Rucete ✏ AP Chemistry In a Nutshell
3. Ionic Compounds, Formulas, and Reactions
This chapter explores how to write chemical formulas, understand different types of reactions, name ionic compounds, and predict ionic interactions using bonding principles and net ionic equations.
Chemical Formulas
• A chemical formula uses atomic symbols and subscripts to represent the elements and number of atoms in a compound.
• Parentheses are used to group atoms and show multiples (e.g., Al(NO₃)₃).
• Hydrates include water molecules, shown after a dot (e.g., CoCl₂·6H₂O).
Types of Formulas
• Empirical formula: simplest whole-number ratio of atoms.
• Molecular formula: actual number of atoms in a molecule (e.g., C₆H₆).
• Structural formula: shows how atoms are connected by covalent bonds.
Balancing Chemical Equations
• Chemical equations must be balanced to satisfy the law of conservation of matter.
• Coefficients are added to equalize atoms on both sides without changing subscripts.
• Use inspection method or ion-electron method (for redox reactions).
Combustion Reaction Example
• CH₃CH₂CH₃ + 5O₂ → 3CO₂ + 4H₂O
• Combustion produces CO₂ and H₂O; balance carbon and hydrogen first, then oxygen.
Double-Replacement Reactions
• Two ionic compounds exchange cations to form new products.
• Balance ions and atoms using coefficients.
• Use tables to track atom counts if needed.
Reaction Types
• Combustion: organic + O₂ → CO₂ + H₂O
• Single-Replacement: element + compound → new element + new compound
• Double-Replacement: cations switch places in two compounds
• Neutralization: acid + base → salt + water
• Synthesis: elements form a compound
• Decomposition: compound breaks into elements or simpler compounds
Net Ionic Reactions
• Focus on ions that actually change; remove spectator ions.
• Use net ionic equations to generalize reactions and show essential chemical change.
Half-Reactions and Redox
• Half-reactions show electron transfer: oxidation (loss) or reduction (gain).
• Redox reaction = oxidation half + reduction half.
• Examples include combustion and single-replacement reactions.
Ionic Bonding and Electron Configurations
• Atoms bond by losing/gaining electrons to resemble noble gases.
• Metals → cations (positive); Nonmetals → anions (negative).
• Ionic bond = electrostatic attraction between oppositely charged ions.
Monatomic Ions
• Representative metals lose s and p electrons to form cations (e.g., Na⁺, Ba²⁺).
• Representative nonmetals gain electrons to form anions (e.g., Cl⁻, O²⁻).
• Ions with identical electron configurations are isoelectronic.
Transition Metals and Charges
• Transition metals can form multiple cations with different charges (e.g., Fe²⁺, Fe³⁺).
• Use Roman numerals to indicate charge in compound names (e.g., iron(III) oxide).
• Exceptions include Ag⁺, Zn²⁺, and Al³⁺, which form only one common ion.
Polyatomic Ions
• Groups of atoms bonded covalently that carry a net charge (e.g., NO₃⁻, SO₄²⁻).
• Polyatomic ions remain intact in reactions; use parentheses in formulas when needed (e.g., Ca(NO₃)₂).
• Memorizing common polyatomic ions helps in naming and formula writing.
Writing Ionic Formulas
• Use ion charges to balance total positive and negative charge.
• Write the metal first, then the nonmetal or polyatomic ion.
• Simplify ratios of subscripts to lowest whole numbers.
Example: Al³⁺ and O²⁻ → Al₂O₃
Naming Ionic Compounds
• Binary compounds: name the cation, then the anion with “-ide” suffix (e.g., NaCl = sodium chloride).
• Use Roman numerals for metals with multiple oxidation states (e.g., CuCl = copper(I) chloride).
• For compounds with polyatomic ions, keep the ion name unchanged (e.g., NaNO₃ = sodium nitrate).
Ionic Solids and Crystal Structures
• Ionic compounds form crystal lattices—a repeating, orderly 3D structure.
• Each ion is surrounded by oppositely charged ions in a pattern that maximizes attractions and minimizes repulsions.
• High melting points, brittleness, and conductivity when molten or dissolved are characteristic of ionic solids.
Properties of Ionic Compounds
• High melting and boiling points due to strong ionic bonds.
• Solids are poor electrical conductors; molten or aqueous forms conduct well.
• Generally soluble in water, forming electrolyte solutions.
In a Nutshell
Ionic compounds form when atoms transfer electrons to achieve noble gas configurations, creating oppositely charged ions held together by electrostatic attraction. Understanding how to name, write, and predict formulas and reactions is essential for describing ionic substances and their behaviors in chemical processes.