Compounds: Properties, Formulas & Models

Compounds and Their Properties

• Compounds possess unique properties, differing from their constituent elements.
• The Law of Constant Composition (Proust's Law) states that all samples of a given compound share the same elemental composition.
• Water (H₂O) has a consistent mass ratio of 8:1 of oxygen to hydrogen.
• Ammonia (NH₃) contains a 4.67:1 mass ratio of nitrogen to hydrogen.

Chemical Formulas

• Chemical formulas indicate the elements present and their relative numbers.
• Empirical formulas show the simplest ratio of elements (e.g., HO for hydrogen peroxide).
• Molecular formulas show the actual number of atoms in a molecule (e.g., H₂O₂ for hydrogen peroxide).
• Structural formulas show the connectivity between atoms in a molecule.

Molecular Models

• Ball-and-Stick models represent atoms as spheres and bonds as sticks.
• Space-Filling Models show the actual size and shape of a molecule.

Classifying Elements and Compounds

• Atomic elements exist as single atoms (e.g., Na, Fe, He).
• Molecular elements exist as diatomic molecules (e.g., O₂, N₂, Cl₂).
• Molecular compounds are composed of two or more nonmetals (e.g., CO₂, H₂O).
• Ionic compounds are composed of metals and nonmetals.
• Example: NaCl (table salt) consists of Na⁺ and Cl⁻.

Ionic and Molecular Compounds

• Type I metals have a fixed charge (e.g., NaCl = Sodium chloride).
• Type II metals have a variable charge, indicated by Roman numerals (e.g., FeCl₃ = Iron(III) chloride).
• Polyatomic ions are groups of atoms that act as a unit (e.g., NO₃⁻ = Nitrate, SO₄²⁻ = Sulfate).
• Prefixes indicate the number of atoms in molecular compounds (e.g., CO₂ = Carbon dioxide, N₂O = Dinitrogen monoxide).

Acids

• Binary acids start with hydro- and end in -ic acid (e.g., HCl = Hydrochloric acid).
• Oxyacids containing polyatomic ions change -ate to -ic acid and -ite to -ous acid (e.g., H₂SO₄ = Sulfuric acid, H₂SO₃ = Sulfurous acid).

Writing Formulas for Compounds

• The total charge must equal zero when writing ionic formulas.
• Charges are balanced using subscripts (e.g., Al³⁺ and O²⁻ → Al₂O₃).
• Parentheses are used when multiple polyatomic ions are present (e.g., Mg(NO₃)₂).

Formula Mass

• Formula mass calculation includes the sum of atomic masses of all atoms in the chemical formula.
• Example: H₂O = (2 × 1.008) + (1 × 16.00) = 18.016 g/mol.

The Mole and Avogadro's Number

• 1 mole = 6.022 × 10^23 units of a substance.
• The mole allows converting between atomic-scale quantities and measurable mass.
• Avogadro's number applies to atoms, molecules, ions, or particles.

Molar Mass and Atomic Mass

• Atomic Mass Unit (amu): 1/12th the mass of a carbon-12 atom.
• Molar mass is the mass of 1 mole of an element (in grams) and equals its atomic mass in amu.
• Example: 1 mol of carbon = 12.01 g, 1 mol of oxygen = 16.00 g.

Converting Between Moles, Mass, and Number of Atoms

• To convert from mass to moles, divide by molar mass.
• To convert moles to mass, multiply by molar mass.
• Number of Moles to Atoms/Molecules conversion: Multiply by Avogadro's number.
• Number of Atoms/Molecules to Moles conversion: Divide by Avogadro's number.

Chemical Formulas as Conversion Factors

• Chemical formulas indicate the relative number of each type of atom.
• Example: CO₂ → 1 C atom per 2 O atoms
• Subscripts can be used as ratios in calculations (e.g., 1 mol CO₂ contains 2 mol O atoms).

Mass Percent Composition

• The percentage of each element's mass in a compound is the element's proportion relative to the compound.
• Equation: Mass % of Element = (mass of element / total mass of compound) X 100
• Calculation is used in NaCl

Molar Mass and Mass Percent Calculations

• The molar mass of Na is 22.99 g/mol.
• The molar mass of Cl is 35.45 g/mol.
• The molar mass of NaCl is 22.99 + 35.45 = 58.44 g/mol.
• The mass % of Na in NaCl = (22.99 g / 58.55 g) X 100 = 39.34%
• The mass % of Cl in NaCl = (35.45 g / 58.55 g) X 100 = 60.66%
• Mass percent is used as a conversion factor to determine the mass of an element in a given mass of a compound
• For example, if NaCl is 39.34% Na, then 100 g of NaCl contains 39.34 g of Na.

Empirical and Molecular Formulas

• The empirical formula represents the simplest whole-number ratio of elements in a compound
• The molecular formula represents the actual number of atoms in a molecule
• Find empirical formula: convert to moles, divide by the lowest number of moles, and multiply a whole number if needed.

Calculation of Molecules

• To determine the molecular formula: Molecular Formula = Empirical Formula × n, where n = (molar mass of molecular formula) / (molar mass of empirical formula)

Evidence of a Chemical Reaction

• Observable changes that suggest a chemical reaction include color change, precipitate formation, gas formation, emission of light, heat emission, and heat absorption.
• Physical changes don't always indicate a chemical reaction.
• Reactants are on the left side of a reaction and products appear on the right

Matter, Equations, and Balancing

• The state of matter abbrevations are: (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous
• Balance equations with the same number of each type of atom on both sides
• Adjust coefficients but not subscripts

Types of Chemical Reactions

• Precipitation reactions occur when an insoluble solid (precipitate) is formed in solution
• Acid-base (neutralization) reactions combine an acid + a base which creates salt and water
• Gas evolution reactions produce a gas as a product

Redox, Combustion, Synthesis, Decomposition Reactions

• Oxidation-reduction (redox) reactions transfer electrons
• Oxidation: losing electrons, Reduction: Gaining Electrons
• OIL RIG (Oxidation Is Loss, Reduction Is Gain).
• Combustion reactions happen when a substance reacts with oxygen, which produces heat and light
• Synthesis reactions occur when two or more reactions combine to form a substance
• Decomposition reactions occur when a compound breaks down to simpler substances

Displacement, Solubility, Equations, and Solutions

• Single-Displacement Reactions happen when one elements replaces another in a compound
• Double-Displacement Reactions are the exchange of ions between two compounds
• Solubility Rules: Some compounds compounds are water soluble while others are insoluble
• Electrolytes can be strong, weak, or none
• Writing Ionic and Net Ionic Equations: Molecular, Complete Ionic, and Net equations

Applications of Chemical Reactions

• Ozone Depletion: Chlorine from CFCs destroys ozone (O3 → 02).
• Acid Rain: Sulfur dioxide (SO₂) reacts with water to form acidic precipitation.
• Stomach Antacids: Neutralize excess stomach acid using bases like Mg(OH)2.

Explore compounds, their unique properties, and the Law of Constant Composition. Learn about chemical formulas, including empirical, molecular, and structural types. Discover molecular models like Ball-and-Stick and Space-Filling, and how to classify different elements.