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

Molecular Structure

Lewis Structures and Bonding

Understanding molecular structure is fundamental in general chemistry. Lewis structures are used to represent the arrangement of electrons in molecules, including bonding and lone pairs.

• Lewis Structures: Diagrams that show the bonding between atoms and the lone pairs of electrons in a molecule.

• Single, Double, and Triple Bonds: Single bonds involve one shared pair of electrons, double bonds involve two, and triple bonds involve three.

• 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 for water (H2O) shows two single bonds between oxygen and hydrogen, with two lone pairs on oxygen.

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.

• Molecular Geometry: The three-dimensional arrangement of atoms in a molecule.

• VSEPR Theory: Electron pairs around a central atom arrange themselves to minimize repulsion, determining the shape of the molecule.

• Example: Methane (CH4) has a tetrahedral geometry.

Sigma and Pi Bonds

Covalent bonds can be classified as sigma (σ) or pi (π) bonds, depending on the type of orbital overlap.

• Sigma (σ) Bonds: Formed by the head-on overlap of atomic orbitals; every single bond is a sigma bond.

• Pi (π) Bonds: Formed by the side-to-side overlap of p orbitals; present in double and triple bonds in addition to a sigma bond.

• Counting Bonds: A double bond consists of one sigma and one pi bond; a triple bond consists of one sigma and two pi bonds.

• Example: Ethylene (C2H4) has a double bond between the carbons (one sigma, one pi).

Hybridization

Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals suitable for bonding.

• Types of Hybridization: sp, sp2, sp3, etc.

• Determining Hybridization: Count the number of electron domains (bonds and lone pairs) around the central atom.

• Example: Carbon in methane (CH4) is sp3 hybridized.

Ionic Bonding and Electron Transfer

Ionic bonding involves the transfer of electrons from one atom (typically a metal) to another (typically a nonmetal), resulting in the formation of ions.

• Electron Transfer Diagrams: Show how electrons move from one atom to another to form ions.

• Example: Sodium (Na) transfers one electron to chlorine (Cl) to form Na+ and Cl-.

Chemical Nomenclature

Writing Names and Formulas

Chemical nomenclature is the system for naming chemical compounds and writing their formulas. It varies depending on the type of compound.

• Ionic Compounds: Composed of cations and anions. 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: Named based on the anion present. For example, HCl (aq) is hydrochloric acid.

• Example: CaCl2 is calcium chloride (ionic); SO2 is sulfur dioxide (covalent).

Calculations

Molarity

Molarity is a measure of concentration, defined as the number of moles of solute per liter of solution.

• Formula:

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

• Example: A solution containing 0.5 moles of NaCl in 1 liter of water has a molarity of 0.5 M.

Dilution Concentrations and Volumes

When a solution is diluted, the amount of solute remains the same, but the volume increases, decreasing the concentration.

• Formula:

$M_1V_1 = M_2V_2$

• Example: To dilute 100 mL of 1.0 M solution to 0.5 M, add enough solvent to reach 200 mL total volume.

Mass and Volume from Molarity

Molarity can be used to calculate the mass of solute required for a given volume of solution.

• Formula:

$\text{mass} = M \times V \times \text{molar mass}$

• Example: To prepare 250 mL of 0.2 M NaCl, calculate the mass needed using the molar mass of NaCl.

Empirical Formulas

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

• Steps to Determine Empirical Formula:

  1. Convert mass of each element to moles.

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

  3. Multiply to get whole numbers if necessary.

• Example: A compound with 40 g C and 6.7 g H has an empirical formula of CH2.

Additional Information

• Periodic Table: You will be provided with a periodic table for reference during calculations and nomenclature.

• Calculator: A scientific calculator is recommended for performing calculations.