Lecture 08_2025

Recap – Covalent Bonding

  • Electrons are shared, bonds are directional.

  • Valence Electrons: Outer shell electrons.

  • Valence of Elements (number of bonds):

    • 1: H, F, Cl, Br, I

    • 2: O

    • 3: N

    • 4: C

Polyatomic Ions

  • Groups of atoms with an overall charge:

    • NH₄⁺

    • H₃O⁺

    • OH⁻

    • NO₃⁻

    • HCO₃⁻

    • CN⁻

    • CO₃²⁻

    • SO₄²⁻

    • PO₄³⁻

Learning Outcomes

By the end of this lecture, you should be able to:

  • Understand the difference between molecular and network solids.

  • Recognize when a polar covalent bond occurs.

  • Recall and use molecular formula and empirical formula in the correct contexts.

  • Identify products and reactants in chemical equations, and balance both atoms and charges.

  • Understand that an acid supplies H⁺ ions and exists in water.

Molecular vs Network Solids

  • Covalently bonded substances may be ‘molecular’ or ‘network’ materials.

    • Example: Iodine (molecular) vs. Carbon (network)

    • The purple vapor above solid iodine is due to I₂ molecules in the gas phase.

Molecular vs Network Solids: Properties

  • Molecular Materials:

    • Soft when solid (e.g., wax)

    • Low melting points

    • Can be gases or liquids at room temperature (e.g., CO₂)

  • Network Materials:

    • Hard solids with high melting points

    • Covalent bonds broken when solid melts (e.g., SiO₂)

Types of Formula

Empirical Formula

  • Lowest ratio of atoms of different types present.

  • Used for:

    • Ionic Compounds: e.g., NaCl, K₂O

    • Network Covalent Compounds: e.g., SiO₂

    • Elements: always used except for molecular elements (H₂, N₂, O₂, etc.)

Molecular Formula

  • Actual number of atoms of each type in a molecule.

    • Example: Glucose has molecular formula C₆H₁₂O₆ (not CH₂O).

    • Implies covalent bonding:

      • Example: Ethyne C₂H₂; Benzene C₆H₆

      • Both have the same empirical formula!

Structural Formula

  • Shows connectivity of atoms.

    • Example:

      • Glucose: C₆H₁₂O₆

      • Hydrogen Peroxide: H₂O₂

Naming

  • Refer to the online module:[Inorganic Nomenclature Module](http://scilearn.sydney.edu.au/fychemistry/iChem/inorganic nomenclature.shtml)

Covalent Bond Formation

  • Wave Theory:

    • Covalent bond formation occurs when the wave functions of neighboring atoms are in-phase, leading to constructive interference and increased electron density between the atoms subject to molecular orbital formation.

Bond Types Comparison

  • Cl₂: Nonpolar Covalent Bond

  • HCl: Polar Covalent Bond

  • NaCl: Ionic Bond

Bonding Summary

  • Ionic Bonds:

    • Electrons transferred

    • Electrostatic attraction between cations and anions

    • Non-directional

  • Polar Covalent Bonds:

    • Electrons shared with unequal sharing

  • Nonpolar Covalent Bonds:

    • Electrons shared equally

Chemical Reactions

  • Reactions occur due to:

    • Products contain less energy than reactants (e.g., burning gas)

    • Energy supplied to force reactants into products of higher energy (e.g., blast furnace)

Chemical Equations

  • Word Equation: Hydrogen plus oxygen forms water

  • Symbolic Equations (unbalanced to balanced):

    • H₂ + O₂ → H₂O (not balanced)

    • 2H₂ + O₂ → 2H₂O (balanced!)

Types of Equations

  • Molecular Equation: H₂(g) + I₂(g) → 2HI(g)

  • Ionic Equations:

    • KI (s) → K⁺(aq) + I⁻(aq)

    • Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s) (precipitate)

Acids

  • The H⁺ cation is essentially a ‘bare proton’ without electrons.

  • In aqueous solutions, H⁺ associates with H₂O to form H₃O⁺ (hydronium ion).

  • Substances that provide H⁺ ions in water are called acids.

Reactions with Acids

Example 1: Metal + Acid

  • Formula Equation:

    • Zn + 2HCl → ZnCl₂ + H₂

  • Complete Ionic Equation:

    • Zn(s) + 2H⁺(aq) + 2Cl⁻(aq) → Zn²⁺(aq) + H₂(g) + 2Cl⁻(aq)

  • Net Ionic Equation:

    • Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g)

Example 2: Carbonate + Acid

  • Formula Equation:

    • CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

  • Complete Ionic Equation:

    • CaCO₃(s) + 2H⁺(aq) + 2Cl⁻(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g) + 2Cl⁻(aq)

  • Net Ionic Equation:

    • CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g)

Questions for Next Lecture

  1. What condition is necessary for a bond to be polar?

  2. Classify the bonds between the following pairs of atoms as ‘ionic’, ‘nonpolar covalent’ or ‘polar covalent’:

  • H and Br

  • O and O

  • Mg and Cl

  • I and I

  1. Balance the following chemical equations:

  • CH₄(g) + O₂(g) → CO₂(g) + H₂O(l)

  • CaCl₂ + AgNO₃ → Ca(NO₃)₂ + AgCl

  • Ag⁺(aq) + CrO₄²⁻(aq) → Ag₂CrO₄(s)

  • Ca(s) + H⁺(aq) → Ca²⁺(aq) + H₂(g)

  • Mg(OH)₂(s) + H⁺(aq) → Mg²⁺(aq) + H₂O(l)

  1. Classify the above equations as ‘molecular’, ‘formula’, or ‘net ionic’.

  2. Would it matter if you used hydrochloric acid or sulfuric acid to perform the reaction represented by equation 3d?