Overview
This lecture covers the foundational concepts of chemistry, including classification of matter, laws of chemical combination, atomic and molecular theory, mole concept, concentration terms, empirical/molecular formulas, stoichiometry, and limiting reagent.
Introduction to Chemistry
- Chemistry is the branch of science that studies the composition, structure, properties, and changes of matter.
- Matter is anything that has mass and occupies space.
Classification of Matter
- Matter can be classified physically as solid, liquid, or gas; these states are interconvertible by changing temperature or pressure.
- Chemically, matter is divided into pure substances (elements and compounds) and mixtures (homogeneous and heterogeneous).
Elements, Compounds, and Mixtures
- Elements contain only one type of atom; classified as metals, non-metals, or metalloids.
- Compounds are pure substances made from two or more elements in a fixed ratio.
- Mixtures are physical combinations of substances without a fixed proportion; can be homogeneous (uniform) or heterogeneous (non-uniform).
Properties of Matter
- Physical properties: measurable without changing the substance (e.g., density, melting point).
- Chemical properties: observed during a chemical change (e.g., reactivity, flammability).
Measurement and SI Units
- Important physical quantities include length (meter), mass (kilogram), time (second), temperature (kelvin), amount of substance (mole), electric current (ampere), and luminous intensity (candela).
- Derived units include area (m²), volume (m³), density (kg/m³), etc.
- SI prefixes (e.g., kilo-, centi-, milli-, nano-) are used to express multiples/fractions of units.
Laws of Chemical Combination
- Law of Conservation of Mass: mass is neither created nor destroyed in a chemical reaction.
- Law of Definite Proportions: a chemical compound always contains the same elements in a fixed ratio by mass.
- Law of Multiple Proportions: elements can combine in different ratios to form different compounds.
- Law of Reciprocal Proportion, Law of Combining Volumes, Avogadro's Law: explain relationships between reacting volumes, masses, and numbers of particles.
Dalton’s Atomic Theory and Limitations
- Matter is made of tiny indivisible atoms, which combine in simple ratios to form compounds.
- Isotopes, isobars, divisibility, and complex ratios are limitations of Dalton’s theory.
Atomic and Molecular Mass
- Atomic mass = sum of protons and neutrons; measured in atomic mass units (u).
- Molecular mass = sum of atomic masses in a molecule.
- Average atomic mass is weighted by isotopic abundance.
- Formula mass is used for ionic compounds.
The Mole Concept
- 1 mole = 6.022 × 10²³ entities (Avogadro’s number).
- Moles relate mass to number of particles and volume (for gases at STP, 1 mol = 22.4 L).
- Number of moles = given mass / molar mass.
Empirical and Molecular Formula
- Empirical formula shows simplest whole number ratio of elements in a compound.
- Molecular formula = (Empirical formula) × n, where n = (Molar mass / Empirical formula mass).
- Calculate empirical formula from percent composition.
Concentration Terms
- Mass percent: (mass of solute / mass of solution) × 100.
- Molarity (M): moles of solute / liters of solution.
- Molality (m): moles of solute / kg of solvent.
- Mole fraction: moles of component / total moles in solution.
Stoichiometry and Limiting Reagent
- Stoichiometry calculates mass/volume relationships in reactions using balanced equations.
- The limiting reagent is the reactant that finishes first, limiting product formation.
Key Terms & Definitions
- Element — A pure substance of one type of atom.
- Compound — Pure substance of elements in a fixed ratio.
- Mixture — Physical combination of substances, variable composition.
- Empirical Formula — Simplest ratio of atoms in a compound.
- Mole — 6.022 × 10²³ entities of a substance (Avogadro’s number).
- Limiting Reagent — Reactant consumed first, limits product amount.
- Molarity (M) — Moles of solute per liter of solution.
- Molality (m) — Moles of solute per kg of solvent.
- Mole Fraction — Ratio of moles of one component to total moles.
Action Items / Next Steps
- Revise all laws of chemical combination and practice example problems.
- Practice empirical and molecular formula calculations.
- Complete assigned homework on mole concept, concentration terms, and stoichiometry.
- Prepare for questions involving limiting reagent identification and calculations.