oxygen atom attached to carbon.
        - Carbon dioxide has two oxygen atoms attached to carbon.

• Diatomic Molecules:
    - Seven specific elements always exist as diatomic molecules:
      1. Hydrogen ($H_2$)
      2. Nitrogen ($N_2$)
      3. Oxygen ($O_2$)
      4. Fluorine ($F_2$)
      5. Chlorine ($Cl_2$)
      6. Bromine ($Br_2$)
      7. Iodine ($I_2$)
    - Crucial for writing equations accurately.

• Types of Compounds:
    - Covalent Compounds: Formed from non-metallic elements.
    - Ionic Compounds: Comprise non-metallic and metallic elements.

5.2.2 Ionic Compounds

• Definition:
    - Composed of cations (positive ions) and anions (negative ions) arranged in giant lattices.

• Formation of Ions:
    - Cations form when an atom loses electrons.
    - Anions form when an atom gains electrons.
    - Common negatively charged ions often end with -ide (e.g., sulfide ($S^{2-}$), nitride ($N^{3-}$), oxide ($O^{2-}$)).

• Examples of Ionic Formulae:
    - Sodium chloride ($NaCl$)
    - Sodium hydroxide ($NaOH$)
    - Zinc nitrate ($Zn(NO_3)$)

• Writing Formulae for Ionic Compounds:
    - Typically begin with the cation, usually a metal (or sometimes ammonium), followed by the anion, generally a non-metal.
    - Example Process: Finding the formula of tin nitrate.

• Common Ionic Compounds Table:
    - | Compound | Formula |
      |-------------------------------|------------|
      | Sodium hydroxide | $NaOH$ |
      | Sodium chloride | $NaCl$ |
      | Sodium sulfate | $Na_2SO_4$ |
      | Copper(II) hydroxide | $Cu(OH)_2$ |
      

• Common Covalent Compounds Table:
    - | Compound | Formula |
      |-------------------------------|------------|
      | Water | $H_2O$ |
      | Carbon dioxide | $CO_2$ |
      | Methane | $CH_4$ |

5.2.3 Chemical Equations

• Purpose of Chemical Equations:
    - Provide a detailed representation of chemical reactions involving reactants and products.
    - More informative than word equations since they include both formulae and states.

• Differentiation from Word Equations:
    - Formulae used instead of names.
    - Physical states of chemicals often noted.
    - Numbers (coefficients) placed in front of formulae to ensure the balance of atoms on both sides.

• Steps for Writing Chemical Equations:
    1. Write the name of the compound (e.g., tin nitrate).
    2. Write the formulae of individual ions.
    3. Balance positive and negative charges (e.g., using two nitrate ions for tin's two lost electrons).
    4. Use subscripts appropriately without changing existing ones in ions.
    5. Ensure no charges remain in the final formula.

• Example of Balancing a Chemical Equation:
    - Respiration Reaction: Glucose and oxygen produce carbon dioxide and water.
    - Step-by-Step Process:
      - Step 1: Write word equation: glucose + oxygen → carbon dioxide + water.
      - Step 2: Identify chemical formulae:
        - Glucose: $C_6H_{12}O_6$
        - Oxygen: $O_2$
        - Carbon dioxide: $CO_2$
        - Water: $H_2O$.
      - Step 3: Replace names in the word equation with the chemical formulae.
      - Step 4: Balance atoms by adding coefficients:
        - For balance on carbon: add a coefficient of 6 to $CO_2$ making it $6CO_2$.
        - Repeat for hydrogen and oxygen accordingly.
      - Step 5: Include physical states in the final balanced equation:
        - $C_6H_{12}O_6(g) + 6O_2(g) → 6CO_2(g) + 6H_2O(l)$.

• Recap on Elements and States:
    - Check the physical states for each component to include:
      - Glucose is a solid ($g$ - gas), oxygen and carbon dioxide are gases ($g$), and water is liquid ($l$).

Activities and Resources

• Activity: Model chemical equations using modeling kits, demonstrating reactions (e.g., between hydrogen and oxygen to form water).

• Suggested eWorkbook Resources:
    - Chemical equations (ewbk-13127)
    - Balancing chemical equations (ewbk-13129)
    - A world of reactions (ewbk-13131)

Quick Quiz

• Questions 1 through 5 to assess understanding of the material covered.