Chapter 5: Molecules and Compounds

Formation and Properties of Compounds

When elements combine to form compounds, their properties change completely. The Law of Constant Composition states that all samples of a given compound have the same proportions of their constituent elements. For example, water is always H2O; changing the number of hydrogen or oxygen atoms would result in a different compound. The mass ratio of oxygen to hydrogen in water is always 8:1, calculated from their atomic masses (O = 16, H = 1).

Polyatomic Ions and Chemical Formulas

Polyatomic ions are groups of atoms that act as a single ion in chemical formulas. Parentheses are used in formulas when there are multiples of a polyatomic ion, such as in Mg(NO3)2. It is important to determine the number of each atom in a formula and understand the use of parentheses.

• Example: In Mg(NO3)2:

  • Mg: 1 atom

  • NO3: 2 groups

  • N: 2 atoms (1 per group × 2 groups)

  • O: 6 atoms (3 per group × 2 groups)

Molecular vs. Empirical Formulas

The molecular formula shows the exact number of atoms of each element in a molecule, while the empirical formula shows the simplest whole-number ratio of the elements.

Classification of Elements and Compounds

Elements and compounds can be classified as atomic, molecular, or ionic. Atomic elements exist as single atoms (e.g., Ne), molecular elements exist as molecules (e.g., O2), molecular compounds are composed of molecules (e.g., H2O), and ionic compounds consist of ions (e.g., NaCl).

Order of Elements in Chemical Formulas

Generally, the less electronegative element is written first in a chemical formula. In organic compounds, carbon is written first, followed by hydrogen, then other elements.

Diatomic Molecules

Diatomic molecules are molecules composed of two atoms of the same element. Common diatomic elements in nature include hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine.

Chapter 6: Chemical Composition

Avogadro’s Number and the Mole

Avogadro’s number (6.022 × 1023) is the number of atoms in one mole of any substance. The mole is a fundamental unit for counting atoms, molecules, or ions in chemistry.

Molar Mass

The molar mass is the mass of one mole of atoms of an element, numerically equal to the atomic mass but expressed in grams. For compounds, the molar mass is the sum of the molar masses of the constituent elements.

• Example: For CO2:

  • C: 12.01 g/mol

  • O: 16.00 g/mol × 2 = 32.00 g/mol

  • Total: 44.01 g/mol

Chapter 7: Chemical Reactions

Types of Chemical Reactions

Chemical reactions can be classified into several types:

• Oxidation-Reduction: Involves transfer of electrons.

• Combustion: Rapid reaction with oxygen producing heat and light.

• Precipitation: Formation of a solid from solution.

• Gas Evolution: Production of a gas during the reaction.

Evidence of Chemical Reactions

Evidence that a chemical reaction has occurred includes color change, formation of a precipitate, gas production, and energy change (heat/light).

States of Reactants and Products

Abbreviations are used to indicate the physical state of substances in chemical equations:

Balancing Chemical Equations

Balancing chemical equations ensures the number of each type of atom is equal on both sides. This is achieved by adjusting coefficients, not subscripts. The process changes the number of molecules but not their identity.

Aqueous Solutions

When compounds dissolve in water, they form aqueous solutions where ions are separated and move independently.

Molecular, Complete Ionic, and Net Ionic Equations

• Molecular equation: Shows all reactants and products as compounds.

• Complete ionic equation: Shows all ions present in solution.

• Net ionic equation: Shows only the ions and molecules directly involved in the reaction.

Classification of Reactions by Atom Movement

Chemical reactions can be classified based on how atoms are rearranged:

Chapter 8: Quantities in Chemical Reactions

Stoichiometry

Stoichiometry is the numerical relationship between chemical quantities in a balanced equation. It allows prediction of product amounts from reactants and determination of required reactant quantities.

Limiting Reactant and Theoretical Yield

The limiting reactant is the reactant that is completely consumed first, limiting the amount of product formed. The theoretical yield is the maximum amount of product that can be formed from the given reactants.

Analogy: Making Pancakes

Stoichiometry can be understood using a recipe analogy. For example, if 1 cup flour + 2 eggs + ½ tsp baking powder makes 5 pancakes, the ingredient that runs out first limits the number of pancakes (limiting reactant), and the maximum possible pancakes is the theoretical yield.

Enthalpy and Reaction Types

Enthalpy is the heat content of a system. Endothermic reactions absorb heat, while exothermic reactions release heat.

• Endothermic: Energy is absorbed; surroundings feel cooler.

• Exothermic: Energy is released; surroundings feel warmer.

Additional info: The notes do not cover calculation of formula mass, naming compounds, mass-to-mass conversions, or percent yield for this exam.