Exam 2 Study Guide: Molecules, Compounds, and Chemical Reactions

Chapter 3: Molecules and Compounds

Bond Types and Compound Classification

Understanding the types of chemical bonds and how substances are classified is fundamental in chemistry. Compounds can be categorized based on the nature of their bonding and their chemical formulas.

• Covalent Bonds: Formed when two nonmetals share electrons. Compounds with only nonmetals are typically molecular compounds.

• Ionic Bonds: Formed when electrons are transferred from a metal to a nonmetal, resulting in oppositely charged ions. Compounds containing metals and nonmetals are usually ionic compounds.

• Classification:

  • Atomic Elements: Exist in nature as single atoms (e.g., Na, He).

  • Molecular Elements: Exist as molecules with two or more atoms of the same element (e.g., O2, N2).

  • Molecular Compounds: Composed of two or more nonmetals (e.g., H2O, CO2).

  • Ionic Compounds: Composed of cations (usually metals) and anions (usually nonmetals or polyatomic ions), e.g., NaCl, Ca(NO3)2.

Naming Compounds

Chemists use systematic rules to name compounds and write their formulas.

• Binary Ionic Compounds: Name the cation (metal) first, then the anion (nonmetal with -ide ending). For transition metals, indicate the charge with Roman numerals (e.g., FeCl2: iron(II) chloride).

• Ionic Compounds with Polyatomic Ions: Name the cation, then the polyatomic ion (e.g., Na2SO4: sodium sulfate).

• Binary Molecular Compounds: Use prefixes to indicate the number of each atom (e.g., CO2: carbon dioxide).

• Molecular Acids: Acids containing hydrogen and a nonmetal (e.g., HCl: hydrochloric acid).

Formulas and Mass Calculations

Formulas represent the composition of compounds, and mass calculations are essential for quantitative chemistry.

• Molecular Mass (Formula Mass): The sum of the atomic masses of all atoms in a molecule or formula unit. $\text{Molecular Mass} = \sum (\text{number of atoms} \times \text{atomic mass})$

• Molar Mass: The mass of one mole of a substance, expressed in g/mol.

• Avogadro’s Number: $6.022 \times 10^{23}$ particles/mol. Used to convert between mass, moles, and number of molecules.

Percent Composition and Empirical/Molecular Formulas

Percent composition helps determine the empirical and molecular formulas of compounds.

• Mass Percent: The percentage by mass of each element in a compound. $\text{Mass Percent} = \frac{\text{mass of element}}{\text{mass of compound}} \times 100\%$

• Empirical Formula: The simplest whole-number ratio of atoms in a compound.

• Molecular Formula: The actual number of atoms of each element in a molecule.

• Combustion Analysis: Used to determine empirical/molecular formulas from the masses of products formed during combustion (typically CO2 and H2O).

Chapter 4: Chemical Reactions and Chemical Quantities

Balancing Chemical Equations

Chemical equations must be balanced to obey the law of conservation of mass.

• Adjust coefficients to ensure the same number of each atom on both sides of the equation.

Stoichiometry and Limiting Reactants

Stoichiometry involves quantitative relationships between reactants and products in a chemical reaction.

• Stoichiometric Calculations: Use balanced equations to convert between moles, mass, and number of particles.

• Limiting Reactant: The reactant that is completely consumed first, limiting the amount of product formed.

• Theoretical Yield: The maximum amount of product that can be formed from the limiting reactant.

• Percent Yield: $\text{Percent Yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100\%$

Types of Chemical Reactions

• Combustion Reactions: A substance reacts with O2 to form CO2 and H2O (for hydrocarbons).

• Reactions of Alkali Metals and Halogens: Alkali metals react vigorously with halogens to form ionic salts (e.g., 2Na + Cl2 → 2NaCl).

Chapter 5: Introduction to Solutions and Aqueous Reactions

Molarity and Solution Calculations

Molarity is a measure of concentration, and is used in dilution and reaction calculations.

• Molarity (M): $\text{M} = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Dilution: $M_1V_1 = M_2V_2$ (where M = molarity, V = volume)

Reactions in Aqueous Solution

• Solubility: Compounds are classified as soluble or insoluble in water.

• Electrolytes: Substances that dissociate into ions in solution (strong/weak electrolytes); nonelectrolytes do not dissociate.

• Precipitation Reactions: Occur when two aqueous solutions form an insoluble product (precipitate).

• Molecular, Complete Ionic, and Net Ionic Equations: Different ways to represent reactions in solution, focusing on the species that actually change.

• Neutralization Reactions: Acid and base react to form water and a salt; can involve strong or weak acids/bases.

• Titration: Analytical technique to determine concentration by reacting a known volume with a standard solution.

• Gas-Evolution Reactions: Reactions that produce a gas as a product (e.g., CO2, H2).

Redox Reactions

• Oxidation State: The hypothetical charge an atom would have if all bonds were ionic.

• Redox Reaction: Involves the transfer of electrons; identify oxidizing and reducing agents.

Summary Table: Key Concepts and Skills

Topic	Key Skills
Bond Types & Classification	Identify covalent/ionic bonds, classify compounds
Naming Compounds	Name/write formulas for ionic, molecular, acids
Mass Calculations	Calculate molecular/molar mass, use Avogadro’s number
Percent Composition	Calculate mass percent, empirical/molecular formulas
Balancing Equations	Balance chemical equations
Stoichiometry	Perform stoichiometric calculations, limiting reactant, yield
Solution Chemistry	Calculate molarity, dilution, classify solubility/electrolytes
Reactions in Solution	Write precipitation, neutralization, gas-evolution equations
Redox Reactions	Assign oxidation states, identify redox agents