Overview
This lecture reviews key concepts in chemical calculations for the OCR Chemistry II syllabus, including ions, compounds, equations, moles, solution calculations, gas laws, yields, atom economy, and empirical formulas.
Forming Compounds & Ions
- Ions form when atoms gain or lose electrons to achieve a full electron shell.
- Metals (e.g., sodium) lose electrons to form positive ions; non-metals (e.g., chlorine) gain electrons to form negative ions.
- Ionic bonds are electrostatic attractions between oppositely charged ions.
- Group 1 forms 1+, Group 2 forms 2+, Group 6 forms 2–, Group 7 forms 1– ions.
- Know common molecular ions: OH–, NO₃–, NH₄+, SO₄²–, CO₃²–.
Writing Ionic Compound Formulas
- Use "swap and drop": swap ion charges, drop signs/numbers, and simplify formula.
Water of Crystallization
- Salts can be hydrated (with water) or anhydrous (without water).
- Water of crystallization: number of water molecules per mole of salt (e.g., CuSO₄·5H₂O).
- To find water of crystallization: measure mass before and after heating, find difference (water lost), calculate moles, and determine ratio.
Ionic and Chemical Equations
- Ionic equations show only ions participating in reactions; spectator ions remain unchanged and are omitted.
- Net ionic equation for neutralization: H⁺ + OH⁻ → H₂O.
The Mole and Avogadro’s Number
- A mole measures amount of substance; 1 mol = 6.02 × 10²³ particles (Avogadro’s number).
- Number of particles = moles × Avogadro’s constant.
Calculating Moles: Solids and Solutions
- For solids: moles = mass (g) / Mᵣ (molecular/atomic mass).
- For solutions: moles = concentration (mol/dm³) × volume (dm³).
- Convert cm³ to dm³ by dividing by 1000 or multiplying by 10⁻³.
Ideal Gas Law
- PV = nRT relates pressure (P, Pa), volume (V, m³), moles (n), gas constant (R = 8.31), temperature (T, K).
- Standard conditions: 298 K, 100 kPa.
- Convert pressure to Pa and volume to m³ before calculations.
State Symbols and Balancing Equations
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
- Use balanced equations to determine masses or volumes of products/reactants.
Empirical and Molecular Formula
- Empirical formula: simplest whole-number ratio of atoms in a compound.
- To find: write masses/percentages, divide by atomic masses, divide by smallest result for ratios.
- For combustion analysis, use CO₂ for C, H₂O for H.
Percentage Yield and Atom Economy
- Percentage yield = (actual yield / theoretical yield) × 100.
- Theoretical yield: calculated from balanced equation, assuming complete reaction.
- Atom economy = (Mᵣ of desired product / sum of Mᵣ of all products) × 100.
- High atom economy indicates efficient use of resources and less waste.
Key Terms & Definitions
- Ionic bond — Electrostatic attraction between oppositely charged ions.
- Water of crystallization — Water molecules integrated in a salt's crystal structure.
- Mole (mol) — SI unit for amount of substance (6.02 × 10²³ particles).
- Avogadro’s constant — Number of particles in one mole (6.02 × 10²³).
- Empirical formula — Simplest ratio of elements in a compound.
- Percentage yield — Efficiency of a reaction given as a percentage of the theoretical maximum.
- Atom economy — Measure of how efficiently reactants are converted to desired products.
Action Items / Next Steps
- Memorize common ions and their charges.
- Practice calculating empirical formulas and percentage yields.
- Review and practice using the ideal gas equation with unit conversions.
- Complete related homework problems on calculations with moles, solutions, and gases.