Overview
This lecture covers conservation of mass in chemical reactions, relative formula mass calculations, mole concepts, balancing equations, limiting reactants, solution concentrations, yield, atom economy, and gas volumes.
Conservation of Mass & Balancing Equations
- The total mass of substances is conserved in chemical reactions; atoms are rearranged, not destroyed or created.
- Equations must be balanced so the same number of each atom appears on both sides.
Relative Atomic and Formula Mass
- Relative atomic mass (RAM) is the weighted average mass of an atom.
- Relative formula mass for a compound is the sum of RAMs of its atoms (e.g., CO₂: 12 + (2×16) = 44).
- Some reactions produce gases, which may escape and cause a measured decrease in mass.
Moles & Calculations
- A mole is a set amount (Avogadro's number) of atoms/molecules; it's a way to compare amounts.
- One mole of a substance has a mass equal to its RAM or formula mass in grams (e.g., 1 mol C = 12g).
- Number of moles = mass (g) ÷ RAM or relative formula mass.
- In calculations, use: moles = mass ÷ RAM; mass = moles × RAM.
- Stoichiometry is the ratio of moles of reactants to products (e.g., CH₄ + 2O₂ → CO₂ + 2H₂O is 1:2:1:2).
Limiting Reactant
- The limiting reactant is the substance that runs out first and limits the amount of product formed.
Concentration of Solutions
- Concentration can be expressed in g/dm³ or mol/dm³ (1 dm³ = 1000 cm³).
- A 1 mol/dm³ solution means 1 mole dissolved per 1 dm³.
Yield & Atom Economy
- Percentage yield = (actual mass of product ÷ theoretical mass) × 100.
- Atom economy = (RAM of desired product ÷ total RAM of reactants) × 100.
- Atom economy measures efficiency of converting reactants into desired products.
Gas Volumes at RTP
- One mole of any gas occupies 24 dm³ at room temperature and pressure (20°C, 1 atm).
- Volume (dm³) = moles × 24.
Key Terms & Definitions
- Relative Atomic Mass (RAM) — the weighted average mass of an atom.
- Relative Formula Mass — sum of RAMs in a compound.
- Mole — quantity containing Avogadro's number of particles.
- Stoichiometry — ratio of reactants and products in a reaction.
- Limiting Reactant — reactant that determines the amount of product.
- Concentration — amount of solute per unit volume of solution.
- Yield — actual versus theoretical product made.
- Atom Economy — efficiency of reactant mass in forming desired product.
- RTP (Room Temperature and Pressure) — 20°C and 1 atm.
Action Items / Next Steps
- Memorize the moles, mass, and RAM formula.
- Practice balancing equations and calculating moles, yield, and atom economy.
- Read ahead on chemical changes for the next topic.