Overview
This lecture covers Chapter 7: Chemical Reactions and Quantities, including writing and balancing chemical equations, types of reactions, oxidation-reduction, the mole, molar mass, stoichiometry, limiting reagents, percent yield, and energy changes in reactions.
Chemical Equations & Balancing (7.1)
- Reactants are on the left, products on the right of the reaction arrow.
- Subscripts in chemical formulas (e.g., O₂) cannot be changed; only coefficients (amounts) can be adjusted to balance equations.
- A balanced equation has the same number of each type of atom on both sides.
- Examples: H₂ + O₂ → H₂O must be balanced using coefficients, not by altering formulas.
Types of Chemical Reactions (7.2)
- Combination: Two or more substances form one product (A + B → AB).
- Decomposition: One compound breaks into two or more substances (AB → A + B).
- Single Replacement: One element replaces another in a compound (A + BC → AC + B).
- Double Replacement: Two compounds exchange parts (AB + CD → AD + CB).
- Combustion: Substance reacts with O₂ producing CO₂, H₂O, and energy.
Oxidation and Reduction (7.3)
- Oxidation: Loss of electrons ("Oil" in OIL RIG).
- Reduction: Gain of electrons ("Rig" in OIL RIG).
- The oxidized element increases in charge; the reduced element decreases in charge.
The Mole & Avogadro’s Number (7.4)
- One mole = 6.02 × 10²³ atoms, molecules, or particles.
- Moles relate the mass of a substance to the number of particles.
- Example: 1 mole of H₂O = 18g = 6.02 × 10²³ molecules.
Molar Mass (7.5)
- Molar mass is the sum of atomic masses (from the periodic table) in grams per mole.
- Example: Glucose (C₆H₁₂O₆) molar mass = 6×12 + 12×1 + 6×16 = 180 g/mol.
Calculations Using Molar Mass (7.6)
- Convert between grams and moles using molar mass.
- Formula: grams = moles × molar mass; moles = grams ÷ molar mass.
Mole Relationships in Chemical Equations (7.7)
- Use coefficients from balanced equations to calculate mole ratios between reactants and products.
- Example: If the equation is 2 Fe + 3 S → Fe₂S₃, then 2 moles Fe react with 3 moles S.
Calculating Mass in Chemical Reactions (7.8)
- Convert given mass to moles, use mole ratio, then convert back to mass as needed.
- Always use conversion factors to move between mass and moles.
Limiting Reagents & Percent Yield (7.9)
- Limiting reagent: The reactant that runs out first and limits the amount of product formed.
- Percent yield = (actual yield ÷ theoretical yield) × 100%.
- Identify limiting reagent by comparing moles that each reactant can produce.
Energy in Chemical Reactions (7.10)
- Exothermic reaction: Releases heat (ΔH negative), products are lower in energy than reactants.
- Endothermic reaction: Absorbs heat (ΔH positive), products are higher in energy than reactants.
Key Terms & Definitions
- Reactants — Substances present before the reaction.
- Products — Substances formed by the reaction.
- Coefficient — Number before a formula, shows quantity.
- Subscript — Small number in a formula, shows number of atoms.
- Mole — 6.02 × 10²³ particles of a substance.
- Molar mass — Grams per mole of a substance.
- Limiting reagent — Reactant used up first in a reaction.
- Percent yield — Actual yield as a percentage of theoretical yield.
- Exothermic — Reaction that releases heat.
- Endothermic — Reaction that absorbs heat.
Action Items / Next Steps
- Practice balancing chemical equations and using coefficients.
- Complete practice problems converting between grams, moles, and molecules.
- Calculate limiting reagents and percent yields for given reactions.
- Review energy diagrams for exothermic and endothermic reactions.
- Read the textbook sections 7.1–7.10 and solve end-of-chapter problems.