General Chemistry: Molecular Structure, Nomenclature, and Calculations Study Guide

Molecular Structure

Lewis Structures and Bonding

Understanding molecular structure is fundamental in General Chemistry. Lewis structures are diagrams that represent the bonding between atoms and the lone pairs of electrons in a molecule.

• Lewis Structures: Visual representations showing how valence electrons are arranged among atoms in a molecule. They help predict molecular shape, reactivity, and properties.

• Single, Double, and Triple Bonds:

  • Single bond: One pair of shared electrons ($\sigma$ bond).

  • Double bond: Two pairs of shared electrons (one $\sigma$ and one $\pi$ bond).

  • Triple bond: Three pairs of shared electrons (one $\sigma$ and two $\pi$ bonds).

• Electron Transfer Diagrams: Used to show the movement of electrons, especially in ionic bonding where electrons are transferred from one atom to another.

Example: The Lewis structure of carbon dioxide (CO2) shows two double bonds between the carbon and each oxygen atom.

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is used to predict the geometry of molecules based on the repulsion between electron pairs around a central atom.

• Key Principle: Electron pairs (bonding and lone pairs) arrange themselves to minimize repulsion, determining the molecular shape.

• Common Geometries: Linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral.

Example: Methane (CH4) has a tetrahedral geometry according to VSEPR theory.

Sigma and Pi Bonds

Multiple bonds in molecules consist of sigma ($\sigma$) and pi ($\pi$) bonds.

• Sigma ($\sigma$) bond: The first bond formed between two atoms, with electron density along the axis connecting the nuclei.

• Pi ($\pi$) bond: Formed from the sideways overlap of p orbitals, present in double and triple bonds.

Example: Ethylene (C2H4) has a double bond between the two carbon atoms (one $\sigma$ and one $\pi$ bond).

Hybridization

Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals suitable for the pairing of electrons to form chemical bonds.

• sp3 hybridization: Tetrahedral geometry (e.g., CH4).

• sp2 hybridization: Trigonal planar geometry (e.g., C2H4).

• sp hybridization: Linear geometry (e.g., C2H2).

Example: In ethyne (acetylene, C2H2), each carbon is sp hybridized.

Chemical Nomenclature

Naming Compounds

Chemical nomenclature is the system for naming chemical compounds. The rules differ for ionic compounds, covalent compounds, and acids.

• Ionic Compounds: Composed of metals and nonmetals. Name the cation first, then the anion (e.g., NaCl is sodium chloride).

• Covalent Compounds: Composed of nonmetals. Use prefixes to indicate the number of atoms (e.g., CO2 is carbon dioxide).

• Acids: Compounds that release H+ ions in solution. Naming depends on the anion (e.g., HCl is hydrochloric acid, H2SO4 is sulfuric acid).

Example: The name for HNO3 is nitric acid.

Calculations

Molarity

Molarity (M) is a measure of concentration, defined as moles of solute per liter of solution.

• Formula: $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

Example: Dissolving 1 mole of NaCl in 1 liter of water gives a 1 M solution.

Dilution Calculations

Dilution involves reducing the concentration of a solution by adding more solvent.

• Formula: $M_1V_1 = M_2V_2$ where $M_1$ and $V_1$ are the initial molarity and volume, and $M_2$ and $V_2$ are the final molarity and volume.

Example: To dilute 100 mL of 2 M HCl to 1 M, add enough water to make the total volume 200 mL.

Mass and Volume from Molarity

Given molarity and volume, you can calculate the mass of solute required.

• Formula: $\text{moles} = M \times V$ $\text{mass} = \text{moles} \times \text{molar mass}$

Example: To prepare 250 mL of 0.5 M NaCl, calculate moles: $0.5 \times 0.250 = 0.125$ moles. Multiply by molar mass of NaCl to get grams needed.

Empirical Formulas

The empirical formula is the simplest whole-number ratio of atoms in a compound.

• Steps to Determine Empirical Formula:

  1. Convert mass percentages to grams (assume 100 g sample).

  2. Convert grams to moles for each element.

  3. Divide by the smallest number of moles to get the simplest ratio.

Example: A compound with 40% C, 6.7% H, and 53.3% O by mass has the empirical formula CH2O.

Additional Information

• Materials to Bring: Number 2 pencil, scientific calculator.

• Materials Provided: Periodic table.