Chemistry — Essential Concepts
Periodic trends, bonding, stoichiometry, gas laws, and acid/base chemistry — every Chemistry CBE essential on one page.
Chemistry CBE quick-reference — periodic trends, bonding, mole math, gas laws, and acid-base chemistry.
Periodic Trends
- Atomic radius: increases down a group, decreases across a period
- Ionization energy: opposite (decreases down, increases across)
- Electronegativity: increases up + right (fluorine highest)
- Metallic character: increases down + left
Bonding
- Ionic: metal + nonmetal; electron transfer (NaCl)
- Covalent: nonmetal + nonmetal; electron sharing (H₂O)
- Metallic: sea of electrons (Fe)
- Polarity: electronegativity difference > 0.4 → polar covalent
Naming & Formulas
- Ionic: name cation first, then anion (-ide for monatomic)
- Polyatomics: NO₃⁻ nitrate, SO₄²⁻ sulfate, PO₄³⁻ phosphate, NH₄⁺ ammonium, OH⁻ hydroxide
- Acids: HCl hydrochloric, H₂SO₄ sulfuric, HNO₃ nitric, CH₃COOH acetic
The Mole
- 1 mole = 6.022 × 10²³ particles (Avogadro's number)
- Molar mass: from periodic table (g/mol)
- Moles = mass / molar mass
- STP: 1 mol gas = 22.4 L
Stoichiometry
- Balance equation
- Convert mass → moles using molar mass
- Use mole ratio from balanced equation
- Convert moles → desired unit (mass, volume, particles)
Limiting reactant: whichever runs out first based on ratios.
Gas Laws
- Ideal gas: PV = nRT (R = 0.0821 L·atm/mol·K)
- Combined: P₁V₁/T₁ = P₂V₂/T₂
- Boyle (constant T): PV = constant
- Charles (constant P): V/T = constant
- Always use Kelvin (K = °C + 273)
Acid-Base
- pH = −log[H⁺]; pH + pOH = 14
- Acid: pH < 7, donates H⁺; Base: pH > 7, accepts H⁺ or donates OH⁻
- Strong acids fully dissociate (HCl, H₂SO₄, HNO₃); weak acids partially
- Neutralization: acid + base → salt + water
Thermochemistry & Kinetics
- Exothermic: releases heat (ΔH < 0)
- Endothermic: absorbs heat (ΔH > 0)
- Activation energy: minimum energy to start reaction; catalysts lower it
- Reaction rate ↑ with temperature, concentration, surface area
Common Test Mistakes
- Forgetting to balance the equation before stoichiometry
- Using °C instead of K in gas laws
- Mixing up ionization energy and electronegativity trends
- Confusing moles and molarity (mol vs mol/L)